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A numerical investigation of heat transfer enhancement in offset strip fin heatexchangers in self-sustained oscillatory flows

Saidi Arash Sundeacuten Bengt

Published inInternational Journal of Numerical Methods for Heat amp Fluid Flow

DOI101108EUM0000000005984

2001

Link to publication

Citation for published version (APA)Saidi A amp Sundeacuten B (2001) A numerical investigation of heat transfer enhancement in offset strip fin heatexchangers in self-sustained oscillatory flows International Journal of Numerical Methods for Heat amp Fluid Flow11(7) 699-716 httpsdoiorg101108EUM0000000005984

Total number of authors2

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Heat transferenhancement

699

International Journal of NumericalMethods for Heat amp Fluid FlowVol 11 No 7 2001 pp 699-716

MCB University Press 0961-5539

Received February 2001Revised June 2001

Accepted June 2001

A numerical investigation ofheat transfer enhancement in

offset strip fin heatexchangers in self-sustained

oscillatory flowsArash Saidi and Bengt SundeAcircn

Division of Heat Transfer Lund Institute of Technology Lund Sweden

Keywords Fins Heat transfer Computational methods

Abstract Numerical analysis of the instantaneous flow and heat transfer has been carried out foroffset strip fin geometries in self-sustained oscillatory flow The analysis is based on the two-dimensional solution of the governing equations of the fluid flow and heat transfer with the aid ofappropriate computational fluid dynamics methods Unsteady calculations have been carried outThe obtained time-dependent results are compared with previous numerical and experimental resultsin terms of mean values as well as oscillation characteristics The mechanisms of heat transferenhancement are discussed and it has been shown that the fluctuating temperature and velocitysecond moments exhibit non-zero values over the fins The creation processes of the temperature andvelocity fluctuations have been studied and the dissimilarity between these has been proved

Nomenclatureb = Fin thicknessdh = Hydraulic diameterf = Fanning friction factorfs = Oscillation frequencyh = Mean heat transfer coefficienth = fin heightj = Colburn factorl = Fin lengthLm = The length of calculation moduleNu = Nusselt numberP = PressurePr = Prandtl number

Re = Reynolds numbers = Fin transverse spacingSt = Strouhal numberT = Mean temperaturet = Fluctuating part of temperatureUc = Mean velocity in the minimum areaUi = Mean velocity components (i = 123)Um = Mean velocityui = Fluctuating part of velocity

components (i = 123)frac34 = Kinematic viscosityraquo = Density

IntroductionThe introduction of compact heat exchangers in gas turbine cycles is increasingOne of the applications is intercooling (combined with recuperators) to achievehigher levels of thermal efficiency The most common way of intercooling is touse a gas-to-liquid compact heat exchanger (Saidi et al 2000) By definition acompact heat exchanger is one which incorporates a heat transfer surface withan area density (or compactness) above 700m2m3 on at least one of the fluidsides which usually is the gas side (Shah et al 1998) In order to increase the

T h e c u r r e n t is s u e a n d f u l l te x t a r c h iv e o f th is jo u r n a l i s a v a i la b le a t

httpwwwemerald-library comft

Financial support from the Swedish Energy Administration (STEM) is kindly acknowledged

HFF117

700

compactness of an intercooler it is necessary to consider closely the types ofextended surfaces being suitable The dominating part of the thermal resistancefor liquid-to-gas heat exchangers occurs on the gas (or air) side and thus it isworthwhile to focus attention on different types of fins for the air-side There aremany types of extended surface concepts that can be used or are being used inintercoolers As an example the extended surface type called offset strip fingeometry (Joshi and Webb 1987) is presented in Figure 1 This geometry is verycommon It provides high heat transfer area per unit volume and high Nunumbers The heat transfer enhancement mechanism is through interruption oflaminar boundary layers on the fins and self-sustained oscillatory flows athigher Re numbers In Figure 1 the geometrical parameters of an offset strip finarrangement are shown These parameters are l s b and h namely fin lengthfin transverse spacing fin thickness and fin height

Jacobi and Shah (1996) describe the flow characteristics in such fins AtReynolds (Re) numbers less than 400 the flow is laminar and steady and theboundary layer dominates the heat transfer process At intermediate Renumbers 400-1000 the flow remains laminar but unsteadiness and vortexshedding tend to dominate At Re numbers above 1000 the flow becomesturbulent A more detailed study of the flow transition including effects ofgeometrical parameters has been presented in Joshi and Webb (1987) and anexperimental correlation for prediction of the transition Re number wassuggested The j and f correlations are also suggested for laminar and turbulentranges but the self-sustained oscillatory flow range is not included in thecorrelations There are also other experimental investigations These worksreport different correlations for heat transfer and pressure drop The firstanalytical effort by Kays (1972) gave a modified laminar boundary layersolution Power-law fitted correlations were presented by Wieting (1975) for 22geometries for Re numbers of laminar and turbulent ranges excluding theintermediate Re numbers Most recently Manglik and Bergles (1995) re-analyzed the existing empirical data for actual cores and suggested designcorrelations for heat transfer and friction factor in the form of single continuousexpressions covering all flow regimes Once again their correlations excludethe intermediate Re numbers

Numerical investigations on the subject are also available Sparrow et al(1977) investigated a case with zero fin thickness They did not capture the

Figure 1Offset strip fin geometry

Heat transferenhancement

701

impingement in front of the fin and the recirculation downstream of it due tothe zero thickness assumption for the fins Patankar and Prakash (1981)investigated the effect of fin thickness on the flow field and heat transfer Theyconcluded that even though thicker fins result in higher pressure drops theheat transfer does not improve significantly Suzuki et al (1982) carried out anumerical investigation for mixed convection in laminar flow throughstaggered arrays of zero thickness offset strip fins That investigation has beenextended to finite thickness of the fins by Suzuki et al (1985) Xi et al (1992)carried out numerical computations for the range of Re numbers (low values)where the flow remained steady and laminar and it was observed that the flowinstability in the wake of the fin is effective to enhance the heat transferdownstream of the fin as well Two-dimensional numerical computations for aperiodically changing unsteady flow regime or so-called second laminar flowregime have been carried out (Suzuki et al 1994 Xi et al 1995) This regime ischaracterized by self-sustained flow oscillations These and other studies havepresented interesting and detailed investigations of the oscillating temperatureand flow fields and obtained results showing the enhancement mechanismsdue to the unsteady character of the flow A detailed analysis of the heat andmomentum transport in a periodic series of fins in a communicating duct usingperiodicity assumption was carried out by Majumdar and Amon (1992) Theystudied the transport phenomena due to oscillations in temperature andvelocity fields

Mercier and Tochon (1997) have carried out a two-dimensional timedependent analysis in the turbulent flow regime Owing to limitations ofcomputer capabilities all of the turbulent scales could not be resolved in theircomputations and the computational method was referred to as a pseudo-directnumerical simulation Two recent and related investigations are those of Zhanget al (1997) and DeJong et al (1998) They showed that inclusion of flowunsteadiness plays a very important role in the accurate prediction of heattransfer They also verified the fact that a two-dimensional unsteady numericalsimulation captures the important features of the flow and heat transfer for arange of conditions

The present investigation focuses attention on the intermediate Re numbersat which the majority of experimental investigations do not providecorrelations The interesting phenomenon of dissimilarity of heat transfer andmomentum transport with respect to heat transfer enhancement is studied forthe case of offset strip fin arrays Such a phenomenon has been studied forother geometries like a series of single fins by Xi et al (1995) Suzuki et al(1994) and for a periodic series of fins in a communicating channel byMajumdar and Amon (1992) Although similar studies have been carried outbut not for the case of an offset strip fin geometry which is the geometry in thisinvestigation one objective is to improve the understanding of theenhancement mechanism in this particular geometry It is not obvious that theresults of the former studies on different geometries are applicable to thisspecific geometry It is also worth mentioning that the considered geometry in

HFF117

702

contrast with those studied in the past is a practical extended surfaceapplication DeJong et al (1998) studied this case but they did not pay attentionto the detailed mechanisms of the heat and momentum transfer processes

Taking these points into consideration it is clear that the current study isunique in dealing with the character of heat transfer enhancement mechanismin offset strip fin geometries The results of this study will show that theenhancement mechanism in a certain Re number range is special and also itwill prove the dissimilarity between the two processes of heat transfer andmomentum transfer in that certain range of Re numbers It is also worthmentioning that the overall goal of this study is not to present new numericalmethods or verify any numerical scheme but instead to reveal the physicalphenomena and the related implications for the heat transfer mechanism Thusan established finite volume method is used in the computational analysis

Assumptions and method of analysisPossible methods of comparison and evaluation of different geometricalconcepts of extended surfaces have been presented by Shah et al (1998)Investigations of the thermal performance of various extended surfacegeometries are commonly experimental but rapid developments of computercapacity and numerical solution methods have implied that a theoreticalanalysis is reliable in many cases So-called CFD-methods (computational fluiddynamics) (see for example Veersteg and Malalasekera (1995)) may offer acheaper and more flexible tool as a variety of extended surfaces or fins is to beanalyzed Such methods will give the opportunity for testing new conceptsbefore experimentation that can be expensive and troublesome In this reportfour different extended surfaces are considered

The computational domain is limited to a basic module shown in Figure 2 Thissimplification is based on the assumption of fully developed flow and thermalfields in the array of fins Two pairs of periodic boundaries in the longitudinal andtransverse directions limit the module Experimental investigations (DeJong et al1998 Joshi and Webb 1987) justify these assumptions

The two-dimensional continuity Navier-Stokes and energy equations are solved

ui

xi

ˆ 0 hellip1dagger

ui

frac12Dagger

xjhellipuiujdagger ˆ iexcl 1

raquo

p

xiDagger frac34

2ui

xjxjhellip2dagger

T

frac12Dagger

xj

hellipTujdagger ˆ frac34

Pr

2T

xjxj

hellip3dagger

The flow is assumed to be incompressible with constant properties Buoyancyforces and viscous dissipation are not considered Periodic boundary conditions

Heat transferenhancement

703

have been applied at the inlet and outlet sections and in the transverse directionThe wall boundary conditions are no-slip for the momentum equations andconstant heat flux for the energy equation

A finite volume multi-block method has been applied In this case anavailable general purpose CFD-code (STAR-CD) has been applied to solve theresulting governing algebraic equations In previous works this code has beenfound to perform well if applied with care The QUICK scheme is used to

Figure 2The grid used for the

calculations

HFF117

704

handle convective-diffusive terms The PISO algorithm is used for treating thecoupling of the pressure and velocity fields The Crank-Nicolson method isused for time discretization The maximum residual tolerance of all equationshas been kept at less than 10plusmn6 and the maximum value of the Courant numberfor all Re numbers was about unity

A multi-block calculation has been carried out with six blocks The numberof grid points in the calculations varied from 162 pound 40 to 270 pound 62 (Figure 2)Further refinement to 324 pound 75 grid points changed the results of the frictionfactor and Nu numbers by less than 1 percent The computer used to carry outthe calculations was a Digital AlphaStation1 255 (with CPU clock rate of233MHz) and the CPU time required for the calculations was 0001857 sec pertime step and mesh point The time step of the calculations is chosen in order toachieve the correct time resolution so that there are a sufficient number of timesteps to capture all the oscillations As a result CFL (Courant) numbers of amaximum value around unity in the whole domain were proved

The Re number for the flow is defined as

Re ˆ Ucdh=frac34 hellip4dagger

Uc is the velocity at the minimum flow area and dh is the hydraulic diameterwhich is defined as

dh ˆ 2hellips iexcl bdaggerl=hellipl Dagger bdagger hellip5dagger

The pressure drop has been related to the Fanning friction factor

f ˆ 2centP

raquoU2c

dh

4Lm

sup3 acutehellip6dagger

and the average heat transfer coefficient is presented in terms of the Colburn jfactor

j ˆ Nu

Re Pr1=3ˆ h Pr2=3

raquocpUmhellip7dagger

ResultsThe case study in this investigation is an offset strip fin geometry shown inFigure 3 with the geometrical dimensions given in Table I These dimensionsare identical to those in a previous investigation (DeJong et al 1998) and theoverall computed results are compared with the numerical results of that studyThe focus point of this study as has already been mentioned has been differentfrom their investigation and attention is paid to the mechanisms of heattransfer enhancement

The results are presented in three sections In the first section the timeaveraged mean values of heat transfer and fluid flow properties friction factorand Colburn j factor are presented The second section includes the time-

Heat transferenhancement

705

dependent features of the flow simulation In this part the time-dependentvelocity field is presented The oscillatory velocity history at a point is depictedand the dominating frequency of the oscillatory flow is determined and the non-dimensional oscillation similarity of the flow for a range of Re numbers isdiscussed In the last section the time-averaged second moments of the velocityand temperature fields are presented The corresponding production terms ofvelocity and temperature fluctuations are provided and discussed

Time-averaged mean valuesThe friction factor results compared with the results of DeJong et al (1998) areprovided in Figure 4 The calculated friction factors of this study are under-predicted (14-16 percent) compared with those of DeJong et al (1998) TheColburn j factor results are compared with corresponding results of thatinvestigation in Figure 5(a) The deviations between the results of this studyand those of DeJong et al (1998) are in the range of 6-8 percent Thesecomparisons show that the present numerical simulation method is able tosatisfactorily reproduce the results obtained for the same geometry by DeJonget al (1998) in terms of time-averaged mean values It should thus be reasonableto interpret the time-dependent characteristics of the flow and temperaturefields

Figure 3Basic module and

computational domain

Table IDimensions of the

offset strip fingeometry

Parameter Value

bl 0117sl 0507l 24mm

HFF117

706

If one considers the ratio jf it is found that a maximum appears around Re =

900-1000 The reason for this might be that laminar self-oscillating flow occurs

and the benefit in overall performance is believed to be caused by dissimilarity

between the mechanism for momentum and heat transfer A more detaileddiscussion will follow

Unsteady velocity field description

Figure 6 shows the velocity field around the fin during a complete period of

oscillations This Figure shows the development of the flow in form of the time-

Figure 4Comparison of frictionfactor results

Figure 5Comparison of Colburn jfactor results

Heat transferenhancement

707

Figure 6Velocity vectors over a

period of oscillations

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

Heat transferenhancement

701

impingement in front of the fin and the recirculation downstream of it due tothe zero thickness assumption for the fins Patankar and Prakash (1981)investigated the effect of fin thickness on the flow field and heat transfer Theyconcluded that even though thicker fins result in higher pressure drops theheat transfer does not improve significantly Suzuki et al (1982) carried out anumerical investigation for mixed convection in laminar flow throughstaggered arrays of zero thickness offset strip fins That investigation has beenextended to finite thickness of the fins by Suzuki et al (1985) Xi et al (1992)carried out numerical computations for the range of Re numbers (low values)where the flow remained steady and laminar and it was observed that the flowinstability in the wake of the fin is effective to enhance the heat transferdownstream of the fin as well Two-dimensional numerical computations for aperiodically changing unsteady flow regime or so-called second laminar flowregime have been carried out (Suzuki et al 1994 Xi et al 1995) This regime ischaracterized by self-sustained flow oscillations These and other studies havepresented interesting and detailed investigations of the oscillating temperatureand flow fields and obtained results showing the enhancement mechanismsdue to the unsteady character of the flow A detailed analysis of the heat andmomentum transport in a periodic series of fins in a communicating duct usingperiodicity assumption was carried out by Majumdar and Amon (1992) Theystudied the transport phenomena due to oscillations in temperature andvelocity fields

Mercier and Tochon (1997) have carried out a two-dimensional timedependent analysis in the turbulent flow regime Owing to limitations ofcomputer capabilities all of the turbulent scales could not be resolved in theircomputations and the computational method was referred to as a pseudo-directnumerical simulation Two recent and related investigations are those of Zhanget al (1997) and DeJong et al (1998) They showed that inclusion of flowunsteadiness plays a very important role in the accurate prediction of heattransfer They also verified the fact that a two-dimensional unsteady numericalsimulation captures the important features of the flow and heat transfer for arange of conditions

The present investigation focuses attention on the intermediate Re numbersat which the majority of experimental investigations do not providecorrelations The interesting phenomenon of dissimilarity of heat transfer andmomentum transport with respect to heat transfer enhancement is studied forthe case of offset strip fin arrays Such a phenomenon has been studied forother geometries like a series of single fins by Xi et al (1995) Suzuki et al(1994) and for a periodic series of fins in a communicating channel byMajumdar and Amon (1992) Although similar studies have been carried outbut not for the case of an offset strip fin geometry which is the geometry in thisinvestigation one objective is to improve the understanding of theenhancement mechanism in this particular geometry It is not obvious that theresults of the former studies on different geometries are applicable to thisspecific geometry It is also worth mentioning that the considered geometry in

HFF117

702

contrast with those studied in the past is a practical extended surfaceapplication DeJong et al (1998) studied this case but they did not pay attentionto the detailed mechanisms of the heat and momentum transfer processes

Taking these points into consideration it is clear that the current study isunique in dealing with the character of heat transfer enhancement mechanismin offset strip fin geometries The results of this study will show that theenhancement mechanism in a certain Re number range is special and also itwill prove the dissimilarity between the two processes of heat transfer andmomentum transfer in that certain range of Re numbers It is also worthmentioning that the overall goal of this study is not to present new numericalmethods or verify any numerical scheme but instead to reveal the physicalphenomena and the related implications for the heat transfer mechanism Thusan established finite volume method is used in the computational analysis

Assumptions and method of analysisPossible methods of comparison and evaluation of different geometricalconcepts of extended surfaces have been presented by Shah et al (1998)Investigations of the thermal performance of various extended surfacegeometries are commonly experimental but rapid developments of computercapacity and numerical solution methods have implied that a theoreticalanalysis is reliable in many cases So-called CFD-methods (computational fluiddynamics) (see for example Veersteg and Malalasekera (1995)) may offer acheaper and more flexible tool as a variety of extended surfaces or fins is to beanalyzed Such methods will give the opportunity for testing new conceptsbefore experimentation that can be expensive and troublesome In this reportfour different extended surfaces are considered

The computational domain is limited to a basic module shown in Figure 2 Thissimplification is based on the assumption of fully developed flow and thermalfields in the array of fins Two pairs of periodic boundaries in the longitudinal andtransverse directions limit the module Experimental investigations (DeJong et al1998 Joshi and Webb 1987) justify these assumptions

The two-dimensional continuity Navier-Stokes and energy equations are solved

ui

xi

ˆ 0 hellip1dagger

ui

frac12Dagger

xjhellipuiujdagger ˆ iexcl 1

raquo

p

xiDagger frac34

2ui

xjxjhellip2dagger

T

frac12Dagger

xj

hellipTujdagger ˆ frac34

Pr

2T

xjxj

hellip3dagger

The flow is assumed to be incompressible with constant properties Buoyancyforces and viscous dissipation are not considered Periodic boundary conditions

Heat transferenhancement

703

have been applied at the inlet and outlet sections and in the transverse directionThe wall boundary conditions are no-slip for the momentum equations andconstant heat flux for the energy equation

A finite volume multi-block method has been applied In this case anavailable general purpose CFD-code (STAR-CD) has been applied to solve theresulting governing algebraic equations In previous works this code has beenfound to perform well if applied with care The QUICK scheme is used to

Figure 2The grid used for the

calculations

HFF117

704

handle convective-diffusive terms The PISO algorithm is used for treating thecoupling of the pressure and velocity fields The Crank-Nicolson method isused for time discretization The maximum residual tolerance of all equationshas been kept at less than 10plusmn6 and the maximum value of the Courant numberfor all Re numbers was about unity

A multi-block calculation has been carried out with six blocks The numberof grid points in the calculations varied from 162 pound 40 to 270 pound 62 (Figure 2)Further refinement to 324 pound 75 grid points changed the results of the frictionfactor and Nu numbers by less than 1 percent The computer used to carry outthe calculations was a Digital AlphaStation1 255 (with CPU clock rate of233MHz) and the CPU time required for the calculations was 0001857 sec pertime step and mesh point The time step of the calculations is chosen in order toachieve the correct time resolution so that there are a sufficient number of timesteps to capture all the oscillations As a result CFL (Courant) numbers of amaximum value around unity in the whole domain were proved

The Re number for the flow is defined as

Re ˆ Ucdh=frac34 hellip4dagger

Uc is the velocity at the minimum flow area and dh is the hydraulic diameterwhich is defined as

dh ˆ 2hellips iexcl bdaggerl=hellipl Dagger bdagger hellip5dagger

The pressure drop has been related to the Fanning friction factor

f ˆ 2centP

raquoU2c

dh

4Lm

sup3 acutehellip6dagger

and the average heat transfer coefficient is presented in terms of the Colburn jfactor

j ˆ Nu

Re Pr1=3ˆ h Pr2=3

raquocpUmhellip7dagger

ResultsThe case study in this investigation is an offset strip fin geometry shown inFigure 3 with the geometrical dimensions given in Table I These dimensionsare identical to those in a previous investigation (DeJong et al 1998) and theoverall computed results are compared with the numerical results of that studyThe focus point of this study as has already been mentioned has been differentfrom their investigation and attention is paid to the mechanisms of heattransfer enhancement

The results are presented in three sections In the first section the timeaveraged mean values of heat transfer and fluid flow properties friction factorand Colburn j factor are presented The second section includes the time-

Heat transferenhancement

705

dependent features of the flow simulation In this part the time-dependentvelocity field is presented The oscillatory velocity history at a point is depictedand the dominating frequency of the oscillatory flow is determined and the non-dimensional oscillation similarity of the flow for a range of Re numbers isdiscussed In the last section the time-averaged second moments of the velocityand temperature fields are presented The corresponding production terms ofvelocity and temperature fluctuations are provided and discussed

Time-averaged mean valuesThe friction factor results compared with the results of DeJong et al (1998) areprovided in Figure 4 The calculated friction factors of this study are under-predicted (14-16 percent) compared with those of DeJong et al (1998) TheColburn j factor results are compared with corresponding results of thatinvestigation in Figure 5(a) The deviations between the results of this studyand those of DeJong et al (1998) are in the range of 6-8 percent Thesecomparisons show that the present numerical simulation method is able tosatisfactorily reproduce the results obtained for the same geometry by DeJonget al (1998) in terms of time-averaged mean values It should thus be reasonableto interpret the time-dependent characteristics of the flow and temperaturefields

Figure 3Basic module and

computational domain

Table IDimensions of the

offset strip fingeometry

Parameter Value

bl 0117sl 0507l 24mm

HFF117

706

If one considers the ratio jf it is found that a maximum appears around Re =

900-1000 The reason for this might be that laminar self-oscillating flow occurs

and the benefit in overall performance is believed to be caused by dissimilarity

between the mechanism for momentum and heat transfer A more detaileddiscussion will follow

Unsteady velocity field description

Figure 6 shows the velocity field around the fin during a complete period of

oscillations This Figure shows the development of the flow in form of the time-

Figure 4Comparison of frictionfactor results

Figure 5Comparison of Colburn jfactor results

Heat transferenhancement

707

Figure 6Velocity vectors over a

period of oscillations

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

HFF117

702

contrast with those studied in the past is a practical extended surfaceapplication DeJong et al (1998) studied this case but they did not pay attentionto the detailed mechanisms of the heat and momentum transfer processes

Taking these points into consideration it is clear that the current study isunique in dealing with the character of heat transfer enhancement mechanismin offset strip fin geometries The results of this study will show that theenhancement mechanism in a certain Re number range is special and also itwill prove the dissimilarity between the two processes of heat transfer andmomentum transfer in that certain range of Re numbers It is also worthmentioning that the overall goal of this study is not to present new numericalmethods or verify any numerical scheme but instead to reveal the physicalphenomena and the related implications for the heat transfer mechanism Thusan established finite volume method is used in the computational analysis

Assumptions and method of analysisPossible methods of comparison and evaluation of different geometricalconcepts of extended surfaces have been presented by Shah et al (1998)Investigations of the thermal performance of various extended surfacegeometries are commonly experimental but rapid developments of computercapacity and numerical solution methods have implied that a theoreticalanalysis is reliable in many cases So-called CFD-methods (computational fluiddynamics) (see for example Veersteg and Malalasekera (1995)) may offer acheaper and more flexible tool as a variety of extended surfaces or fins is to beanalyzed Such methods will give the opportunity for testing new conceptsbefore experimentation that can be expensive and troublesome In this reportfour different extended surfaces are considered

The computational domain is limited to a basic module shown in Figure 2 Thissimplification is based on the assumption of fully developed flow and thermalfields in the array of fins Two pairs of periodic boundaries in the longitudinal andtransverse directions limit the module Experimental investigations (DeJong et al1998 Joshi and Webb 1987) justify these assumptions

The two-dimensional continuity Navier-Stokes and energy equations are solved

ui

xi

ˆ 0 hellip1dagger

ui

frac12Dagger

xjhellipuiujdagger ˆ iexcl 1

raquo

p

xiDagger frac34

2ui

xjxjhellip2dagger

T

frac12Dagger

xj

hellipTujdagger ˆ frac34

Pr

2T

xjxj

hellip3dagger

The flow is assumed to be incompressible with constant properties Buoyancyforces and viscous dissipation are not considered Periodic boundary conditions

Heat transferenhancement

703

have been applied at the inlet and outlet sections and in the transverse directionThe wall boundary conditions are no-slip for the momentum equations andconstant heat flux for the energy equation

A finite volume multi-block method has been applied In this case anavailable general purpose CFD-code (STAR-CD) has been applied to solve theresulting governing algebraic equations In previous works this code has beenfound to perform well if applied with care The QUICK scheme is used to

Figure 2The grid used for the

calculations

HFF117

704

handle convective-diffusive terms The PISO algorithm is used for treating thecoupling of the pressure and velocity fields The Crank-Nicolson method isused for time discretization The maximum residual tolerance of all equationshas been kept at less than 10plusmn6 and the maximum value of the Courant numberfor all Re numbers was about unity

A multi-block calculation has been carried out with six blocks The numberof grid points in the calculations varied from 162 pound 40 to 270 pound 62 (Figure 2)Further refinement to 324 pound 75 grid points changed the results of the frictionfactor and Nu numbers by less than 1 percent The computer used to carry outthe calculations was a Digital AlphaStation1 255 (with CPU clock rate of233MHz) and the CPU time required for the calculations was 0001857 sec pertime step and mesh point The time step of the calculations is chosen in order toachieve the correct time resolution so that there are a sufficient number of timesteps to capture all the oscillations As a result CFL (Courant) numbers of amaximum value around unity in the whole domain were proved

The Re number for the flow is defined as

Re ˆ Ucdh=frac34 hellip4dagger

Uc is the velocity at the minimum flow area and dh is the hydraulic diameterwhich is defined as

dh ˆ 2hellips iexcl bdaggerl=hellipl Dagger bdagger hellip5dagger

The pressure drop has been related to the Fanning friction factor

f ˆ 2centP

raquoU2c

dh

4Lm

sup3 acutehellip6dagger

and the average heat transfer coefficient is presented in terms of the Colburn jfactor

j ˆ Nu

Re Pr1=3ˆ h Pr2=3

raquocpUmhellip7dagger

ResultsThe case study in this investigation is an offset strip fin geometry shown inFigure 3 with the geometrical dimensions given in Table I These dimensionsare identical to those in a previous investigation (DeJong et al 1998) and theoverall computed results are compared with the numerical results of that studyThe focus point of this study as has already been mentioned has been differentfrom their investigation and attention is paid to the mechanisms of heattransfer enhancement

The results are presented in three sections In the first section the timeaveraged mean values of heat transfer and fluid flow properties friction factorand Colburn j factor are presented The second section includes the time-

Heat transferenhancement

705

dependent features of the flow simulation In this part the time-dependentvelocity field is presented The oscillatory velocity history at a point is depictedand the dominating frequency of the oscillatory flow is determined and the non-dimensional oscillation similarity of the flow for a range of Re numbers isdiscussed In the last section the time-averaged second moments of the velocityand temperature fields are presented The corresponding production terms ofvelocity and temperature fluctuations are provided and discussed

Time-averaged mean valuesThe friction factor results compared with the results of DeJong et al (1998) areprovided in Figure 4 The calculated friction factors of this study are under-predicted (14-16 percent) compared with those of DeJong et al (1998) TheColburn j factor results are compared with corresponding results of thatinvestigation in Figure 5(a) The deviations between the results of this studyand those of DeJong et al (1998) are in the range of 6-8 percent Thesecomparisons show that the present numerical simulation method is able tosatisfactorily reproduce the results obtained for the same geometry by DeJonget al (1998) in terms of time-averaged mean values It should thus be reasonableto interpret the time-dependent characteristics of the flow and temperaturefields

Figure 3Basic module and

computational domain

Table IDimensions of the

offset strip fingeometry

Parameter Value

bl 0117sl 0507l 24mm

HFF117

706

If one considers the ratio jf it is found that a maximum appears around Re =

900-1000 The reason for this might be that laminar self-oscillating flow occurs

and the benefit in overall performance is believed to be caused by dissimilarity

between the mechanism for momentum and heat transfer A more detaileddiscussion will follow

Unsteady velocity field description

Figure 6 shows the velocity field around the fin during a complete period of

oscillations This Figure shows the development of the flow in form of the time-

Figure 4Comparison of frictionfactor results

Figure 5Comparison of Colburn jfactor results

Heat transferenhancement

707

Figure 6Velocity vectors over a

period of oscillations

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

Heat transferenhancement

703

have been applied at the inlet and outlet sections and in the transverse directionThe wall boundary conditions are no-slip for the momentum equations andconstant heat flux for the energy equation

A finite volume multi-block method has been applied In this case anavailable general purpose CFD-code (STAR-CD) has been applied to solve theresulting governing algebraic equations In previous works this code has beenfound to perform well if applied with care The QUICK scheme is used to

Figure 2The grid used for the

calculations

HFF117

704

handle convective-diffusive terms The PISO algorithm is used for treating thecoupling of the pressure and velocity fields The Crank-Nicolson method isused for time discretization The maximum residual tolerance of all equationshas been kept at less than 10plusmn6 and the maximum value of the Courant numberfor all Re numbers was about unity

A multi-block calculation has been carried out with six blocks The numberof grid points in the calculations varied from 162 pound 40 to 270 pound 62 (Figure 2)Further refinement to 324 pound 75 grid points changed the results of the frictionfactor and Nu numbers by less than 1 percent The computer used to carry outthe calculations was a Digital AlphaStation1 255 (with CPU clock rate of233MHz) and the CPU time required for the calculations was 0001857 sec pertime step and mesh point The time step of the calculations is chosen in order toachieve the correct time resolution so that there are a sufficient number of timesteps to capture all the oscillations As a result CFL (Courant) numbers of amaximum value around unity in the whole domain were proved

The Re number for the flow is defined as

Re ˆ Ucdh=frac34 hellip4dagger

Uc is the velocity at the minimum flow area and dh is the hydraulic diameterwhich is defined as

dh ˆ 2hellips iexcl bdaggerl=hellipl Dagger bdagger hellip5dagger

The pressure drop has been related to the Fanning friction factor

f ˆ 2centP

raquoU2c

dh

4Lm

sup3 acutehellip6dagger

and the average heat transfer coefficient is presented in terms of the Colburn jfactor

j ˆ Nu

Re Pr1=3ˆ h Pr2=3

raquocpUmhellip7dagger

ResultsThe case study in this investigation is an offset strip fin geometry shown inFigure 3 with the geometrical dimensions given in Table I These dimensionsare identical to those in a previous investigation (DeJong et al 1998) and theoverall computed results are compared with the numerical results of that studyThe focus point of this study as has already been mentioned has been differentfrom their investigation and attention is paid to the mechanisms of heattransfer enhancement

The results are presented in three sections In the first section the timeaveraged mean values of heat transfer and fluid flow properties friction factorand Colburn j factor are presented The second section includes the time-

Heat transferenhancement

705

dependent features of the flow simulation In this part the time-dependentvelocity field is presented The oscillatory velocity history at a point is depictedand the dominating frequency of the oscillatory flow is determined and the non-dimensional oscillation similarity of the flow for a range of Re numbers isdiscussed In the last section the time-averaged second moments of the velocityand temperature fields are presented The corresponding production terms ofvelocity and temperature fluctuations are provided and discussed

Time-averaged mean valuesThe friction factor results compared with the results of DeJong et al (1998) areprovided in Figure 4 The calculated friction factors of this study are under-predicted (14-16 percent) compared with those of DeJong et al (1998) TheColburn j factor results are compared with corresponding results of thatinvestigation in Figure 5(a) The deviations between the results of this studyand those of DeJong et al (1998) are in the range of 6-8 percent Thesecomparisons show that the present numerical simulation method is able tosatisfactorily reproduce the results obtained for the same geometry by DeJonget al (1998) in terms of time-averaged mean values It should thus be reasonableto interpret the time-dependent characteristics of the flow and temperaturefields

Figure 3Basic module and

computational domain

Table IDimensions of the

offset strip fingeometry

Parameter Value

bl 0117sl 0507l 24mm

HFF117

706

If one considers the ratio jf it is found that a maximum appears around Re =

900-1000 The reason for this might be that laminar self-oscillating flow occurs

and the benefit in overall performance is believed to be caused by dissimilarity

between the mechanism for momentum and heat transfer A more detaileddiscussion will follow

Unsteady velocity field description

Figure 6 shows the velocity field around the fin during a complete period of

oscillations This Figure shows the development of the flow in form of the time-

Figure 4Comparison of frictionfactor results

Figure 5Comparison of Colburn jfactor results

Heat transferenhancement

707

Figure 6Velocity vectors over a

period of oscillations

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

HFF117

704

handle convective-diffusive terms The PISO algorithm is used for treating thecoupling of the pressure and velocity fields The Crank-Nicolson method isused for time discretization The maximum residual tolerance of all equationshas been kept at less than 10plusmn6 and the maximum value of the Courant numberfor all Re numbers was about unity

A multi-block calculation has been carried out with six blocks The numberof grid points in the calculations varied from 162 pound 40 to 270 pound 62 (Figure 2)Further refinement to 324 pound 75 grid points changed the results of the frictionfactor and Nu numbers by less than 1 percent The computer used to carry outthe calculations was a Digital AlphaStation1 255 (with CPU clock rate of233MHz) and the CPU time required for the calculations was 0001857 sec pertime step and mesh point The time step of the calculations is chosen in order toachieve the correct time resolution so that there are a sufficient number of timesteps to capture all the oscillations As a result CFL (Courant) numbers of amaximum value around unity in the whole domain were proved

The Re number for the flow is defined as

Re ˆ Ucdh=frac34 hellip4dagger

Uc is the velocity at the minimum flow area and dh is the hydraulic diameterwhich is defined as

dh ˆ 2hellips iexcl bdaggerl=hellipl Dagger bdagger hellip5dagger

The pressure drop has been related to the Fanning friction factor

f ˆ 2centP

raquoU2c

dh

4Lm

sup3 acutehellip6dagger

and the average heat transfer coefficient is presented in terms of the Colburn jfactor

j ˆ Nu

Re Pr1=3ˆ h Pr2=3

raquocpUmhellip7dagger

ResultsThe case study in this investigation is an offset strip fin geometry shown inFigure 3 with the geometrical dimensions given in Table I These dimensionsare identical to those in a previous investigation (DeJong et al 1998) and theoverall computed results are compared with the numerical results of that studyThe focus point of this study as has already been mentioned has been differentfrom their investigation and attention is paid to the mechanisms of heattransfer enhancement

The results are presented in three sections In the first section the timeaveraged mean values of heat transfer and fluid flow properties friction factorand Colburn j factor are presented The second section includes the time-

Heat transferenhancement

705

dependent features of the flow simulation In this part the time-dependentvelocity field is presented The oscillatory velocity history at a point is depictedand the dominating frequency of the oscillatory flow is determined and the non-dimensional oscillation similarity of the flow for a range of Re numbers isdiscussed In the last section the time-averaged second moments of the velocityand temperature fields are presented The corresponding production terms ofvelocity and temperature fluctuations are provided and discussed

Time-averaged mean valuesThe friction factor results compared with the results of DeJong et al (1998) areprovided in Figure 4 The calculated friction factors of this study are under-predicted (14-16 percent) compared with those of DeJong et al (1998) TheColburn j factor results are compared with corresponding results of thatinvestigation in Figure 5(a) The deviations between the results of this studyand those of DeJong et al (1998) are in the range of 6-8 percent Thesecomparisons show that the present numerical simulation method is able tosatisfactorily reproduce the results obtained for the same geometry by DeJonget al (1998) in terms of time-averaged mean values It should thus be reasonableto interpret the time-dependent characteristics of the flow and temperaturefields

Figure 3Basic module and

computational domain

Table IDimensions of the

offset strip fingeometry

Parameter Value

bl 0117sl 0507l 24mm

HFF117

706

If one considers the ratio jf it is found that a maximum appears around Re =

900-1000 The reason for this might be that laminar self-oscillating flow occurs

and the benefit in overall performance is believed to be caused by dissimilarity

between the mechanism for momentum and heat transfer A more detaileddiscussion will follow

Unsteady velocity field description

Figure 6 shows the velocity field around the fin during a complete period of

oscillations This Figure shows the development of the flow in form of the time-

Figure 4Comparison of frictionfactor results

Figure 5Comparison of Colburn jfactor results

Heat transferenhancement

707

Figure 6Velocity vectors over a

period of oscillations

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

Heat transferenhancement

705

dependent features of the flow simulation In this part the time-dependentvelocity field is presented The oscillatory velocity history at a point is depictedand the dominating frequency of the oscillatory flow is determined and the non-dimensional oscillation similarity of the flow for a range of Re numbers isdiscussed In the last section the time-averaged second moments of the velocityand temperature fields are presented The corresponding production terms ofvelocity and temperature fluctuations are provided and discussed

Time-averaged mean valuesThe friction factor results compared with the results of DeJong et al (1998) areprovided in Figure 4 The calculated friction factors of this study are under-predicted (14-16 percent) compared with those of DeJong et al (1998) TheColburn j factor results are compared with corresponding results of thatinvestigation in Figure 5(a) The deviations between the results of this studyand those of DeJong et al (1998) are in the range of 6-8 percent Thesecomparisons show that the present numerical simulation method is able tosatisfactorily reproduce the results obtained for the same geometry by DeJonget al (1998) in terms of time-averaged mean values It should thus be reasonableto interpret the time-dependent characteristics of the flow and temperaturefields

Figure 3Basic module and

computational domain

Table IDimensions of the

offset strip fingeometry

Parameter Value

bl 0117sl 0507l 24mm

HFF117

706

If one considers the ratio jf it is found that a maximum appears around Re =

900-1000 The reason for this might be that laminar self-oscillating flow occurs

and the benefit in overall performance is believed to be caused by dissimilarity

between the mechanism for momentum and heat transfer A more detaileddiscussion will follow

Unsteady velocity field description

Figure 6 shows the velocity field around the fin during a complete period of

oscillations This Figure shows the development of the flow in form of the time-

Figure 4Comparison of frictionfactor results

Figure 5Comparison of Colburn jfactor results

Heat transferenhancement

707

Figure 6Velocity vectors over a

period of oscillations

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

HFF117

706

If one considers the ratio jf it is found that a maximum appears around Re =

900-1000 The reason for this might be that laminar self-oscillating flow occurs

and the benefit in overall performance is believed to be caused by dissimilarity

between the mechanism for momentum and heat transfer A more detaileddiscussion will follow

Unsteady velocity field description

Figure 6 shows the velocity field around the fin during a complete period of

oscillations This Figure shows the development of the flow in form of the time-

Figure 4Comparison of frictionfactor results

Figure 5Comparison of Colburn jfactor results

Heat transferenhancement

707

Figure 6Velocity vectors over a

period of oscillations

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

Heat transferenhancement

707

Figure 6Velocity vectors over a

period of oscillations

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

HFF117

708

Figure 6

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

Heat transferenhancement

709

dependent velocity vectors in a series of six time steps during this period (at Re= 993) The flow structure shows a wavy-oscillatory pattern This Figure andthe flow pattern show that at this Re number the flow has become unsteadyThe wavy pattern shows that the flow between fins is not bounded in thechannel type area just downstream of the fin the so-called ` communicatingrsquorsquoregion as has also been observed in other studies for example Majumdar andAmon (1992) As is obvious the flow is bouncing up and down out of this areaand promoting the mixing process between the area downstream of the fin andthe air in the vicinity of the upper fin

The structure of the flow field over the fin needs consideration as well Incontrast with a simple boundary layer type pattern over the fin there arecertain kinds of circulation bubbles over the fin two of which are readilyobserved These bubbles are commuting over the fin during the period ofoscillation and over a certain finite length of the fin At t = frac12 6 they are at theupstream part of the fin they move further downstream and the secondcirculation bubble (or vortex) is absorbed in the main flow stream at t = 4frac12 6However another pair is built up already at t = 5frac126 and the cycle continues

The velocity time history at point X (Figure 2) is depicted in Figure 7 It isobviously a very orderly time variation that suggests a pure oscillatory motionthat is not chaotic A way to highlight this point even more is to look at the fastFourier transformation (FFT) of this time history (Figure 8) Thistransformation to frequency shows a very strong oscillation frequency at fs =68Hz With the Strouhal number based on the transverse dimension of the finits thickness (b) is defined as

St ˆfsb

iquest

Uc

Figure 7Time history of velocity

at point X Re = 1124

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

HFF117

710

and is equal to 02 This value is equal to the experimentally reported value ofStrouhal number for a staggered tube bank with the same transversedimension (Fitz-Hugh 1973) It is also worth mentioning that this frequencydominates the whole flow structure and the whole flow pattern is repeatedwith the same frequency (the time period shown in Figure 6 is identical to theinverse of this frequency)

Second moments of velocity and temperature and their interpretationThe time-averaged U-velocity and temperature contours are presented inFigure 9 Despite the fluctuating character of the unsteady flow field the time-averaged patterns of velocity and temperature fields are symmetrical Highestvelocities occur in the contraction area between fins Considering the unsteadyflow behavior no boundary layer type flow can be found but in the time-averaged flow picture it is found and can be seen in Figure 9(a) This boundarylayer forms over the fin and thickens downstream A somewhat thinnerthermal boundary layer also exists in the time-averaged structure as seen inFigure 9(b) The higher temperature levels are found over the fin inside thisboundary layer and just downstream of the fin

Figure 9(a) shows the contour plot of the second moment correlation betweenthe two fluctuating velocity components uv This corresponds to a Reynoldsshear stress component in a turbulent flow Although the flow is not turbulentand shows quite regular oscillatory behavior (Figures 7 and 8) non-zero valuesof uv exist everywhere in the flow field As expected the values show an anti-symmetrical pattern as well It is interesting to analyze the distribution of thesecond moment of the velocity fluctuations Two different parts of the flowfield in Figure 9(a) can be recognized One is the flow area over the fin surfaceand the second one is the area downstream of the fin As can be seen themaximum spots of the fluctuating moment occur in the flow area downstream

Figure 8FFT of the time historyof velocity at point XRe = 1124

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

Heat transferenhancement

711

of the fin while the values over the fin surface are considerably lower Thissuggests that the main production of these moments and the mixing process ofthe momentum due to fluctuations take place in the wake of the fin and not inthe boundary layer over the fin surface

The second moment of the temperature-velocity fluctuations vt is shown inFigure 10(b) The distribution of this second moment shows a similar pattern tothe previous one but there is a major difference as well The hot spots

Figure 9Time-averaged

U-velocity (a) andtemperature contours (b)

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

HFF117

712

downstream of the fin are found in this Figure as well and in a very similarway This suggests that the mixing process due to velocity-temperaturefluctuations occurs in this region The heat transfer process is then enhancedThe difference is on the area over the fin surface In contrast with the secondmoment of the velocity fluctuations there is a kernel of positive values of vtover the fin surface that is not convected from the upstream part as it is not

Figure 10Second momentcorrelation of velocitycomponents uv contour(a) temperaturefluctuations moment vtcontour (b)

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

Heat transferenhancement

713

clearly attached to the contours upstream This means that the production of vttakes place over the fin surface as well

The above mentioned observations and reflections suggest a certaindissimilarity between the process of momentum transfer and the heat transferprocess To make this discussion even clearer one may consider the productionterms for velocity and temperature fluctuations The production of thefluctuating kinetic energy is equal to

Pk ˆ iexcluiujUi

xjhellip8dagger

This value is calculated and depicted in Figure 11(a) The positive values of thisterm show the regions where the mixing process will promote the momentumtransfer This Figure shows that positive production occurs only downstream ofthe fin in the area where the fluctuations exist A small hot spot of positiveproduction exists in the corner of the fin at the leading edge The whole boundarylayer area over the fin shows a negative production of kinetic energy whichmeans that the mixing of the momentum in this area will be damped Obviouslythere is a clear contradiction to the mechanism in a turbulent boundary layerwhere the bursting process in the near wall region provides the production ofkinetic energy Now attention will be paid to the production of the temperaturevariance (similar to fluctuating kinetic energy) This production is given by

Ptt ˆ iexclujtT

xjhellip9dagger

Two hot spots of production are observed downstream of the fin in Figure 11(b)These are related to the same fluctuations that cause the momentum mixing inthe same area However in contrast with what was observed in the previousproduction plot the values over the fin surface area are not solely negative andan area of positive production of temperature fluctuations is observed here Thispositive area is generated locally and is convected downstream and strengthensthe hot spots downstream of the fin That explains the small upward shift of thisspot compared with that in the production of kinetic energy while there thenegative production over the fin surface pushed the hot spot towards the centerThis also clarifies the observation made earlier on the existence of positive valuesof vt over the fin surface

This comparison and the comparison between the second moments in theprevious Figure show a clear dissimilarity between the heat transfer andmomentum transfer processes While both production terms have positivevalues just downstream of the fin they have different signs over the fin surfaceNegative values of the production of fluctuating kinetic energy indicate asuppression of momentum transfer in this area while the temperature varianceproduction has a positive value which reveals enhancement of heat transfer

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

HFF117

714

ConclusionsA numerical analysis of the time-dependent flow over an offset strip fingeometry was carried out The results were presented in three sections In thefirst section time-averaged mean values of the friction factor and the Colburn jfactor were compared with the results of another numerical investigation

Figure 11Production of velocityfluctuations due tovelocity gradients (a)production oftemperature fluctuationsdue to temperature andvelocity gradients (b)

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

Heat transferenhancement

715

available in the literature This comparison ensured that the present numericalinvestigation provided satisfactory accuracy

In the second section the unsteady flow structure has been considered Itwas shown that the flow is not bounded in the channel type area justdownstream of the fin or in the so-called ` communicatingrsquorsquo region It was alsoshown that the velocity field has a pure oscillating motion A dominatingfrequency of the oscillations is valid in the whole flow domain

The contour plots of the second moment correlation of the fluctuatingvelocity components uv and the second moment of the temperature-velocityfluctuations vt were presented The locations of maximum observed in the wakeregion for these moments occurred at the same spot but unlike the uv momentpositive values of vt also exist in the region over the fin surface Positiveproduction of the fluctuating kinetic energy occurs only in the wake regionwhile in the boundary layer region over the fin surface negative production isfound In contrast the production of the temperature variance possesses positivevalues even in the area over the fin surface By comparing the second momentsof the velocity and temperature-velocity fluctuations and the production offluctuating kinetic energy and temperature variance the dissimilarity betweenthe processes of heat transfer and momentum transport was identified Thisdissimilarity is obviously beneficial as the heat transfer enhancement is notcoupled with an increased momentum transfer or pressure drop

In summary the results of this study showed

That the mechanism of heat transfer enhancement revealed byfluctuating temperature and velocity fields for oscillating laminar flowsituations can be studied by numerical solution methods of thegoverning equations

Evidence of the dissimilarity between heat transfer and momentumtransfer which has not been observed or studied for offset strip fingeometries before

The clear difference between the variances of velocity fluctuations inlaminar self-oscillating flow and turbulent flow

References

DeJong NC Zhang LW Jacobi AM Balachandar S and Tafti DK (1998) `Acomplementary experimental and numerical study of flow and heat transfer in offset strip-fin heat exchangersrsquorsquoASME J Heat Transfer Vol 120 pp 690-8

Fitz-Hugh JS (1973) `Flow induced vibration in heat exchangersrsquorsquo Oxford University ReportRS57 AERE-P7238

Jacobi AM and Shah RK (1996) `Air-side flow and heat transfer in compact heat exchangersa discussion of physicsrsquorsquo Process Enhanced and Multiphase Heat Transfer plusmn A Festschriftfor AE Bergles Begell House NY pp 379-90

Joshi HM and Webb RL (1987) `Heat transfer and friction in the offset strip fin heatexchangerrsquorsquo Int J Heat Mass Transfer Vol 30 pp 69-84

Kays WM (1972) ` Compact heat exchangersrsquorsquoAGARD Lecture Ser No 57 on Heat ExchangersAGARD-LS-57-72 NATO Paris

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41

HFF117

716

Majumdar D and Amon CH (1992) ` Heat and momentum transport in self-sustainedoscillatory viscous flowsrsquorsquo ASME J Heat Transfer Vol 114 pp 866-73

Manglik RM and Bergles AE (1995) ` Heat transfer and pressure drop correlations for therectangular offset strip fin compact heat exchangersrsquorsquo Exp Thermal and Fluid Sci Vol 10pp 171-80

Mercier P and Tochon P (1997) `Analysis of turbulent flow and heat in compact heatexchanger by pseudo-direct numerical simulationrsquorsquo Compact Heat Exchangers for theProcess Industries Begell House NY pp 223-30

Patankar SV and Prakash C (1981) `An analysis of the effect of plate thickness on laminarflow and heat transfer in interrupted-plate passagesrsquorsquo Int J Heat Mass Transfer Vol 24pp 1801-10

Sahnoun A and Webb RL (1992) ` Prediction of heat transfer and friction for the louver fingeometryrsquorsquo ASME J Heat Transfer Vol 114 pp 893-900

Saidi A SundeAcircn B and Eriksson D (2000) ` Intercoolers in gas turbine systems and combi-processes for production of electricityrsquorsquo ASME paper 2000-GT-234

Shah RK Heikal MR and Thonon B (1998) `Advances in numerical analysis of heat transferand flow friction characteristics of compact heat exchangersrsquo surfacersquorsquo CHTrsquo97 Advancesin Computational Heat Transfer Begell House NY pp 68-87

Sparrow EM Baliga BR and Patankar SV (1977) ` Heat transfer and flow analysis ofinterrupted-wall channels with application to heat exchangersrsquorsquo ASME J Heat TransferVol 99 pp 4-11

Suzuki K Hirai E Miyaki T and Sato T (1985) ` Numerical and experimental studies on atwo-dimensional model of an offset-strip-fin type compact heat exchanger used at lowReynolds numberrsquorsquo Int J Heat Mass Transfer Vol 28 pp 823-36

Suzuki K Hirai E SatoT and Kieda S (1982) ` Numerical study of heat transfer system withstaggered array of vertical flat plates used at low Reynolds numberrsquorsquo Proc 7th Int HeatTransfer Conf Vol 3 pp 483-8

Suzuki K Xi G Inaoka K and Hagiwara YH (1994) `Mechanism of heat transferenhancement due to self-sustained oscillation from an in-line fin arrayrsquorsquo Int J Heat MassTransfer Vol 37 pp 83-96

Veersteg HK and Malalasekera W (1995) An Introduction to Computational Fluid DynamicsThe Finite Volume Method Longman Scientific ampTechnical Publishers

Wieting R (1975) `Empirical correlations for heat transfer and flow friction characteristics ofrectangular offset-fin plate-fin heat exchangersrsquorsquo ASME J Heat Transfer Vol 97pp 488-90

Xi G Hagiwara Y and Suzuki K (1992) ` Effect of fin thickness on flow and heat transfercharacteristics of fin array plusmn an offset-fin array in the low Reynolds number rangersquorsquo HeatTransfer-Japanese Res Vol 22 pp 1-19

Xi G Hagiwara Y and Suzuki K (1995) ` Flow instability and augmented heat transfer of finarraysrsquorsquo J Enhanced Heat Transfer Vol 2 pp 23-32

Zhang LW Tafti DK Najjar FM and Balachandar S (1997) `Computations of flow and heattransfer in parallel-plate fin heat exchangers on the CM-5 effects of flow unsteadiness andthree-dimensionalityrsquorsquo Int J Heat Mass Transfer Vol 40 pp 1325-41