performance investigation of artificially roughened duct used in solar...

Performance Investigation of Artificially Roughened Duct used in Solar Air HeatersF F

45

Performance Investigation of ArtificiallyRoughened Duct used in Solar Air Heaters

Tarun Mahajan1, Ranjit Singh2 and Brij Bhushan3

Department of Mechanical Engineering, Beant College ofEngineering and Technology, Gurdaspur, Punjab-143521,India, 1E-mail: [email protected].

ABSTRACT: Experimental investigation has been reported in the presentpaper for fully developed turbulent flow in the rectangular duct havingabsorber plate roughened by fixing plain-woven square wire mesh. Effect ofartificial roughness on heat transfer coefficient and friction has beeninvestigated for a range of system and operating parameters. It has beenobserved that roughened absorber plate results into higher heat transfercoefficient at the cost of frictional penalty. In order to predict performance ofthe system, Nusselt number and friction factor correlations have beendeveloped by using experimental data.Keywords: Solar air heater, artificial roughness, heat transfer coefficient,Nusselt number and friction factor.

NomenclatureAc cross-sectional area of duct, m2.Ao area of orifice plate at the throat, m2.Cd coefficient of discharge.Cp specific heat of the air, J kg–1 K–1.D hydraulic diameter of duct, m.e height of roughness element, m.f friction factor (dimensionless).fs friction factor for smooth plate (dimensionless).g acceleration due to gravity, m2 s–1.

International Journal of Product DesignJanuary-June 2011, Volume 1, Number 1, pp. 45–62

mailto:[email protected]

International Journal of Product DesignF F

46

H height of the duct, m.∆ht difference of manometric fluid levels in micro-manometer, m.∆hwdifference of water column in U-tube manometer, m.h heat transfer coefficient, W m–2 K–1.k thermal conductivity of air, W m–1 K–1.L length of the test section, m.m mass flow rate of air, kg s–1.

Nu Nusselt number (dimensionless).NusNusselt number for smooth plate (dimensionless).p pitch of roughness element, m.Pr Prandlt number (dimensionless).∆Po pressure drop at the orifice plate, Nm–2.∆Pt pressure drop at the test section, Nm–2.q heat transfer rate, W.Re Reynolds number (dimensionless).Ti inlet air temperature of the test section, K.To outlet air temperature of the test section, K.Tam mean temperature of air, K.Tpm mean temperature of absorber plate, K.V velocity of air, ms–1.W width of the duct, m.

Greek Lettersρ density of air, kg m–3.ρk density of manometric fluid in micro-manometer, kg m–3.ρw density of water, kg m–3.β ratio of orifice and pipe diameter (dimensionless).θ inclination of U-tube manometer, degree.µ dynamic viscosity of air, kg/s-m.


47

1. INTRODUCTIONEnergy has been needed and used by human being at an increasingrate for his substance and well being on the planet earth. Continuoususe of fossil fuels have resulted energy crisis and environmentalthreats. Now the need has been felt to have inexhaustible and cleanenergy resources. Solar energy is considered to be primary energysource for all forms of energy. It is free and available almosteverywhere. Simplest method to utilize solar radiation is to convertit into thermal energy for heating applications by using solarcollectors. Solar air heaters because of their intrinsic simplicity andminimum use of materials are cheap and are used for manyapplications like space heating, timber seasoning, crop drying etc.Thermal efficiency of solar air heaters in comparison of solar waterheaters is generally considered poor because of their inherently lowheat transfer capability between the absorber plate and air flowingin the duct. Methodology of creating artificial roughness on thesurface of absorber plate is considered to be an effective techniquefor enhancing heat transfer coefficient in order to increase heattransfer rate between absorber plate and air flowing through theduct. However, it results into an increase in friction loss. Therefore,turbulence must be created only in the region very close to theabsorber surface i.e. in laminar sub-layer only. It can be producedby several methods such as by wire fixation in the form of transversecontinuous ribs, transverse broken ribs, inclined and V-shaped orstaggered ribs; rib formation by machining process in the form ofchamfered ribs, wedge shaped ribs, combination of different integralrib roughness elements and by using expanded metal mesh ribs ashas been described by Bhushan and Singh [1]. Prasad and Mullick[2] utilized artificial roughness in the duct used in solar heater inthe form of small diameter wires to increase heat transfer coefficient.Prasad and Saini [3] investigated fully developed turbulent flow inthe duct with a small diameter protrusion wire on the absorber plate.Muluwork et al. [4] compared thermal performance of staggereddiscrete V-apex up and down with corresponding transversestaggered discrete ribs. Karwa et al. [5] performed experimentalstudy to predict effect of rib head chamfer angle and duct aspect


48

ratio on heat transfer and friction loss in a rectangular duct rough-ened with integral chamfered ribs. Bhagoria et al. [6] performedexperimental investigation to determine the effect of relativeroughness pitch, relative roughness height and wedge angle on heattransfer and friction loss in a solar air heater roughened duct havingwedge shaped rib roughness. Momin et al. [7] experimentallyinvestigated effect of geometrical parameters of V-shaped ribs onheat transfer and fluid flow characteristics in rectangular duct usedin solar air heaters. Jaurker et al. [8] experimentally investigatedheat transfer and friction characteristics of rib-grooved artificialroughness. Varun et al. [9] presented a review on artificial roughnessinvestigations reported in literature. It has been observed fromvarious experimental investigations on artificial roughness thatcreating artificial roughness on absorber plate is a tedious task andmay not be economically feasible. A suitable geometry of roughnesselement therefore needs to be selected, which besides being easilyavailable should be easy to fix on the absorber plate. In the samedirection, Saini and Saini [10] used expanded metal mesh to createartificial roughness on absorber plate and investigated effect ofsystem and operating parameters on heat transfer and friction loss.Similar to expanded metal mesh, plain-woven square wire meshavailable in common market can be used for generating artificialroughness on the absorber plate. It has been observed that heattransfer and pressure drop data for such type of a wire mesh is notreported in the literature. Therefore, it was decided to carry outexperimental investigation to study performance of artificiallyroughened duct with plain-woven square wire mesh of two types.In the present paper an experimental investigation has beenreported, in which heat transfer and pressure drop data have beengenerated for artificially roughened duct and used to developNusselt number and friction factor correlations for predictingperformance of the system.

2. EXPERIMENTAL SET-UP AND PROCEDUREIn order to carry out present experimental investigation, a test rigwas designed and fabricated as per guidelines proposed in literature


49

for similar experimental investigations. Schematic and photo-garaphic view of experimental set-up are shown in Fig. (1) and Fig.(2) respectively. It consists of an air duct along with temperatureand pressure drop measuring instruments, electric heater assembly,pipe line, flow measuring device, centrifugal blower and gate valves.Air duct comprised 20 mm thick wooden ply board and size of theduct was 2400 × 300 × 30 mm. Length of entry, test and exit sectionswere kept 900 mm, 1000 mm and 500 mm respectively. Length ofconverging plenum on exit side of the duct was 600 mm. An electricheater having size of 1500 × 300 mm was fabricated by combiningseries and parallel loops of heating wire on mica sheet to get uniformheat flux. Backside of the heater was insulated with glass wool tominimize the heat loss. Heater was placed on top of the duct andvariac was provided to control electric supply to it.

Figure 1: Schematic of Experimental Set-Up.


50

Figure 2: Photographic View of Experimental Set-Up.

Absorber plate being a 20 SWG GI sheet of 2400 × 350 mm sizewas painted black on heater side and artificial roughness wasprovided on air duct side by fixing wire mesh. Mass flow rate of theair was measured by means of an orifice-meter connected with a U-tube manometer. Flow of air through duct was smoothly controlledby means of two gate valves provided at entry and exit of centrifugalblower. Thermocouples made from copper-constantan wire wereused to measure temperature of air and absorber plate at differentlocations as shown in Figs. (3) and (4).

Figure 3: Different Locations of Thermocouples used to MeasureAbsorber Plate Temperature.


51

Figure 4: Different Locations of Thermocouples used to MeasureAir Temperature in the Duct.

Figure 5: Photographic View of P-1 and P-2 Type of Plain-Woven Square WireMesh used in Experimental Investigation.

Micro-manometer was used to measure pressure drop acrosstest section of the duct. Photographic views of two types of wiremesh used in the present experimental investigation are shown inFig. (5). Specifications of wire meshes are given in Table 1. Table 2shows range/value of parameters used in present experimentalinvestigation. Wire mesh was glued on to the underside of theabsorber plate and ends of the wire mesh were fixed with screws toensure good contact between the absorber plate and wire mesh.Measuring instruments like orifice meter, temperature indicator,temperature selector switch, micro manometer and U-tube mano-meter were properly checked and calibrated before starting theexperimentation. All joints in the test rig were thoroughly checkedto avoid any leakage. Five values of flow rate were used for each setand following data were collected at an interval of one hour in eachset of experimentation:

(i) Pressure drop across orifice meter to measure flow rate ofair.

(ii) Pressure drop across test section of duct.


52

(iii) Temperature of absorber plate at various locations in testsection of the duct.

(iv) Temperature of air at various locations in test section of theduct.

(v) Voltage and current supplied to electric heater.

Table 1Specifications of Wire Mesh used as Roughness Element.

Plate No. Geometry Specificationse(mm) p(mm)

P-1 Plain-woven square wire mesh 1.20 2.10P-2 Plain-woven square wire mesh 1.15 6.5

Table 2Range/Value of Parameters.

S. No. Parameter Range/Value

1. Reynolds number 4000-200002. Mass flow rate of air 0.0128-0.0627kg/s3. Duct aspect ratio (W/H) 104. Relative roughness height (e/D) 0.022, 0.0215. Relative roughness pitch (p/e) 1.75, 5.65

Accuracy of experimental data was verified by conductingexperiments for a conventional smooth duct. Nusselt number andfriction factor values were determined from experimental data andcompared with the values obtained from the following Nusseltnumber and friction factor correlations reported by Momin et al. [7]for rectangular smooth duct.

Modified Dittus-Boelter correlation for Nusselt number is

Nus = 0.023 Re0.8 Pr0.4

–0.2

2 av

e

RD ...(1)

where2 av

e

R

D =(1.156 / –1)

/

H W

H W

+

Modified Blasius equation for friction factor is

fs = 0.085 Re–0.25 ...(2)


53

Figure 6: Comparison of Experimental and Predicted Data of Nusselt Numberfor Smooth Absorber Plate.

Figure 7: Comparison of Experimental and Predicted Data of Friction Factor forSmooth Absorber Plate.


54

Fig. (6) and Fig. (7) shows comparison of experimental andpredicted data of Nusselt number and friction factor for smoothabsorber plate. A reasonably good agreement between experimentaland predicted data ensures accuracy of the data being collected fromself designed and fabricated experimental set-up. Error analysisbased on the procedure described by Holman [11] has been carriedout to find out uncertainties in measured/calculated values ofexperimental data. Uncertainty in Reynolds number, Nusseltnumber and friction factor values has been estimated as 1.65%, 1.73%,and 3.28% respectively.

3. DATA REDUCTIONFollowing equations were used for calculating pressure drop acrosstest section and orifice plate (∆Pt and ∆Po), mass flow rate of air (m ),velocity of air (V), heat transfer rate (q), heat transfer coefficient (h),Nusselt number (Nu) and friction factor (f):

∆Pt = ρk g(∆ht) ...(3)

∆Po = ρw g (∆hw) ...(4)

m = Cd Ao

0.5

4

2

1–op Sin ρ∆ θ

β ...(5)

V =c

m

Aρ

...(6)

q = mCp (To – Ti) ...(7)

Also

q = hAc (Tpm – Tam) ...(8)

Therefore, from Eqs. (7) and (8)

h = ( – )c pm am

q

A T T...(9)

Where Tpm and Tam are mean temperature of absorber plate andair. These were determined from temperature values recorded forabsorber plate and air at different locations along test section of the


55

duct. Reynolds number, Nusselt number and friction factor valueswere calculated by using the following relationships:

Re =VDρµ ...(10)

Nu =hD

k...(11)

f = 2

2

4tPD

LV

∆ ρ

...(12)

4. RESULTS AND DISCUSSIONPresence of wire mesh in the flow regimes cause vortices, help inincreasing turbulence and hence enhancing heat transfer as well asfriction loss. Effect of various flow and roughness parameters onheat transfer and friction characteristics has been investigated andresults have been reported and discussed in the present section.Fig. (8) represents variation of heat transfer coefficient as a functionof mass flow rate of air. It can be observed that for a given type ofwire mesh, heat transfer coefficient increases monotonously withincrease of mass flow rate of air for smooth as well as roughenedplate. Experimental data show that heat transfer coefficient forroughened plate is considerably higher as compared to smooth plate.It may happen due to turbulence caused by breakage of laminarsub-layer with application of artificial roughness. It has also beenobserved from these results that P-1 type of plain-woven squarewire mesh has higher heat transfer coefficient than that of P-2 typeof plain-woven square wire mesh. Fig. (9) shows variation of Nusseltnumber as a function of Reynolds number for smooth androughened absorber plates. Test results show that enhancement inNusselt number for roughened plates is less at lower values ofReynolds number. However, enhancement in Nusselt numberincreases at higher rate for higher values of Reynolds number forboth types of roughened absorber plates. It may be due to higherlocal distur-bances and secondary flow formation by roughnesselements in flow regimes as reported by Saini and Saini [10].


56

Fig. (10) shows variation of pressure drop as a function of massflow rate for smooth and roughened absorber plates. Due to localdisturbances and generation of secondary flows by roughenedelements, increase in pressure drop has been observed as comparedto smooth plate. Fig. (11) shows variation of friction factor as afunction of Reynolds number for smooth and roughened absorberplates. Friction factor decreases with increase of Reynolds numberas has been expected. With increase of Reynolds number boundarylayer thickness decreases and roughness element begins to projectbeyond laminar sub-layer. This reduction in the boundary layerthickness increases heat transfer coefficient as well as friction lossas reported by Karwa et al. [5]. The shedding of vortices also causesadditional loss of energy resulting in increased friction loss. It hasalso been observed in the present experimental investigation thatheat transfer coefficient and friction loss decrease with increase inrelative roughness pitch (p/e) as reported by Verma and Prasad[13].

Figure 8: Variation of Heat Transfer Coefficient with Mass Flow Ratefor Smooth and Roughened Absorber Plates.


57

Figure 9: Variation of Nusselt Number with Reynolds Number forSmooth and Roughened Absorber Plates.

Figure 10: Variation of Pressure Drop with Mass Flow Rate for Smoothand Roughened Absorber Plates.


58

Figure 11: Variation of Friction Factor with Reynolds Number for Smooth andRoughened Absorber Plates.

5. DEVELOPMENT OF NUSSELT NUMBER ANDFRICTION FACTOR CORRELATIONS

It has been observed that Nusselt number and friction factor arestrong functions of Reynolds number. Functional relationships forNusselt number and friction factor can therefore be written as:

Nu = λ(Re) ...(13)f = λ(Re) ...(14)

As per procedure described by Singh et al. [12] and using Sigmaplot software following Nusselt number and friction factorcorrelationswere developed corresponding to experimental data as shown inFigs. (8) and (10) in order to predict performance of the system:

for P-1 type of plain-woven square wire meshNu = 0.0105 Re0.8685 ...(15)

f = 68 Re–0.8851 ...(16)and for P-2 type of plain-woven square wire mesh

Nu = 0.01334 Re0.8374 ...(17)f = 42.79 Re–0.8415 ...(18)


59

Figure 12: Comparison of Predicted and Experimental Data of Nusselt Numberfor P-1 Type of Plain-Woven Square Wire Mesh.

Figure 13: Comparison of Predicted and Experimental Data of Nusselt Numberfor P-2 Type of Plain-Woven Square Wire Mesh.


60

Figure 14: Comparison of Predicted and Experimental Data of Friction Factor forP-1 Type of Plain-Woven Square Wire Mesh.

Figure 15: Comparison of Predicted and Experimental Data of Friction Factorfor P-2 Type of Plain-Woven Square Wire Mesh.


61

Figs. (12), (13), (14) and (15) show comparison of experimentaldata and that predicted from above developed Nusselt number andfriction factor correlations for both types of roughened absorberplates. Average absolute percentage deviations between experi-mental and predicted values of Nusselt number and friction factorfor P-1 type of plain-woven square wire mesh have been found tobe ± 2.5 % and ± 16 % respectively, whereas the corresponding valuesfor P-2 type of plain-woven square wire mesh have been found tobe ± 3.5 % and ± 14 % respectively.

6. CONCLUSIONSAn experimental investigation on performance of artificiallyroughened duct used in solar air heaters has been reported in thepresent paper. Effect of artificial roughness (created by fixing plain-woven square wire mesh on absorber plate) on heat transfer andfriction has been investigated for Reynolds numbers range of 4000–20000. It has been observed that roughened absorber plate resultsinto higher heat transfer coefficient at the cost of frictional penalty.In order to predict performance of the system Nusselt number andfriction factor correlations have been developed by using experimentaldata.

REFERENCES[1] B. Bhushan, R. Singh, “A Review on Methodology of Artificial Roughness

Used in Duct of Solar Air Heaters”, Energy, 35 (2010) pp. 202-212.[2] K. Prasad, S.C. Mullick, “Heat Transfer Characteristics of a Solar Air Heater

Used for Drying Purposes”, Appl. Energy, 13 (1983), pp. 83-98.[3] B.N. Prasad, J.S. Saini, “Effect of Artificial Roughness on Heat Transfer and

Friction Factor in Solar Air Heater”, Solar Energy, 41 (1988), pp. 555-560.[4] K.B. Muluwork, J.S. Saini, S.C. Solanki, “Studies on Discrete Rib Roughened

Solar Air Heaters”, In: Proceedings of National Solar Energy Convention-98 ,Roorkee, India, pp. 75-84, 1998.

[5] R. Karwa, S.C. Solanki, J.S. Saini, “Heat Transfer Coefficient and Friction FactorCorrelations for the Transitional Flow Regime in Rib Roughened RectangularDucts”, International Journal of Heat and Mass Transfer, 42 (1999), pp. 1597-1615.

[6] J.L. Bhagoria, J.S. Saini, S.C. Solanki, “Heat Transfer Coefficient and FrictionFactor Correlations for Rectangular Solar air Heater Duct Having Transverse


62

Wedge Shaped Rib Roughness on the Absorber Plate”, Renewable Energy, 25(2002), pp. 341-369.

[7] A.M.E. Momin, J.S. Saini, S.C. Solanki, “Heat Transfer and Friction in SolarAir Heater Duct with V-shaped Rib Roughness on Absorber Plate”,International Journal of Heat and Mass Transfer, 45 (2002), pp. 3383-3396.

[8] A.R. Jaurker, J.S. Saini, B.K. Gandhi, “Heat Transfer and FrictionCharacteristics of Rectangular Solar Air Heater Duct using Rib-GroovedArtificial Roughness”, Solar Energy, 80 (2006), pp. 895-907.

[9] Varun, R.P. Saini, S.K. Singal, “A Review on Roughness Geometry used inSolar Air Heaters”, Int. J. Solar Energy, 81 (2007), pp.1340-1350.

[10] R.P. Saini, J.S. Saini, “Heat Transfer and Friction Factor Correlations forArtificially Roughened Ducts with Expanded Metal Mesh as RoughenedElement”, International Journal of Heat and Mass Transfer, 40 (1997), pp. 973-986.

[11] J.P. Holman, “Experimental Methods for Engineers”, Tata McGraw-Hill, NewDelhi, 2004.

[12] R. Singh, R.P. Saini, J.S. Saini, “Nusselt Number and Friction FactorCorrelations for Packed Bed Solar Energy Storage System Having Large SizedElements of Different Shapes”, Solar Energy, 80 (2006), pp. 760-771.

[13] S.K. Verma, B.N. Prasad, “Investigation for the optimal Thermo-HydraulicPerformance of Artificially Roughened Solar Air Heaters”, Renewable Energy,20 (2000), pp. 19–36.

performance investigation of artificially roughened duct used in solar...

Documents