Vol 88, May 2007 13

Recent Advances in Experimental and Numerical Analysis ofScramjet Combustor Flow FieldsM Deepu, Associate Member

S S Gokhale, Fellow

S Jayaraj, Fellow

A survey of experimental and numerical efforts to model the complex flow field generated by mixing andreaction of fuel injection in supersonic combustor has been made and presented in this paper. Recentinterest in developing air–breathing propulsion devices incorporating supersonic combustor has forcedmany researchers to develop a configuration giving efficient mixing and combustion, also meeting therequirements of flame holding and completion of combustion with sufficient stabilisation in the flow field.Many experimental and numerical analyses have been reported during the last few decades regarding thecharacteristics of the complex flow field resulting due to fuel–air mixing and combustion. Computer aidedpost–processing of the data from measurement or computation could provide realistic picture of processoccurring in combustors. Experimental results widely used in CFD code validations are discussed indetail.

KeywordsKeywordsKeywordsKeywordsKeywords::::: Scramjet; Supersonic combustion; Turbulent reacting flows

NOTNOTNOTNOTNOTAAAAATIONTIONTIONTIONTION

Aj : area of jet

c : velocity of sound

p : pressure

uj : velocity of jet

U : convective velocity

V : vortex swirl velocity

YH 2: mass fraction of hydrogen

Γ : vortex circulation

ρ : density

ω : vorticity

Ω : vortex angular velocity

INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION

The recent interest in single stage to orbit trans–atmospheric vehicle has lead to the development of ahypersonic flight1,2, which incorporates a supersoniccombustor. Supersonic Combustion Ramjet engine(SCRAMJET) benefits from the better performance of air–breathing propulsion system. Scramjets need a combustorthat should have efficient mixing and combustion of fuelwith air at supersonic speeds without much pressure loss.Many experimental and numerical analyses have beenreported during the last few decades regarding thecharacteristics of the complex flow field resulting due to

fuel–air mixing and combustion. Many fuel injection andflame holding techniques which can have efficient fueloxidant mixing with accurate burning rate have beendeveloped but there is always a trade–off between mixingeffectiveness and pressure drop occurring inside thecombustor affecting the total propulsive thrust availableat the nozzle. Effective design3 of supersonic combustors,based on their mixing and flame holding capabilities, byvarious analytical and experimental studies have been theobject of many investigators. Fundamental configurationsemploying direct injection of fuel to supersonic flow fromcircular, elliptical holes or slots are found to be ineffectivedue to the higher pressure loss associated with them.Various new concepts4 leading to increase in mixingeffectiveness within the short residence time of fuel–airmass inside the combustor with reduced shock losses havebeen devised recently, such as, swept ramps, aerodynamicramps and cantilever injectors. Many numericalinvestigations could accurately demonstrate the complexflow field with reaction, well supported by experimentalfindings. Recent advances in experimental and numericalanalyses of supersonic turbulent reacting and non–reacting flow fields are reviewed here. Computational fluiddynamics (CFD) has proven to be an invaluable tool forthe analysis of such complex flow fields5. Althoughexperimental techniques and results are briefly discussed,the main objective is to give insight to the contributionsfrom CFD community towards portraying such a complexflow situation with the support of available experimentaldata. Mixing and combustion process in supersoniccombustors are generally modelled for basic geometries,which will serve the purpose of an ideal combustor.Experimental data can be used for the validations for suchbasic geometries and some numerical analyses that couldsurvive the ordeal of standard experimental result havebeen discussed in detail.

M Deepu is with the Department of MechanicalM Deepu is with the Department of MechanicalM Deepu is with the Department of MechanicalM Deepu is with the Department of MechanicalM Deepu is with the Department of MechanicalEngineering, N S S College of Engineering, PalakkadEngineering, N S S College of Engineering, PalakkadEngineering, N S S College of Engineering, PalakkadEngineering, N S S College of Engineering, PalakkadEngineering, N S S College of Engineering, Palakkad678 008, Kerala; S S Gokhale is with the Department of678 008, Kerala; S S Gokhale is with the Department of678 008, Kerala; S S Gokhale is with the Department of678 008, Kerala; S S Gokhale is with the Department of678 008, Kerala; S S Gokhale is with the Department ofAerospace Engineering, IIT Madras, Chennai 600 036;Aerospace Engineering, IIT Madras, Chennai 600 036;Aerospace Engineering, IIT Madras, Chennai 600 036;Aerospace Engineering, IIT Madras, Chennai 600 036;Aerospace Engineering, IIT Madras, Chennai 600 036;and S Jayaraj is with the Department of Mechanicaland S Jayaraj is with the Department of Mechanicaland S Jayaraj is with the Department of Mechanicaland S Jayaraj is with the Department of Mechanicaland S Jayaraj is with the Department of MechanicalEngineering; NITEngineering; NITEngineering; NITEngineering; NITEngineering; NIT, Calicut 673 601, Kerala., Calicut 673 601, Kerala., Calicut 673 601, Kerala., Calicut 673 601, Kerala., Calicut 673 601, Kerala.

This paper (modified) was received on April 20, 2006. Writtendiscussion on this paper will be entertained till July 31, 2007.

14 IE(I) Journal–AS

FUNDAMENTFUNDAMENTFUNDAMENTFUNDAMENTFUNDAMENTALALALALAL REQUIREMENTS OF REQUIREMENTS OF REQUIREMENTS OF REQUIREMENTS OF REQUIREMENTS OFSUPERSONIC COMBUSTORSSUPERSONIC COMBUSTORSSUPERSONIC COMBUSTORSSUPERSONIC COMBUSTORSSUPERSONIC COMBUSTORS

Various injection schemes of different geometricalconfigurations6 and flow conditions have been investigatedin the past two decades. With increasing combustor Machnumber, the degree of fuel–air mixing that can be achievedthrough the natural convective and diffusive processes isreduced, leading to an overall decrease in combustionefficiency and thrust. Because of these difficulties, attentionturned to the development of techniques for enhancingthe rate of fuel–air mixing in the combustor. To a largeextent, for given conditions, the net heat release achievedin a scramjet combustor is driven by the efficiency andeffectiveness of the fuel injection.

Performance of a supersonic combustor system dependson efficient injection and complete burning. Futurehypersonic vehicles are expected to require the performanceand operability benefits from air–breathing propulsionsystems as it is providing high specific impulse. The useof supersonic combustors in such vehicles requires efficientsupersonic combustion in combustor lengths short enoughto be compatible with practical engine sizes. Supersoniccombustion process is controlled by both chemical kineticsand mixing. Turbulent mixing is produced by the decay ofconcentration difference between eddies of differingchemical composition. Thus, there is always a trade–offbetween the thrust available at engine nozzle due to lowcombustion efficiency and losses due to mixing energy.Two important aspects of greater concern from fuelinjection process are the degree of turbulent fuel mixingand its transport. Injection effectiveness measures thedegree of fuel penetration and injection efficiency measuresthe degree of turbulent fuel mixing and transport withinthe short residence time. In addition to the key issues,there are other obstacles in the way of development ofsupersonic combustor. Among these, wall friction andheating at high Mach number on combustor body affectingaerodynamic performance and durability of combustor isof greater importance. Also, there are material limitationsin terms of maximum temperature produced by burningof a typical fuel injected. Some of the important termsused in the analysis of scramjet combustor flow fields aredescribed here.

Momentum Flux Ratio (Momentum Flux Ratio (Momentum Flux Ratio (Momentum Flux Ratio (Momentum Flux Ratio (J J J J J )))))

Gruber, et al 7 conducted a series of experiments todemonstrate injectant penetration depth as a function ofvarious flow parameters. Various studies8,9 have used aprincipal controlling parameter, known as the momentumflux ratio of the jet to the free stream. This is essentially,the dynamic pressure ratio given by

(1)

Schetz and Billing9, 10 suggested that the injection pressure

matched condition that is where the static pressure of thejet is equal to the effective backpressure. It is found thatit produced a more optimum penetration than simply overpressurising the jet as it is resulting only a reduced amountof shock loss.

Mixing Efficiency (Mixing Efficiency (Mixing Efficiency (Mixing Efficiency (Mixing Efficiency (ηηηηηmixmixmixmixmix)))))

Mao, et al 11 defined mixing efficiency as that fraction ofthe least available reactant that would react if the fuel–air mixture were brought to a chemical equilibriumwithout additional local or global mixing.

(2)

where

where 2

s

HY is the stochiometric hydrogen mass fraction;

and f is the stochiometric hydrogen fuel/air mass ratio.

TTTTTotal Pressure Loss Parameterotal Pressure Loss Parameterotal Pressure Loss Parameterotal Pressure Loss Parameterotal Pressure Loss Parameter ( ( ( ( (ΠΠΠΠΠ)))))

In the analysis of scramjet combustor flow fields, the totalpressure loss is quantified by the mass averaged parameterintroduced by Fuller, et al 8, defined as

(3)

where .

A high velocity field significantly offsets the reduction inthe total pressure loss parameter due to the total pressurelosses, since it would imply a higher mass flow in thearea under consideration.

VVVVVorticity in the Flow Fieldorticity in the Flow Fieldorticity in the Flow Fieldorticity in the Flow Fieldorticity in the Flow Field

When a supersonic flow passes over a ramp or a backwardfacing step, it generates shock or expansion waves or itscombination. Rogers, et al 12 argued that this leads to thegeneration of the vorticity by baroclinic torque, sincedensity and pressure gradients are not parallel. Theformulation which gives vorticity is

(4)

Leibovich13 has developed a criterion that permits one to

Vol 88, May 2007 15

select ramp angles that shed unstable vorticity, and isgiven by

(5)

INJECTOR CONFIGURAINJECTOR CONFIGURAINJECTOR CONFIGURAINJECTOR CONFIGURAINJECTOR CONFIGURATIONS USED INTIONS USED INTIONS USED INTIONS USED INTIONS USED INSCRAMJETSSCRAMJETSSCRAMJETSSCRAMJETSSCRAMJETS

The fundamental aspects bring success to a supersoniccombustor are efficient injection, mixing and reactionprocesses occurring inside the chamber. The transversejet injected to supersonic cross flow represents a possibleconfiguration for fuel delivery in supersonic combustors.Many investigations14–16 of the transverse jet insupersonic cross flow analyse the under expanded injectionflow field and analytical description of the injectantpenetration as a function of various flow parameters.

WWWWWall Injection into Supersonic Flowall Injection into Supersonic Flowall Injection into Supersonic Flowall Injection into Supersonic Flowall Injection into Supersonic Flow

At high speeds, the normal injection configuration is moreuseful in facilitating mixing and tangential injection ispreferred in low speeds. Schlieran photographs and otherflow visualisation aids, such as, particle image velocymetry(PIV) and Raleigh/Mie scattering imaging reveal thepresence of a bow shock wave upstream of the jet, Machdisc formed after the turning of the jet in the direction ofthe cross flow, the recirculations formed in the vicinity ofjet expansion and also in the region where the bow shockis interacting with the upcoming boundary layer on theplate upstream of the jet. Other interesting factors arethe formation of a pair of counter rotating vortices nearthe jet, horse shoe vortices formed in the near wall regionfrom the vorticity within the cross flow boundary layerand vorticity generated by wall pressure gradient resultingfrom the jet–free stream interaction and also the wake–vortex system. A perspective view of the jet cross flowinteraction as given by Lee, et al 17 is shown in Figure 1.

The pattern of the flow at the circular injector base can beunderstood from the oil flow images18,19. Circular and non–

circular jets exhibit different flow structures. Gruber,et al 20 conducted a series of experiments to show thedifferences in flow pattern of injection from circular andelliptical nozzles where one finds clear evidences of theinfluence of transport of shear layer eddies and nature ofthe upstream bow shock and its associated separationregion.

An experimental analysis of injectant penetration andmixing of angled injection to supersonic stream wereconducted by Bayley, et al 21. The authors used planarlaser–induced iodine fluorescence for the measurementsof injectant mole fraction. It was found that the injectantpenetrates more as angle of injection is increased andvortex region dominates in the region of angled jet. Figure2 gives a comparison of injectant penetration for variousangles of injection.

Injection Downstream of a Rearward FacingInjection Downstream of a Rearward FacingInjection Downstream of a Rearward FacingInjection Downstream of a Rearward FacingInjection Downstream of a Rearward FacingStepStepStepStepStep

Flame holding is an important requirement in supersoniccombustors. A backward facing step is helpful in creatingthe necessary recirculations for holding the sufficientquantity of fuel from the injected stream of fuel. Berman,et al 22 conducted numerical analysis of supersonic flowover a rearward facing step with hydrogen injection. Thehydrogen injection is capable of changing the wave patternof the incoming flow and the mechanism of combinedconvection and diffusion cause hydrogen to spread in theregion between step and injector. This manifests the useof step in combustor. This supplements the earlier reactingflow studies of various scramjet combustors byDrummond23. Wind tunnels will never be obsolete, asnumerical models must always be validated using theexperimental data. McDaniel, et al 24, 25 reported theirefforts to provide an extensive database for a unitsupersonic combustor. In its first part, the authors providethe data for non–reactive mixing flows and of thehydrogen–air combustion data set in second part. The testcase chosen was Mach 2 flow over rearward facing stepwith staged transverse injection down stream of the step.Measurements in supersonic flows require the use of non–

Figure 2 Comparison of injectant penetration for variousFigure 2 Comparison of injectant penetration for variousFigure 2 Comparison of injectant penetration for variousFigure 2 Comparison of injectant penetration for variousFigure 2 Comparison of injectant penetration for variousangles of injectionangles of injectionangles of injectionangles of injectionangles of injection2121212121

0 2 4 6 8 10X/D

Maxim

um

inje

ctan

t m

ole

fract

ion 1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

60° 45° 30°

Figure 1 Perspective view of the jet cross flowFigure 1 Perspective view of the jet cross flowFigure 1 Perspective view of the jet cross flowFigure 1 Perspective view of the jet cross flowFigure 1 Perspective view of the jet cross flowinterac t ioninterac t ioninterac t ioninterac t ioninterac t ion 1 71 71 71 71 7

Approach flow(M>1)

Separationshock

Three–dimensionalbow shock

Horse shoevortex region

Wake region

Counterrotating

vortex pair

Jet shock structure(barrel shock and Mach disk)

Jet boundary

16 IE(I) Journal–AS

intrusive diagnostic technique to avoid the disturbancesgenerated by the physical probe. Non–reacting flowmeasurements techniques were based on laser–inducedfluorescence from iodine molecules seeded in tomainstream. Iodine has a strong visible emissionspectrum, easily accessible by laser systems. Seeding withiodine will not perturb the flow field thermodynamic statesand does not attenuate the laser beam or induce tappingof fluorescence. The schematic representation ofexperimental set up used by McDaniel, et al 24 is shownin Figure 3. In an attempt to provide a quality data setappropriate to the validation of CFD models, Ebrahimi26,27

provides experimental data for primary flow parametersassociated with basic geometries of scramjet.

Experiments in Reacting and Non–reactingExperiments in Reacting and Non–reactingExperiments in Reacting and Non–reactingExperiments in Reacting and Non–reactingExperiments in Reacting and Non–reactingMixing LayersMixing LayersMixing LayersMixing LayersMixing Layers

Experiments on non–reacting free shear layers is todetermine the mixing rate for free shear layers of twodifferent gas species. Experiments have shown that a rapiddecrease in mixing efficiency as Mach number is increasedinto the supersonic regime. Winant and Browand28

proposed the vortex–pairing model for low speed mixinglayer which is applicable only in subsonic regions in flowfield. Effect of density on decreased mixing efficiency withMach number is described by Brown and Roshko29. Later,Papamouschou and Roshko30 extended their study. Sarkarand Balakrishnan31 attempted to explain decreased mixingefficiency to neglect the fluctuating dilatation in thepressure strain correlation that appears in the turbulentkinetic energy transport equation of standard two–equationReynolds averaged Navier–Stokes flow solvers.

Ramp based Fuel InjectorsRamp based Fuel InjectorsRamp based Fuel InjectorsRamp based Fuel InjectorsRamp based Fuel Injectors

Ramped fuel injectors are found to cause better mixing ofthe fuel32. Tangential injection is more preferred incombustors as it suffers only less pressure loss at theexpense of less fuel penetration. As the flow crosses theramp an oblique shock wave is produced which get reflectedfrom the top wall and interact with the injected streamfrom the ramp. A mach disc is formed in the injected streamwhich blocks further expansion of the jet. Other interestingfactors are the formation of a pair of counter rotatingvortices near the jet and horse shoe vortices formed due tothe stream wise vorticity. A schematic of the same is givenin Figure 4.

This design of combustor allows for nearly parallelinjection of the fuel which enhances the available thrustof the engine since shock losses are minimum comparedto combustors using normal injection. Mixing is enhancedby creating stream–wise vorticity by two mechanisms.Vorticity is first created as the high pressure flows abovethe ramps. The spillage further enhanced by creating aspan–wise sweep to the ramps. Downstream of the ramps,vorticity is created through the baroclinic torquemechanism by the interaction of oblique shocks withinjected fuel. Aso, et al 34 conducted experimental andnumerical tests to study the effect of swept angle of rampin supersonic combustors. For experiments, the authorsused supersonic wind tunnel, whose nominal Machnumber is 4.0 and total pressure is 1.3 MPa. In the testsection, the flat plate model is installed, whose diagram isshown in Figure 5. Interchangeable mount is set on themodel and by changing the amount, various types ofinjection are tested. From their volume fractionmeasurements, it is concluded that the oblique injectionis more effective in the mixing than the parallel injection.Furthermore, it turned out that oblique injection generatesmaller disturbance on the flat plate surface flow thanparallel injection from the surface flow visualisationpictures.

Abdel–Salam, et al 35, 36 conducted a series of numericaltests related to ramp fuel injectors. An importantcontribution in this regard was directed towards theanalysis of the effect of ramp swept angle in theperformance of combustor. Increase in side sweep angle isfound to have an active role in enhanced mixing rates.Mohieldin, et al 37 conducted a numerical study ofsupersonic mixing and combustion using unstructuredgrid for exploring more physical aspects related to theinjection from a swept ramp in to supersonic flow. Their

Figure 4 Schematic representation of the flow field ofFigure 4 Schematic representation of the flow field ofFigure 4 Schematic representation of the flow field ofFigure 4 Schematic representation of the flow field ofFigure 4 Schematic representation of the flow field ofinjection from rampinjection from rampinjection from rampinjection from rampinjection from ramp3333333333

Reflected ramp induced shock

Mα

Ramp lip shock

Fuel plume

Recirculation regionMach diskRamp induced shock

Expansionregion

H2 jet

Figure 3 Schematic representation of the experimentalFigure 3 Schematic representation of the experimentalFigure 3 Schematic representation of the experimentalFigure 3 Schematic representation of the experimentalFigure 3 Schematic representation of the experimentalset up used by McDaniel, set up used by McDaniel, set up used by McDaniel, set up used by McDaniel, set up used by McDaniel, et al et al et al et al et al 2424242424

P0 = 274 kPa

T0 = 300 K

d = 1.93 mm

30.48 mm

21

.29

mm

3.18 mm

– 50

+5 XZ

Y

X/D

Figure 5 Details of swept ramp injector used for mixingFigure 5 Details of swept ramp injector used for mixingFigure 5 Details of swept ramp injector used for mixingFigure 5 Details of swept ramp injector used for mixingFigure 5 Details of swept ramp injector used for mixingstudies of studies of studies of studies of studies of Aso, Aso, Aso, Aso, Aso, et alet alet alet alet al3434343434

Mount

y

13

0

265

520

z

x

15

0

39

74.9φ 3

5

13

10°°°°°

10°°°°°

10°°°°°

x

24

All dimensions are in mm

Vol 88, May 2007 17

analysis uses the famous commercial CFD code FLUENT®

as it is widely used for midspeed applications due to itscapability to handle high speed as well as low speed flows.

Aerodynamic InjectorsAerodynamic InjectorsAerodynamic InjectorsAerodynamic InjectorsAerodynamic Injectors

Aerodynamic ramp is the new generation fuel injectiontechnique with lower pressure loss. Advantage of such amodified hole for fuel injection is that, the flow fieldproduced by the injection leaves a secondary core of thejet plume nearer to the wall. An attempt to capitalise onthe effects of an aerodynamic injector array in a scramjetcombustor were first numerically studied by Eklund andGruber38 and then experimentally by Gruber, et al 39.Latest design comprising an array of nine suchaerodynamic fuel injection ports, were designed so as tomaximise axial jet induced vorticity and to use this to liftthe plume into the flow. Qualitative understanding of suchflows were done using shadowgraph and surface flowvisualisation pictures given by Jacobson, et al 40. Theaerodynamic fuel injector40 is shown in Figure 6.

Computational study of an ethylene fueled scramjetcombustor employing an aerodynamic ramp fuel injectorand a cavity flame holder has been performed by Eklund,et al 41. This was aimed to focus on hydrocarbon fuelsrather than hydrogen fuel and capitalise on the higherdensity and easier storage of hydrocarbon fuels. Studiesof Stouffer and Gruber42 show that the performance ofthe aerodynamic ramp fuel injector was disappointing. Itis because the individual fuel jets of the injector rapidlymerged into a single plume that resulted in a relativelypoor fuel–air distribution over the cavity flame holder, andoverall reduced mixing within the combustor. The aero–ramp injector showed somewhat higher local total pressurelosses than the single–hole injector. This was due to thehigher composite angle injection of the aero–ramp arrayand the multiple shock structures from the two rows ofjets. Although the total pressure losses appeared moresubstantial, the mass averaged total pressure lossparameter shows the aero–ramp to have only slightlyhigher overall losses in the area studied. The aero–rampproduced larger separation zones in front, in between, andbehind the injector jets. This would allow more opportunityfor flame holding in a high enthalpy flow, but would alsocreate hot spots on the surface near the injector. Theplume of the aero–ramp had a larger plume area than thesingle–hole injector due to lateral spreading.

Cantilever Fuel InjectorsCantilever Fuel InjectorsCantilever Fuel InjectorsCantilever Fuel InjectorsCantilever Fuel Injectors

Parent and Sislian43 conducted numerical studies ofmixing efficiencies of cantilevered ramp and Waitz rampinjector44. For the analysis the authors used Favreaveraged Navier–Stokes equations for multiple specieswith κ–ω turbulence model. The study shows the mixingefficiency variation with convective Mach number.Cantilevered design has the advantage that shock isformed under the injectors providing contiguous shocksurface span–wise direction of the injector array, whichwill increase the baroclinic effect and hence largermixing efficiency. Figure 7 gives the geometry andcompares the mixing efficiency of planar, free andcantilevered jets.

The cantilever injection geometry is considered that isthought to embody the characteristics of both injectiontechniques. Shock B is responsible for the cross–streamshear, and shock A for the baroclinic effect, both of whichgenerate strong longitudinal vortices. However, in thepresent design, in addition to the side wall vorticesgenerated by the cross–stream shear, strong vortices willbe produced behind the ‘bluff–body’ of the injector, as inthe case of a low–angle wall fuel injector. These vorticeswill further enhance the mixing process. Although it canbe considered as a candidate for fuel injection in scramjetcombustors, the proposed cantilevered ramp injector isprimarily considered for use in shock–induced combustionramjets, where fuel–air mixing should take place withoutcombustion until a specific location in the propulsive ductof the engine.

Cavity based Fuel InjectorsCavity based Fuel InjectorsCavity based Fuel InjectorsCavity based Fuel InjectorsCavity based Fuel Injectors

Microscale mixing is essential as it promotes rapidreaction, but mixing alone does not initiate combustionprocess. Once ignition is established, combustion can beprolonged efficiently with the assistance of proper mixing.Cavity flame holders designed by the Central Institutionof Aviation Motors (CIAM) in Moscow, were used for thefirst time in a joint French/Russian dual mode scramjetflight test. Many recent 45,46 studies reveal that use ofcavities have improved flame stabilisation and combustionefficiency. A cavity exposed to a supersonic flow producessustained oscillations which will considerably alter theflow properties in its premises. This has actually motivatedmany researchers47– 49 in carrying out experimental andnumerical studies to reveal its complete physics. Therecirculations inside cavity increase residence time of the

Figure 7 Geometry of cantilevered rampFigure 7 Geometry of cantilevered rampFigure 7 Geometry of cantilevered rampFigure 7 Geometry of cantilevered rampFigure 7 Geometry of cantilevered ramp4343434343

Shock A

Shock B

C

Fuel

CFigure 6 Figure 6 Figure 6 Figure 6 Figure 6 Aerodynamic fuel injectorAerodynamic fuel injectorAerodynamic fuel injectorAerodynamic fuel injectorAerodynamic fuel injector4040404040

18 IE(I) Journal–AS

fluid entering in it. It is known that the growth rate ofthe mixing layer between supersonic air and gaseous fuelin a scramjet combustor decreases as the convective Machnumber increases due to compressibility effects. Studiesof Yu and Schadow50 and Kumar, et al 51 suggested thatthe high frequency oblique oscillating shock waveemanating from cavities exposed to the supersonic flowfield (reacting and non–reacting) are capable of improvingthe mixing. Shape of the cavity can be controlled so as toreduce the pressure losses associated with oscillations, asthis is essential in providing the required thrust at thenozzle. The idea of providing cavity in supersoniccombustors improves its flame holding capabilities andreduction in induction time (which enhance autoignition)as it creates a recirculation region with a hot pool ofradicals.

Basically, there are two types of cavity flow, namely, openand closed. The open cavity flow is said to occur whenlength–to–depth ratio, L /D < 10. These cavities are foundto be dominated either by longitudinal or transversepressure oscillations depending on L /D ratio and the Machnumber. If the cavity is filled by a large single vortex,transverse mechanism controls the vortex, whereas,mixing will be controlled by longitudinal mechanism ifmany vortices are filled in a lengthy cavity. For a crossflow at M = 2.5 such a transition can be seen if L /D ischanged between 2 and 3. The longitudinal cavityoscillations are produced by the impingement of shear layerat the cavity rear wall, which produces an increase in thewall pressure. This impingement of shear layer createsan acoustic wave of velocity equal to local sound speedand hits on the aft wall, creating a small recirculation atits top. This recirculation will later convect to downstreamof the combustor producing oscillations in flow field. Thus,it results in mass addition and removal at cavity rearwall. ForL /D > 10, the cavity flow is termed as ‘closed’,because here the free shear layer attaches to the lowerwall. Such an arrangement will have high drag lossesdue to high pressure near rear end and comparatively lowpressure towards the front. Here, the shear layer is unableto span entire length of the cavity, and hence these areless preferred in scramjet combustors. Flow fieldschematics45 of open cavity flow for L /D < 7 and closedcavity flow for L /D > 10 are shown in Figure 8.

Ben–Yakar, et al 52 conducted experiments with OH-PLIIFvisualisation of hydrogen transverse jet injection intoreacting and non–reacting supersonic flow. Study showedthe indication of the autoignition of hydrogen jet in aircross–flow simulating flight Mach10 conditions. In thisstudy, the OH fluorescence appears first in therecirculation region upstream of the jet and extends alongthe outer edge of the jet plume.

Settles, et al 53 conducted an experiment to study the effectof supersonic free shear layer reattachment over rampbody placed after a cavity. Here the planar shear layerimpinging on the ramp inclined by 20° behind a cavity.Lee and Kim17 suggested different methods for mixing

augmentations of the transverse injection in a scramjetcombustor, based on the fact that the main factorcontrolling the mixing characteristics in transverseinjection is the effective backpressure and the thestreamwise vorticity generated by baroclinic torque. Athree dimensional Navier–Stokes code adopting was usedthe upwind method of Roe’s flux difference splittingscheme. The authors have studied the main parametersof fuel–air mixing, such as, mixing rate, penetrationdistance and stagnation–pressure loss. The schematic andpressure variation plots of Settles experiment53 areshown in Figure 9.

Fuel Injection from StrutsFuel Injection from StrutsFuel Injection from StrutsFuel Injection from StrutsFuel Injection from Struts

Various studies54–56 have shown that incomplete mixing,shock waves and viscous effects are the main factorsleading to the thrust loss in supersonic combustors, thoughthese effects aid mixing. Strut injectors offer a possibilityfor parallel injection without causing much blockage tothe incoming stream of air and also the fuel can be injectedat the core of the stream. Tomioka, et al 57 studied theeffects of staged injection from struts. Gerlinger andBruggemann58 conducted a numerical investigation ofhydrogen injection from strut to foresee the effects of lipthickness of the injector in mixing. It was concluded thatincrease in lip thickness causes an increase in mixinglayer due to the enhanced diffusivity associated with itbut it do not have much effect on mixing efficiency. Totalpressure loss in the combustor is less affected by the heightof the strut.

EXPERIMENTEXPERIMENTEXPERIMENTEXPERIMENTEXPERIMENTALALALALAL DIAGNOSTIC TOOLS USED DIAGNOSTIC TOOLS USED DIAGNOSTIC TOOLS USED DIAGNOSTIC TOOLS USED DIAGNOSTIC TOOLS USEDIN SCRAMJET COMBUSTORSIN SCRAMJET COMBUSTORSIN SCRAMJET COMBUSTORSIN SCRAMJET COMBUSTORSIN SCRAMJET COMBUSTORS

The experimental condition in supersonic combustion flowis extremely hostile. The measuring process should notperturb the flow field for making the accuratemeasurement in such compressible turbulent reacting andhigh enthalpy flows. The advanced diagnostic tools usedrecently for such applications are reviewed here.

Coherent anti–stokes Raman scattering (CARS) is anexcellent technique for measurement in high enthalpysupersonic test facilities, because of its capability to provide

Figure 8 Flow field schematicsFigure 8 Flow field schematicsFigure 8 Flow field schematicsFigure 8 Flow field schematicsFigure 8 Flow field schematics4545454545 of (a) open cavity flow of (a) open cavity flow of (a) open cavity flow of (a) open cavity flow of (a) open cavity flowfor for for for for L/D L/D L/D L/D L/D < 7 and (b) closed cavity flow for < 7 and (b) closed cavity flow for < 7 and (b) closed cavity flow for < 7 and (b) closed cavity flow for < 7 and (b) closed cavity flow for L/DL/DL/DL/DL/D > 10 > 10 > 10 > 10 > 10

Transverse mechanism Longitudinal mechanism

Transition at L/D ≅ ≅ ≅ ≅ ≅ 2 ~~~~~ 3

(a)

(b)

D

L

Vol 88, May 2007 19

instantaneous information regarding pressure,temperature, concentration of species, vibrational androtational spectra of molecules in flow etc. In non–intrusivemeasurement technique like CARS, data acquisition andits analysis is the prime task. Charged couple devices(CCD) cameras and various types of spectrometers acquiredata. Electronic signal processors process these signals.Computers gather these processed information and providerealistic plots of various quantities by doing inbuiltmathematical operations, such as, probability densityfunction (PDF) based methods. Such plots depict the truephysics inside combustors. Yu, et al59 made measurementsusing CARS in supersonic combustors employing strutinjectors. Unstable resonator spatially enhanced detectionmethod was used in which a two–beam three–dimensionalphase matching configuration is used for alignment.

Particle image velocimetry (PIV) is a planar techniqueproviding instantaneous two–dimensional velocity fields.Development of double frame CCD cameras extend theapplication of PIV. It is widely used in analysis of non–reacting supersonic flow fields. The application of PIV tohigh–speed flows with high velocity gradients requires theuse of submicron tracer particles to minimise the particleslip velocity. Since, the scattering cross–section of theseparticles is very small, an intensive light source isnecessary to obtain Mie scattering signals sufficientlystrong for detection. Another requirement for high velocityflows is that the illuminating system must generatesuccessive pulses reproducibly within very short timedelays to resolve the speed accurately. Weisgerber, et al 60

made measurements using PIV in a supersonic tunnelwith and without combustion. The structure of the reactivemixing layer between the fuel and the air has beeninvestigated by means of PIV. Measurements are basedon two successive images of tracer particles seeding theflow. The correlation between the two images yields thequasi–instantaneous two–dimensional velocity field.

Nano–shadowgraphs uses spark shadowgraphs takenusing a Nano–pulser spark with an exposure time ofnanoseconds and it is possible to see the turbulent eddystructures in the flow significantly larger than 0.01 mm.Surface oil flow visualisation uses silicone oil mixed withtwo colours of fluorescent dye. Studies of Jacobsen, et al 40

used a thin layer of fluorescent green oil, which was placedall around the fuel injector. The surface oil flow patterns

were also recorded on videotape during tunnel operation.The authors also employed pressure sensitive paint (PSP)measurements of the static pressure fields around theinjector. The PSP employed is fluoroacrylic copolymer (FIB)binder and incorporates the fluorinated platinum porphyrinspecies. Planar laser induced iodine fluorescence (PLIIF)is an established24, 25 technique used in the analysis ofscramjet combustor. Menon, et al 61 used Nd:YAG laserand acetone (introduced using a fine atomising sprayinjector) to measure the fuel distribution and the fuel–airmixing downstream of the injector. Studies of McDaniel,et al 24 also employed simpler techniques, such as,thermographic phosphor wall temperature imaging andlaser induced iodine fluorescence (LIIF)

NUMERICAL STUDIES OF TURBULENTNUMERICAL STUDIES OF TURBULENTNUMERICAL STUDIES OF TURBULENTNUMERICAL STUDIES OF TURBULENTNUMERICAL STUDIES OF TURBULENTREACTING FLOWS IN SCRAMJETSREACTING FLOWS IN SCRAMJETSREACTING FLOWS IN SCRAMJETSREACTING FLOWS IN SCRAMJETSREACTING FLOWS IN SCRAMJETS

Computational fluid dynamics (CFD) has proven to be aninvaluable tool for the design and analysis of high speedpropulsion devices. Massively parallel computing, togetherwith the maturation of robust CFD codes, has made itpossible to perform simulations of complete engine flowpaths. Navier–Stokes simulations are now widely used inthe determination of optimum fuel injection configurations.Because the smallest scales of turbulence are very largecompared to molecular dimensions, turbulence is acontinuum phenomenon. Consequently, the Navier–Stokes, energy and mass–conservation equations containall of the physics of turbulent fluid motion. The difficultystems from the fact that turbulence is extremelycomplicated. Sustained by vortex stretching, thephenomenon is inherently three dimensional and timedependent. Modified κ–ε model, called RenormalisationGroup (RNG) is widely used in most of the Navier–Stokessolvers for scramjet combustor flow field analysis, and isdemonstrated accurately by Papp and Ghia62. Hydrogenand hydrocarbon fuels, such as, kerosene are most suitablefor hypersonic propulsion systems, because of its highpotential of heat release and rapid mixing with air.

Modelling of Modelling of Modelling of Modelling of Modelling of Air–fuel Mixing in ScramjetAir–fuel Mixing in ScramjetAir–fuel Mixing in ScramjetAir–fuel Mixing in ScramjetAir–fuel Mixing in ScramjetCombustorCombustorCombustorCombustorCombustor

Due to the inherent difficulty in handing two phenomenaof differing time scales (flow and reaction) and limitedcomputer resources to handle more variables, earlierattempts in modelling scramjet combustor were directedtowards studying the mixing and fuel transportcorresponding to different geometries working in differentflow conditions. Along with their experiments, Fujimori,et al 63 did numerical studies of gas injection from circularand slot ports. Implicit formulations involving Cebeci–Smith turbulence model and Harteen type TVD schemewere employed for discretising convective terms. The wallpressure distributions obtained were in close agreementwith their own experimental values, which are widely usedfor code validation. Aso, et al 19 recently analysed similartwo–dimensional injection from slot injection using highresolution flux splitting schemes. Numerical study of a

4

3

2

1

Figure 9 The schematic and pressure variation plots ofFigure 9 The schematic and pressure variation plots ofFigure 9 The schematic and pressure variation plots ofFigure 9 The schematic and pressure variation plots ofFigure 9 The schematic and pressure variation plots ofSettles experimentSettles experimentSettles experimentSettles experimentSettles experiment5353535353

TurbulentBL

Recirculation

Free shear layer

Shear layer station

Compression Redeveloping BL

Shock

Ramp station

R20 30

4050

60

20 30 40 50 60

cm

20°°°°°

M =2.82

20 IE(I) Journal–AS

circular jet in supersonic cross flow done by Tam, et al 64

analyse the fluid dynamic mechanisms inherent fromcircular jet injection into a supersonic crossflow. Thecomputational results were obtained using the VULCAN(viscous upwind algorithm for complex flow analysis)Navier–Stokes code. Three different types of two–equationturbulence models were used to predict the fuel mixingprocess, including the new version of the Wilcox model,the Menter–SST model and the Menter–BSL model. Inaddition, a top–hat profile boundary condition for the jetexit using the κ–ω model was tested. The Wilcox and BSLmodels have better overall predictions in terms of surfacepressure comparisons. The Menter–SST modeloverpredicted the separation region upstream of the jetinjector, while the top–hat profile calculation produced ahigh concentration fuel plume at the lateral side whichmight be caused by high vorticity. Donohue andMcDaniel65 studied injection from ramp injector usingSPARK code. Madabhushi, et al 66 did the computationalmodelling of mixing process of ramp injector usingupwinding based time–dependent Navier–Stokes solver(UTNS) and it could predict the mixing phenomena. Someof the numerical studies on mixing processes associatedwith different geometries are discussed earlier.

Numerical Modelling of Reacting Flows inNumerical Modelling of Reacting Flows inNumerical Modelling of Reacting Flows inNumerical Modelling of Reacting Flows inNumerical Modelling of Reacting Flows inScramjet CombustorScramjet CombustorScramjet CombustorScramjet CombustorScramjet Combustor

Supersonic reacting flow field can be simulated by addingfinite rate chemistry to standard compressible Navier–Stokes equations. Both turbulence and chemical kineticsare important, since the residence time is much smaller.Explicit treatment of all conservation terms with reactionchemistry results in stiff equations and it will degradethe performance of numerical method flow field andchemical kinetics with differing time scales need to besolved simultaneously. Existence of several non–equilibrium states creates challenges in solution procedure.Bussing and Murman67 introduced the method of pre–conditioning the conservation equations in conjunctionwith chemical source terms alone being treated implicitly.Such a method has the advantage of both explicit andimplicit methods. Use of appropriate reaction kinetics isvery important in combustion modelling. Jachimowski68

developed a detailed reaction mechanism and is widelyused in combustor analysis.

Yoon69 developed a finite element solution algorithm forhigh speed flows and compressible chemical reacting flows.This method is based on Taylor–Galerkin finite elementmethod, allows flow equations and chemical source termssolved separately on the physical time scales. Frozen,equilibrium and finite rate chemistry, based on two–stepmodel and eighteen–step model for hydrogen air, aretreated. This technique is used for solving various basicscramjet combustor flow fields.

Wilson and MacCormack70 developed an axisymmetric,fully implicit finite volume solver for detailed hydrogen–air mechanism and validated using a ballistic range

experiment with shock induced combustion, can be usedfor verification of supersonic combustor test cases. Animplicit finite volume, lower upper symmetric overrelaxation scheme was developed by Shuen and Yoon71 forthe study of mixing and chemical reactions in the flowfields of ramjets and scramjets. The code has been used tostudy the sonic hydrogen injection from a slot on top wallof the combustor to Mach 4 air stream.

A lower–upper symmetric Guass–Seidel finite volumemethod was developed by Gerlinger and Algermissen72,in which the combustion with 20 step (nine species) finiterate chemistry coupled implicitly with fluid motion. Forturbulence closure, an algebraic Baldwin Lomax, as wellas κε–qω low Reynolds number model were used. For thetest case of non–reactive flow with air wall injection tosupersonic stream, detailed wall pressure analysis hasbeen conducted and validated with experimental results.Next a channel flow with wall injection and suction areinvestigated. Also the pressure and density variations areanalysed. Another test case with mixing and combustionof hydrogen wall injections are studied by plottingconcentration plots of species produced in reaction.

Kammath and Mao73 reported that the SHIP 3D PNScode provides the capability to perform three dimensionalcomputations of scramjet combustors efficiently at highflight Mach numbers of 10 and above, where the combustorflow is largely supersonic. The code is having FavreAveraged, parabolised equations for conservations of massmomentum and total energy. Solutions are computed usingthe famous traditional SIMPLE, SIMPLER or SIMPLECalgorithms. Here, the discretised equations are solved lineby line using tridiagonal matrix solver. Turbulence ismodelled at two equation level by high and low Re models.It is capable of taking hydrogen–air chemistry either offrozen, one step complete, partial reaction, four reactionor seven species equilibrium model. SHIP 3D PNS solverwas used for solving the fuel injection from ramp insupersonic combustor.

Probability Density Function Modelling ofProbability Density Function Modelling ofProbability Density Function Modelling ofProbability Density Function Modelling ofProbability Density Function Modelling ofReacting Flow FieldsReacting Flow FieldsReacting Flow FieldsReacting Flow FieldsReacting Flow Fields

The assumed probability density function (PDF) approachhas been widely used74 in the numerical study of supersonicreacting free shear flows. Pope75 described the procedurefor obtaining the Reynolds stresses for the two dimensionalflows. In this type of analysis the turbulence chemistryinteractions are treated using the assumed joint PDF thataccounts for fluctuations in both temperature and speciesproduction rates. Baurle, et al 76 and Girimaji77 used thistype of modelling for predicting the fluctuations. The lawof mass action and the joint PDF formulations fortemperature and species composition at each location canbe written as

(6)

Vol 88, May 2007 21

Here, wm represents species production rates and ρ1, ρ2,...,ρns represent densities of each species. PDF temperatureis often described by the Gaussian distribution as

(7)

Recent studies presumes the statistical independence oftemperature and composition. Frankel, et al 78 used bothbeta and Gaussian distribution for describing temperature.Works of Gaffney, et al 79 used the same type of relationshipand showed that PDF has a significant role in getting theignition point. Narayan and Girimaji80 combined amoment method to account for temperature variations withmulti variate beta distribution. Baurle, et al 81 used thisformulation for describing the flow field generated by rampfuel injector. Raju82 successfully demonstrated theapplication of various PDF formulation in standardturbulent reacting flow situations. Chen, et al 83 havecomputed for all available benchmark using NationalCombustion Code (NCC), which is based on PDF models.Some of the numerical studies on combustion processassociated with different geometries are discussed earlier.

A detailed survey of various experimental and numericalattempts to analyse scramjet combustor has been alreadypresented. A survey was performed to identify experimentalmeasurement programs with sufficient documentation ofthe geometry and flow conditions required for CFD codevalidation. The validation of CFD codes appropriate for

subsonic through hypersonic applications requires carefulconsideration of the physical processes encountered in flightregimes, and detailed comparison with qualityexperimental data sets that simulate these processes. Thenecessary data of such celebrated publications issummarised in Table 1.

CONCLUSIONSCONCLUSIONSCONCLUSIONSCONCLUSIONSCONCLUSIONS

Mixing, penetration and combustion characteristics ofinjected fuel and air in scramjet combustor for differenttypes of injectors have been reviewed. Various experimentalstudies on mixing and injectant penetration with differentimaging techniques were considered. Increase in jet to freestream momentum flux ratio will result in the increase ofjet penetration to free stream for all kinds of jets. Withelliptical shaped wall jet, one can get about 25% morelateral spreading when compared to the circular jet ofsame kind. Injector orientation plays an important rolein the strength of the bow shock, with the shocks createdby oblique injector being substantially weak compared totransverse injector. Works of related interest, other thanthose mentioned here reveal the same facts. Mixingefficiency and penetration will have opposite effect on totalpressure loss occurring within the combustor affectingthrust at nozzle. Therefore, optimising the injector shapeand its orientation with optimum performance are the maininterest of the researchers working in this field presently.

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TTTTTable 1 Details of experiments widely used for CFD codeable 1 Details of experiments widely used for CFD codeable 1 Details of experiments widely used for CFD codeable 1 Details of experiments widely used for CFD codeable 1 Details of experiments widely used for CFD codeval idat ionval idat ionval idat ionval idat ionval idat ion

E x p e r i m e n tE x p e r i m e n tE x p e r i m e n tE x p e r i m e n tE x p e r i m e n t G e o m e t r yG e o m e t r yG e o m e t r yG e o m e t r yG e o m e t r y Interes tedInteres tedInteres tedInteres tedInteres ted R e f e r e n c eR e f e r e n c eR e f e r e n c eR e f e r e n c eR e f e r e n c ei t e mi t e mi t e mi t e mi t e m n u m b e rn u m b e rn u m b e rn u m b e rn u m b e r

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Aso Circular and Wall pressure 34, 63slot injection distribution

Henry and Axi symmetric Mixing, ignition 86,87Beach nozzle and product

concentrationprofile

McDaniel Unswept Wall pressure 88ramp distribution

McDaniel Staged Pressure 24injection distribution and

concentrationprofile

Cheng Coaxial H2 – Product 89air jet concentration

and temperatureprofiles

Settle Cavity Reattachment 53pressure

Baurle and Cavity Wall pressure 48Gruber distribution

22 IE(I) Journal–AS

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