important for combustion setting properties

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Study of Supersonic Combustion Characteristics in a Scramjet Combustor

T. M. Abdel-Salam 1 , S . N. Tiwari 2 , T. O. Mohieldin 3 College of Engineering and Technology

Old Dominion UniversityNorfolk, Virginia 23529

1 Adjunct Assistant Professor, Department of Mechanical Engineering, Member AIAA.2 Eminent Professor/Scholar, Department of Mechanical Engineering, Associate Fellow AIAA.3 Professor, Mechanical Engineering Technology Dept., Member ASME, AIAA.

ABSTRACT

Numerical calculations have been performed to studythe flowfield of a dual-mode scramjet combustor.Results are obtained with a finite volume CFD codeand using unstructured grids. A 3-D dual-modecombustor has been investigated with two differenttypes of fuels. Only half of the physical domain aresolved by forcing the symmetry condition at thecenterplane of the model. Results are presented forhydrogen combustion and ethylene combustion.

INTRODUCTION

During the last three decades, a great deal ofresearch toward development of airbreathinghypersonic vehicles has been conducted. A criticalelement in the design of the scramjet engine is detailedunderstanding of the complex flow field present indifferent regions of the system over a range ofoperating conditions. A considerable amount ofresearch has been done on the components of

scramjets, with the inlet, the combustion chamber, andthe thrust nozzle all receiving attention. Thecomponents have also been coupled together, to makea complete scramjet engine, and various forms of thistype of engine have been subjected to experimentalscrutiny. 1 With increasing combustor Mach number,the degree of fuel-air mixing that can be achievedthrough the natural convective and diffusive processesis reduced, leading to an overall decrease incombustion efficiency and thrust. Because of thesedifficulties, attention turned to the development oftechniques for enhancing the rate of fuel-air mixing in

the combustor. To a large extent, for given conditions,the net heat release achieved in a scramjet combustor isdriven by the efficiency and effectiveness of the fuelinjection. 2 Various injection schemes of differentgeometrical configurations and flow conditions have

been investigated in the past two decades. Selected

methods that have been used to enhance the mixing process in the scramjet engines are summarized andreported in Ref.[3]

In the dual-mode scramjet engine, a constant areadiffuser (isolator) is placed upstream the combustor toisolate the inlet flowfield from any combustor-generated upstream interaction in order to prevent theinlet unstart. A constant area duct combustor followsthe isolator. Fuel can be injected by different methodsinside the combustor. The heat release due tocombustion eventually expands the flow back to sonicconditions (thermally choked condition). An expandingduct is placed after the combustor in order to maintainflow expansion to supersonic conditions and delay theformation of thermal choke. In recent years, dual-modecombustion has received attention because of itsapplication in particular flights. The dual-modecombustion is a very challenging problem forcomputational fluid dynamics (CFD). This is due to thenature of the highly turbulent flow field associatedwith the extensive upstream interaction, and thedownstream mixing and combustion at low Machnumber. Moreover, the mixed supersonic and subsonicregions of the combustor require large sections of theflow to be solved simultaneously, forcing the use ofefficient CFD codes and suitable turbulence andcombustion models. Past research on dual-modecombustion has generally been focused on studyinginlets, isolators, combustors, fuels, and fuel injection.Waltrup [4] reviewed extensively the past research upto 1987 on hypersonic inlets, isolators, liquid fuels,wall fuel injection, axial fuel injection, combustors,and exit nozzle. Billing et al. [5-6], and Waltrup andBilling [7-9] first provided analysis of experiments andanalytical tools allowing the prediction of upstreaminteraction, required isolator length for mid-speedscramjet combustor configurations. Differentexperimental and numerical studies have been

16th AIAA Computational Fluid Dynamics Conference23-26 June 2003, Orlando, Florida

AIAA 2003-3550

Copyright 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


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conducted to study a dual-mode combustor model withaerodynamics ramp fuel injectors [10-13].

A dual-mode combustor with high upstreaminteraction was proposed and investigated at the

National Aerospace Laboratory (NAL) in Japan. Fuelis injected normal to the airstream behind a backwardfacing step. Experimental studies have been performedin Refs. 14 and 15. This geometry is similar (but notidentical) to the dual-mode models investigated in thecurrent study. Moreover, different numerical studieshave been conducted to study the same geometry usinghydrogen [16-20]. The effect of the turbulencetemperature fluctuation on the combustion process wasinvestigated by Mizobuchi et al. [16]. The numericalresults of Reggins[17] showed the development ofsubstantial upstream interaction consisting of anoblique shock/expansion train. This shock train isgenerated by recirculation zones on both top and

bottom isolator walls. Olynciw et al.[18] numericallyinvestigated the possibility of scaling thecomputational domain to accelerate the convergence ofthe numerical solution in order to reduce thecomputational time. The study supports the usefulnessof the numerical scaling in simulating dual-modecombustor flowfields. Rodriguez et al. [19] studiedgrid convergence, turbulence modeling, and walltemperature effects in terms of wall pressure. Severalcomputational cases were examined; these casesinclude jet-to-jet symmetry and half duct modeling.Results showed the development of a large side-wall

separation zone extending much further upstream thanthe separation zone at the duct centerline. Mohieldin etal. [20] studied numerically the same model. They haveinvestigated both two-dimensional and three-dimensional models. Their results showed highupstream interaction in the isolator section. Also, it wasfound that the symmetric flow structure no longerexists in the isolator as the length of the upstreaminteraction exceeds the isolator height.

The present study is an extension of previous effortsat Old Dominion University to simulate the mixing andcombustion processes in scramjet engines. The main

objective is to investigate the mixing and thecombustion characteristics of one of the dual-modecombustors using ethylene and hydrogen as fuels.

P HYSICAL M ODEL

The dual-mode combustor model is shown inFig.1. This model is similar to that investigatedexperimentally by Kumaro et al. [14] but with a

different arrangement of the fuel injectors. The dual-mode combustor is a 147.3mm constant widthrectangular duct. It consists of three parts, constantarea duct isolator with aspect ratio 4.7, constant areaduct combustor and expanding duct. The isolator is 32mm in height and 220 mm in length. There is a 3.2 mmsteps on both upper and lower walls of the combustor.The length of combustor is 96 mm followed by a 350mm expanding duct with expansion angle of 1.7 o onthe upper and the lower walls. Fuel is injected throughwall injectors on the upper and lower walls. Thecombustor has 18 injectors , all injectors have thesame diameter of 2.8 mm. Injectors are placed in tworows on each wall. The fuel injectors in each row areequally spaced. The injectors on both rows are putstaggered. The two rows are located at 128 mm, and192 mm downstream of the steps respectively. Thearrangement of the fuel injectors is intended to providethe same fuel flow rate on both walls in addition toincreasing the surface area of the fuel in order toachieve good mixing and complete combustion. Figure2 shows the distribution of the fuel injectors inside thecombustor.

N UMERICAL D ETAILS

In the present study the numerical analysis wascarried out using the CFD code Fluent. The Fluent isa finite volume code for solving the Reynolds-averaged

Navier-Stokes equations. The code uses first/secondorder finite volume discretization method coupled withexplicit/implicit solver. Further details of the numericalmethods used in the code can be found in the Fluentusers guide 21. The governing equations for this studyare Navier-Stokes equations and species continuityequation. The turbulence model used is the RNG k- model. The grids were adopted such that the value ofthe y + at the wall is less than 100, which is suitable forthe use of wall function as reported in Ref. [19]. Thedensity, thermal conductivity, and viscosity of both thehydrogen-air and the ethylene-air mixtures are definedwith ideal gas mixing law. The specific heat capacities,

thermal conductivity, and viscosity of species aredefined as polynomial function of the temperature.

Chemical kinetics were modeled using finite-ratemodel. A one-step, 4-species reduced mechanism wasused for hydrogen, while a 3-step, 6-species reducedmechanism [13] is used for ethylene.


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B OUNDARY C ONDITIONS A ND C ONVERGENCE

A fully developed turbulent flow is assumed for theair inlet and the fuel jet. No-slip conditions are usedalong the nozzle and the combustor walls. Thetemperature of the walls is set equal to 500 K. Along

the supersonic inflow boundaries, uniform conditionsare used for both the freestream and the jet. Thesymmetry condition is used for the central plane of thecombustor. The pressure and other flow quantities atthe outlet section are extrapolated from the interiordomain. Initial conditions are obtained by specifyingfreestream conditions throughout the flowfield. The

boundary conditions at the inlets are similar to that ofRef.[17] and are shown in Table 1.

The convergence of the solution is monitored and judged by four different criteria namely; the residualsof the flow properties, the mass conservation, and the

profile of the wall static pressure. The convergedsolution is assumed to be achieved after satisfying thefollowing four conditions:

1- The residuals of the flow properties are lessthan 10 -3.

2- No changes in the wall static pressure profileare seen with the iterations.

3- Global mass balance at the inlets and the outletsis satisfied.=

out in mm (1)4-Conservation of mass flow rates inside thecomputational domain is satisfied.

R ESULTS

In this section, results are presented for bothhydrogen and ethylene combustion. The mass flow rateof the fuel (hydrogen and ethylene) injected from thelower wall is equal to that injected from the upper wallof the combustor.

Figure 3 compares the lower-wall static pressure for both types of fuel. The x-axis is normalized with theisolator height h. In both cases, the pressureincreases slightly inside the isolator section, thenreaches its maximum value just after the combustorinlet and very close to the location of the injectors. Themaximum value of the pressure inside the combustor isabout 6 and 5 times the inlet value for hydrogen andethylene respectively. The pressure then decreasesinside the combustor and the expanding duct. Noupstream interaction is seen inside the isolator in thetwo cases since uniform boundary conditions are usedat the inlets. It was proven in pervious study 23 that the

profile at inlet affects the flowfield inside the isolator but it does not affect the flowfield inside the combustoror the expanding duct.

Figure 4 shows the axial distribution of Machnumber. A non significant decrease is noted in theisolator section followed by a rapid decrease near thecombustor inlet. At the combustor inlet the flow

becomes mixed subsonic/supersonic flow. After thatthe Mach number recovery starts inside the combustor,however, the combustion process takes place in asubsonic/supersonic stream. The Mach numberincreases again in the expanding duct to supersonicspeeds. Both cases show the same trend however, itcan be seen that inside the expanding duct the Machnumber reaches a value, in the ethylene case higherthan that in the hydrogen case. Contours of Machnumber presented in Fig. 5 show clearly the supersonicregions in the isolator duct and in the expanding duct.Also the figure shows the mixed flow inside theconstant area combustor. The subsonic flow inside thecombustor represents a significant portion of the flow.This region is seen adjacent to the injection walls andextends in the axial direction with the flow till the endof the combustor duct.

Figures 6 and 7 show the axial distribution of theaveraged static temperature and temperatures contoursat different Y-planes. The temperatures are normalizedwith the value at the inlet section. Again, no significantchange is noted in the isolator part. A rapid increase inthe temperatures is noted inside the combustor due to

the chemical reaction. The temperature reaches itsmaximum value near the exit of the combustor then itdecreases very slightly in the expanding duct. Highertemperatures are obtained from the combustion ofethylene. Also, it can be seen that high temperaturesexist in two regions near the injection walls.

Two overall parameters depicting effectiveness offuel combustion over duration, and equivalently, alength of combustor are the heat released and theamount of reference element such as hydrogen that isconverted to water indicating completion ofcombustion of that element. The latter may also be

expressed in terms of the amount of hydrogen thatremains in any form other than that of water at alocation of interest in the combustor in relation to theamount of hydrogen that available initially, i.e.,

supplied hydrogenof amountwater toconverted hydrogenof amount

=C (2)


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Equation (2) implicitly includes the fuel to air ratioused and the effects of mixing of air and fuel ininitially non-premixed cases. At the same time, Eq.(2)does not refer explicitly to the effects of ignition

process or changes in combustion rate. 2 Thecombustion efficiency for the ethylene case iscalculated based on the depletion of the fuel 13 asfollows:

injm

minjm

H C

H C H C c

,

,

42

4242

= (3)

Figure 8 shows the combustion efficiency for thetwo cases, hydrogen combustion and ethylenecombustion. The combustion efficiencies are calculatedwith Eqs.2 and 3. The X-coordinate is normalized withthe isolator height h. The combustion efficiencyshows higher values for the case of hydrogen than thatwith ethylene. Combustion of hydrogen is 58%complete at the end of the combustor duct while, theefficiency is about 40% in the ethylene case. Also, it isfound that the combustion efficiency is slightlyaffected by the flow inside the isolator when comparedwith previous results. The combustion efficiency isaffected mainly by the method of fuel injection and thetype of the fuel. Further increase in the combustionefficiency is seen in the expanding duct.

The benefits obtained in mixing performance must be weighted against the losses incurred. Averaged total pressure is presented in Fig. 9. The pressure isnormalized with the total pressure at the inlet. Thefigure shows the total pressure throughout thecombustor for both fuels. The total pressure loss insidethe isolator is about 10% and 12% of its initial valuefor ethylene and hydrogen respectively. The figureshows clearly that major losses in the total pressure aredue to the chemical reaction which occurred at thecombustor inlet and it is about 48% in both cases at thecombustor. Further slight decrease in the total pressureis noted inside the expanding duct.

In Fig. 10 integrated stream thrust F x is shown for both cases. Thrust is calculated for planes perpendicular to the X-axis as [22]

+= dAu pF x )(2

(4)

Inside the isolator, the thrust force has the same valuein both cases. Slight decrease is seen because offriction. The stream thrust increases just after thecombustor inlet caused by the momentum of the jets. It

is seen that combustion of ethylene causes increase inthe stream thrust inside the combustor and theexpanding duct than that of the hydrogen. It can beseen that the thrust inside the isolator is not affected bythe combustion process or the type of fuel used.

C ONCLUSIONS

A numerical investigation is conducted to study theflowfield characteristics of a dual-mode scramjetcombustor. Two different fuels are used, hydrogen andethylene. The solution is obtained with a finite volumeCFD code and with unstructured grids. Results showno upstream interaction in the isolator since uniform

boundary conditions are used at the inlets. Higherefficiency is obtained with the combustion ofhydrogen. Further work is needed to use profile at theinlets and use a detailed chemistry model for hydrogen.

A CKNOWLEDGEMENT

This work, in part, was supported by NASA LangleyResearch Center through Cooperative Agreement

NCC1-349. The Cooperative Agreement was managedthrough the Institute for Scientific and EducationalTechnology (ISET) of Old Dominion University.

R EFERENCES

[1] Stalker, R. J., Simmons, J. M., Paull, A., andMee, D. J., Measurement of Scramjet Thrust inSock Tunnels, AIAA 18 th Aerospace Ground

Testing Conference, AIAA Paper No. 94-2516,June 1994.[2] Curran, E. T., and Murthy, S. N. B., Scramjet

Propulsion, AIAA Progress in Astronautics andAeronautics, Vol. 189, 2001.

[3] Seiner, J. M., Dash, S. M., and Kenzakowski, D.C., Historical Survey on Enhanced Mixing inScramjet Engines, AIAA Journal of Propulsionand Power, Vol. 17, No. 6, November-December2001, pp. 11273-1286.

[4] Waltrup, P. J., Liquid-Fueled SupersonicCombustion Ramjets: A Research Perspective,

AIAA Journal of Propulsion and Power, Vol. 3, No. 6, Nov.-Dec. 1987, pp. 515-524.[5] Billing, F. S., and Dugger, G. L. The Interaction

of Schock Waves and Heat Addition in theDesign of Supersonic Combustors, Proceedingsof 12 th Symposium on Combustion, CombustionInstitute, Pittsburgh, PA, 1969, pp. 1125-1134.

[6] Billing, F. S., Dugger, G. L., and Waltrup, P. J.,Inlet-Combustor Interface Problems in ScramjetEngines, Proceeding of the 1 st International


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Symposium on Airbreathing Engines, Marseilles,France, June 1972.

[7] Waltrup, P. J., and Billing, F. S., Prediction ofPrecombustion Wall Pressure Distribution inScramjet Engines, Journal of Spacecraft andRockets, Vol. 10, No. 9, 1973, pp. 620-622.

[8] Waltrup, P. J., and Billing, F. S., Structure ofShock Waves in Cylindrical Ducts, AIAAJournal, Vol. 11, No. 9, 1973, pp. 1404-1408.

[9] Billing, F. S., Combustion Processes inSupersonic Flow, Journal of Propulsion andPower, Vol. 4, No.3, May-June 1988, pp. 209-216.

[10] Emami, S., Trexler, C., Auslender, A., andWeidner, J. P., Experimental Investigation ofInlet-combustor Isolator for a Dual- ModeScramjet at a Mach Number of 4, NASATechnical Paper 3502, May 1995.

[11] Eklund, D. R., and Gruber, M. R., Study of aSupersonic Combustor Employing anAerodynamic Ramp Pilot Injector, 35 th AIAAJoint Propulsion Conference, Paper No. 99-2249,June 1999.

[12] Gruber, M., Donbar, J., Jackson, T., Mathur, T.,Eklund, D., Billing, F., Performance of anAerodynamic Ramp Fuel Injector in a ScramjetCombustor, AIAA 36 th Joint PropulsionConference, AIAA Paper No. 2000-3708, July2000.

[13] Eklund, D. R., Baurle, R. A., and Gruber, M. R.,

Numerical Study of a Scramjet CombustorFueled by an Aerodynamic Ramp Injector in aDual-Mode Combustion, 39 th AIAA AerospaceSciences Meeting, Paper No. 2001-0379, January2001.

[14] Kumauro, T., Kudo, K., Masuya, G., Chinzei, N.,Murakami, A., and Tani, K., Experiment on aRectangular Cross Section Scramjet Combustor,(in Japanese), National Aerospace Lab., NALTR-1068, Tokyo, Japan.

[15] Murakami, A., Kumauro, T., and Kudo, K.,Experiment on a Rectangular Cross Section

Scramjet Combustor (II) Effects of Fuel InjectorGeometry, (in Japanese), National AerospaceLab., NAL TR-1220, Tokyo, Japan.

[16] Mizobuchi, M., Matsuo, Y., and Ogawa, S.,Numerical Estimation of TurbulenceTemperature Fluctuation Effect on Hydrogen-Oxygen Reaction Process, 35 th AerospaceSciences Meeting, AIAA Paper No. 97-0910,January 1997.

[17] Riggins, D. The Numerical Investigation of aDual-Mode Scramjet Combustor, JANNAFJoint Meetings, Tucson, AZ, December 10, 1998,

pp. 409-426.[18] Olynciw, M. J., Mohieldin, T. O., McClinton, C.

R., and Tiwari, S. N., Effects of Scaling on Numerical Modeling of Transverse Jet intoSupersonic Cross Flows, AIAA 14 th Computational Fluid Dynamics Conference,Paper No. 99-3368, June 1999.

[19] Rodriguez, C. G., White, J. A., and Riggins, D.W., Three-Dimensional Effects in Modeling ofDual-Mode Scramjets, 36 th AIAA/ASME/SAE/ASEE Joint PropulsionConference and Exhibit, Paper No. 2000-3704,July 2000.

[20] Mohieldin, T. O, Tiwari, S. N., and Olynciw, M.J., Asymmetric Flow-Structures in Dual ModeScramjet Combustor with Significant UpstreamInteraction, 37 th AIAA Joint PropulsionConference, AIAA Paper No. 2001-3296, July2001.

[21] Fluent Version 5 Users Guide, 1999, Fluent Inc., New Hampshire.

[22] Eklund, D. R., Stouffer, S. D., and Northam, G.B, Study of a Supersonic Combustor EmployingSwept Ramp Fuel Injectors, AIAA Journal ofPropulsion and Power, Vol. 13, No. 6,

November-December 1997, pp. 697-704.[23] Abdel-Salam, T. M., Tiwari, S. N., and

Mohieldin, T. O., Three-Dimensional NumericalStudy of a Scramjet Combustor, 40 th AIAAAerospace Sciences Meeting and Exhibit, AIAAPaper No. 2002-0805, January 2002.

Parameter Freestream Injectant

Po [kPa] 1000 664

To [K] 2000 280

M2.5

1.0Turbulent Intensity 1.0% 1.0%

H2 mass fraction 0 1.0

H2O mass fraction 0.17315 0

O2 mass fraction 0.24335 0

N 2 mass fraction 0.5283 0

Table 1: Inlet flow conditions


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Fig. 1 Schematic of the 18-injector dual-mode model

Fig. 2 Details of the 18-injectors combustors
http://arc.aiaa.org/action/showImage?doi=10.2514/6.2003-3550&iName=master.img-001.png&w=351&h=252http://arc.aiaa.org/action/showImage?doi=10.2514/6.2003-3550&iName=master.img-000.png&w=517&h=202


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X/h

M a c h n u m b e r

-5 0 5 10 150

0.5

1

1.5

2

2.5

3

EthyleneHydrogen

X/h

P r e s s u r e

, k P a

-5 0 5 10 15

50

100

150

200

250

300

350

HydrogenEthylene

Fig. 3 Lower wall pressure profile Fig. 4 Axial distribution of mass-weighted Machnumber

Fig. 5 Mach number distribution at two y planes (ethylene)a) Y/h=0 (symmetry plane), b) Y/h=1

Subsonic regions

a) Y/h=0

b) Y/h=1

Sonic lines


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X/h

N o r m a l i z e d t e m p e r a t u r e

, K

-5 0 5 10 150

0.5

1

1.5

2

2.5

3

EthyleneHydrogen

Fig. 6 Axial distribution of mass-weighted static

1600 1800 2000 2100 2500 2600 2700 2900 3000

Fig. 7 Static temperature distribution at different y-planes (ethylene)a) Y/h=0 (symmetry plane), b) Y/h=1, c) Y/h=1.5

a) Y/h=0

b) Y/h=1

c) Y/h=1.5


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X/h

N o r m a l i z e d p r e s s u r e

-5 0 5 10 150

0.5

1

EthyleneHydrogen

X/h

S t r e a m t h r u s t ( N )

-5 0 5 10 15800

850

900

950

1000

1050

1100

1150

1200

1250

1300

1350

1400

1450

1500

HydrogenEthylene

X/h

C o m b u s t i o n e f f i c i e n c y

0 5 10 150

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

HydrogenEthylene

Fig. 8 Axial distribution of the combustion efficiency Fig. 9 Distribution of averaged total pressure

Fig. 10 Distribution of axial thrust


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important for combustion setting properties

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