Jeong-Yeol Choi, Fuhua Ma and Vigor Yang- Dynamics Combustion Characteristics in Scramjet Combustors with Transverse Fuel Injection

8/3/2019 Jeong-Yeol Choi, Fuhua Ma and Vigor Yang- Dynamics Combustion Characteristics in Scramjet Combustors with Tran

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41st AIAA/ASME/SAE/ASEE Joint Propulsion

Conference & Exhibit 10 - 13 July 2005, Tucson, Arizona

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American Institute of Aeronautics and Astronautics

Dynamics Combustion Characteristics in ScramjetCombustors with Transverse Fuel Injection

Jeong-Yeol Choi*

Pusan National University, Pusan 609-735, Korea

and

Fuhua Ma, Vigor Yang

The Pennsylvania State University, University Park, PA 16802, U.S.A.

*Corresponding Author, Associate Professor, Department of Aerospace Engineering, [email protected]

Post-doctoral Research Fellow, Department of Mechanical Engineering, [email protected] Distinguished Professor, Department of Mechanical Engineering, [email protected]

Abstract

A comprehensive DES quality numerical analysis hasbeen carried out for reacting flows in constant-area and

divergent scramjet combustor configurations with and

without a cavity. Transverse injection of hydrogen is

considered over a broad range of injection pressure.

The corresponding equivalence ratio of the overall

fuel/air mixture ranges from 0.167 to 0.50. The work

features detailed resolution of the flow and flame

dynamics in the combustor, which was not typically

available in most of the previous studies. In particular,

the oscillatory flow characteristics are captured at a

scale sufficient to identify the underlying physical

mechanisms. Much of the flow unsteadiness is related

not only to the cavity, but also to the intrinsicunsteadiness in the flowfield. The interactions

between the unsteady flow and flame evolution may

cause a large excursion of flow oscillation. The roles

of the cavity, injection pressure, and heat release in

determining the flow dynamics are examined

systematically.

Introduction

The success of future high-speed air transportation will

be strongly dependent on the development of

hypersonic air-breathing propulsion engines.Although there exist many fundamental issues,

combustor represents one of the core technologies that

dictate the development of hypersonic propulsion

systems. At a hypersonic flight speed, the flow

entering the combustor should be maintained

supersonic to avoid the excessive heating and

dissociation of air. The residence time of the air in a

hypersonic engine is on the order of 1 ms for typical

flight conditions. The fuel must be injected, mixed

with air, and burned completely within such a short

time span.A number of studies have been carried out

worldwide and various concepts have been suggested

for scramjet combustor configurations to overcome the

limitations given by the short flow residence time.

Among the various injection schemes, transverse fuel

injection into a channel type of combustor appears to

be the simplest and has been used in several engine

programs, such as the Hyshot scramjet engine, an

international program leaded by the University of

Queensland [1]. For the enhancement of fuel/air

mixing and flame-holding, a cavity is often employed.

For example, the CIAM of Russia introduced cavities

into its engines [2] and U.S. Air Force also employedcavities in the supersonic combustion experiments [3].

From the aspect of fluid dynamics, transverse

injection of fluid into a supersonic cross flow and flow

unsteadiness associated with a cavity are of significant

interest topics due to their broad applications in many

engineering devices. Extensive efforts have been

applied to study these phenomena, and much of the

results have great relevance relevant to scramjet

combustors. Papamoschou and Hubbard[4] observed

the fluid dynamic instability of injector flow. Ben-

Yakar et al.[5] also observed essentially the same

unstable injection jet in their supersonic combustion

experiment and the showed that supersonic combustionis overlaid with the large eddy motions of the unstable

injection jet. The unsteady nature of transverse injector

flow has been first studied numerically by von Lavante

et al.[6] but its physical nature has been discussed less

importantly. A comprehensive study directly applied to

combustor dynamics, however, is rarely found. The

obstacles lie in the difficulties in conducting high-

fidelity experiments and numerical simulations to

AIAA 2005-4428


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characterize the flow transients at time and length

scales sufficient to resolve the underlying mechanisms.

Choi et al.[7] has studies a two-dimensional

scramjet combustor configuration with transverse fuelinjection and a cavity flame holder with highly refined

computations. They found that RichtmyerMeshkov

shear layer instability or cavity-driven instability, those

are unavoidable in supersonic combustor,[8,9] triggers

the injector flow instability, and the disturbed injector

flow greatly enhanced the fuel/air mixing and

combustion. However, stabilized combustion has not

been observed and overall combustion characteristics

have not been justified. It is because of the constant-

area combustor configuration and ambiguously defined

combustor inlet. Thus, the present study attempts to

achieve improved understanding of the unsteady flow

and flame dynamics in a realistic scramjet combustorconfiguration employing a modified combustor

configuration and Reynolds averaging of the flow field.

Theoretical Formulation and Numerical Treatment

Governing Equations

The flowfield is assumed to be two-dimensional for

computational efficiency, and can be described with the

conservation equations for a multi-component

chemically reactive system. The coupled form of the

species conservation, fluid dynamics, and turbulent

transport equations can be summarized in aconservative vector form as follows.

WGFGFQ vv +

+

=

+

+

yxyxt (1)

where the conservative variable vector, Q, convective

flux vectors, F and G, diffusion flux vectors, Fv and Gv,

and reaction source term W are defined in Eq. (2).

Details of the governing equations and thermo-physical

properties are described in Ref. 10.

2

2

+

=

+

=

=

v

kv

Hv

pv

uv

v

u

ku

Hu

uv

pu

u

k

e

v

u

iii

GFQ (2a)

=

=

=

s

s

w

y

yk

u

x

xk

u

k

i

k

y

yy

xy

d

ii

k

x

xy

xx

d

ii

0

0

0

WGF vv(2b)

where, i=1~N.

Numerical Methods

The governing equations were treated numericallyusing a finite volume approach. The convective fluxes

were formulated using Roe's FDS method derived for

multi-species reactive flows along with the MUSCL

approach utilizing a differentiable limiter function.

The spatial discretization strategy satisfies the TVD

conditions and features a high-resolution shock

capturing capability. The discretized equations were

temporally integrated using a second-order accurate

fully implicit method. A Newton sub-iteration method

was also used to preserve the time accuracy and

solution stability. Detailed descriptions of the

governing equations and numerical formulation are

documented in a previous work [11].

Chemistry Model and Turbulence Closure

The present analysis employs the GRI-Mech 3.0

chemical kinetics mechanism for hydrogen-air

combustion [12]. The mechanism consists of eight

reactive species (H, H2, O, O2, H2O, OH, H2O2 and

HO2) and twenty-five reaction steps. Nitrogen is

assumed as an inert gas because its oxidation process

only has a minor effect on the flame evolution in a

combustor. Turbulence closure is achieved by means

of Mentor's SST (Shear Stress Transport) model

derived from the k- two-equation formulation [13].

This model is the blending of the standard k- modelthat is suitable for a shear layer problem and the

Wilcox k- model that is suitable for wall turbulence

effect [14]. Baridna et al.[15] reported that the SST

model offers good prediction for mixing layers and jet

flows, and is less sensitive to initial values in numerical

simulations.

Code Verification

The overall approach has been validated against a

number of steady and unsteady flow problems

including shock-induced combustion oscillation. Good

agreement has been obtained with experimental data

[11,16,17]. In addition, numerical study was carried

out to validate the present turbulence modeling and to

justify the grid resolution for simulating transverse gas

injection across the supersonic flow over a flat plate.

The analysis simulates the experiment described in Ref.

[18] with a static pressure ratio of 10.29, for which

several numerical studies have been previously carried

out [19, 20]. In this case, choked nitrogen flow is

vertically injected through a 1-mm-wide slot locating

33 cm behind the leading edge into a supersonic

airflow with a Mach number of 3.75. The present

study used the same computational domain as that of


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Chenault and Beran [20]. Computations were carried

out for various combinations of grid systems having 71

to 351 points near the injection port in the streamwise

direction and 41 to 251 points clustered near the wall inthe transverse direction. Furthermore, a parametric

study was performed on the effects of numerical and

turbulence modeling parameters. The numerical

parameters were optimized to maintain numerical

stability and solution convergence. The turbulence

parameters of the SST model [13] have negligible

effects on the solutions for the grid systems employed

herein.

x/L

pw

/p

0.8 0.9 1.0 1.1 1.20.0

1.0

2.0

3.0

4.0Aso et. al., Experiment(1991)

Rizzetta (1992)

Chenault & Beran (1998)

141x101

211x151

281x201351x251

Fig. 1 Wall pressure distribution of the two-

dimensional transverse injection across the

supersonic flow over a flat plate.

Figure 1 compares the wall-pressure distributions

between the numerical and experimental results. A

coarse grid results in a longer separation distance ahead

of the injection port and cannot predict the pressure

picks near the injection port, although the solution

seems to better match the experimental result. The

281201 and 351251 grids have nearly identicalresults within 5% relative error range over the entire

wall. In comparison with previous results, the present

turbulence model predicts the same separation distance

and peak pressure as the k- model while maintaining

smooth pressure increase in the front separation region.Also, pressure variation behind the injector is more

closely predicted by the SST model. The 281201grid was then applied to the scramjet combustor

simulation. The minimum vertical spacing is y+ 5,and 21 grid points are employed in the injector port.

Issues on Turbulence and Chemistry

Present computational formulation constitutes an

unsteady Reynolds averaged Navier-Stokes analysis

(URANS), but showed highly refined combustion

characteristics overlaid with large eddy motions of

unstable injector flow, using a massive computational

grid for two-dimension.[7] These kinds of results were

comparable to the quality of DES (Detached Eddy

simulation), a seamless hybrid approach of LES andRANS, easily attainable by present SST turbulence

model by using local grid size instead of wall distance.

Another important issue is the closure problems for

the interaction of turbulence and chemistry in

supersonic conditions. Recently, there were many

attempts to address this issue using LES methods, PDF

approaches, and other combustion models extended

from subsonic combustion conditions. Although

much useful advances were achieved, the improvement

was insignificant in comparison with the results

obtained from laminar chemistry and experimental data,

as discussed by Mbus et al [21]. A careful review of

existing results, such as Norris and Edwards [22]suggests that the solution accuracy seems to be more

dependent on grid resolution than the modeling of

turbulence-chemistry interaction. In view of the lack of

reliable models for turbulence-chemistry interactions,

especially for supersonic flows, the effect of turbulence

on chemical reaction rate is ignored in the present work.

Highly refined turbulent flows were post-processed

by two kinds of time averaging techniques, Reynolds

average and discrete time average. Reynolds average is

a continuous time average of the flow variables at a

given location and its visualization result is equivalent

to the image taken with long-time exposure. The

discrete time average is an average of severalintermediate results within a given time frame, and its

image is equivalent to the averaged image of motion

pictures. Presently the continuous average was taken

for final 20,000 iterations of computation equivalent to

final 1.2 ms physical time period, and discrete average

was taken from 20 intermediate results among the same

time period.

Configuration of Scramjet Combustor

Combustor Configuration

The supersonic combustor considered in this study

is shown in Fig. 1. The channel type combustor of 10

cm height and 131 cm length is composed of transverse

fuel injection and a cavity. This combustor

configuration is quite similar to the Hyshot test model,

except for the cavity, in which a swallowing slot is

employed to remove the boundary layer from the inlet

and the combustor starts with a sharp nose [1]. A cavity

of 20 cm length and 5cm depth, having an aspect ratio,

L/D of 4.0, is employed at 20 cm downstream of the

injector. In the present computation, intake region was

also considered for the physical assessment of the

thermal choking condition,


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Operating Conditions

The incoming air flow to the combustor is set to

Mach number 3 at 600 K and 1.0 MPa. This combustor

inlet condition roughly corresponds to a flight Mach

number 5-6 at an altitude of 20 km, although the exact

condition depends on the inlet configuration. Gaseoushydrogen is injected vertically through a slot of 0.1 cm

width to the combustor through a choked nozzle. The

fuel temperature is set to 151 K. The injector exit

pressures are 0.5, 0.75, 1.0 and 1.5 Mpa, and the

overall equivalence ratios are from 0.167 to 0.5.

Combustor Conditions

A total of 800160 grids are used for the main-combustor flow passage, 5050 for inlet cowl and159161 grids for the cavity. The grids are clusteredaround the injector and the solid surfaces and the

injector. 54 grid points are included in the injector slot

and the minimum grid size near the wall is 70 m. All

the solid surfaces are assumed to be no-slip and

adiabatic, except for the upper boundary. For

convenience and reduction of the number of grid points

required to resolve the boundary layer, the upper

boundary is assumed to be a slip wall, which is

equivalent to the flow symmetric condition in the

present configuration. Extrapolation is used for the exit

boundary. Time step is set to 6 ns according to the

minimum grid size and the CFL number of 2.0. Four

sub-iterations are used at each time step. Fig. 2 is a

magnified plot of the computational grid around the

injector, cavity and the fore part of the combustor.

Summary of Results

Numerical simulations were carried out for 32 cases

of different configurations and fuel injection pressure

ratios: 1) divergent nozzle-type combustor and constant

area combustor, 2) with and without cavity, 3) reactingand non-reacting flows and 4) fuel injection pressure

ratios of 5.0, 7.5, 10.0 and 15.0. All the cases were run

for 12 ms starting from the initial condition, which is

longer than the typical test time of the ground based

experiments. The plots of the instantaneous flowfields

shown in the followings were taken at 12 ms, and

averaged images were taken from next 1.2 ms.

Figure 4 to 7 shows the instantaneous and averaged

results of reacting flows for different configurations

with dependency on fuel injection pressure. The images

are overlaid temperature and pressure contours to

account for the combustion and shock structures. Black

curve is the sonic line to show the flow choking. Fig. 8is the pressure histories of each case at reference

pressure probing point at x=59 cm along the bottom

wall, and Fig. 9 is its frequency spectrum obtained by

FFT (fast Fourier transform) analysis. Only 4 to 12 ms

time period was considered in FFT analysis to exculde

the transient effects at initial phase. In the following

only the overall characteristics will be discussed in this

paper since the detailed feature of unsteady combustion

was discussed in the previous paper.

In Fig. 4, the computational results for divergent

nozzle configuration without cavity, the flow is nearly

undisturbed and close to steady state. It is though that

the present grid resolution is not enough to capture theflow instability and eddy motions for this flow

conditions. It is also considered that RANS may not

sufficient to account for the supersonic fuel-air mixing.

The eddy motions are captured for large injection

pressure of 10.0 and 15.0. Averaged results for these

cases are quite different to that of undisturbed low

injection pressure cases by showing greatly enhanced

fuel-air mixing and combustion.

The averaged field shows that most of the

combustor is maintained at supersonic condition, and

there is a significant delay in active combustion. The

active combustion begins roughly at 80 cm from cowl

Fig. 2 Scramjet combustor configuration

H2 10cm14cm5cm20cm

20cm131cm

no-slip adiabatic

sli wall

supersonic exit

x = 59cm, reference pressure probing point

Air

Fig. 2 Magnified plot of computational grid around

the injector and the fore part of combustor.

H2


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(a) Instantaneous Field

(b) Averaged Field of Discrete Frames

(c) Continuously Averaged Field

Fig. 4 Nozzle configuration without cavity




Fig. 5 Nozzle configuration with cavity


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Fig. 6 Channel configuration without cavity




Fig. 7 Channel configuration with cavity


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Fi . 8 Bottom wall ressure histories at x=59cm

Fig. 9 Frequency spectrum of bottom wall pressure at x=59cm


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lip for injection pressure ratio of 10.0 and 50 cm for

injection pressure ratio of 15.0. Both cases showed the

unsteady but stabilized combustion. However, pressure

build-up for the case of 10.0 was insignificant butpressure at reference probing point was maintained at

about 0.3MPa for the case of 15.0. It is considered

combustion efficiency would be low for these cases,

especially for the case of 10.0.

In Fig. 5, flow is disturbed for all injection pressure

ratios due to the presence of cavity, and pressure

histories in Fig. 8 also show higher levels than the

cases of same flow conditions without cavity. Thus

combustion is considered to be enhanced greatly.

Especially at pressure ratio 15.0, the active combustion

region is anchored over the cavity where flow is

choked locally.

Fig. 6 and 7 show the results of the constant areachannel type combustors with and without cavity.

Except for the low injection pressure ratio case of 5.0,

where unsteady but stabilized combustion is observed,

pressure is continuously build-up due to the heat

addition in constant area combustor. Although there is

a certain amount of time delay, the cases of injection

pressure ratio of 7.5 also showed the pressure build-up.

Therefore, most of the cases go eventually to thermal

choking condition, though there are time differences.

For the cases of injection pressure of 10.0, detached

bow shocks are shown ahead of the cowl lip, and the

bow shock ran against the inflow boundary.

The role of cavity, enhancing the combustion, isalso observed in these results. For low injection

pressure conditions, where combustion is stabilized, the

pressure level is higher for the case with cavity.

However, the cavity makes little differences for the

higher injection pressure condition except the time

difference in thermal choking.

To account for the turbulence frequencies that

dominate the unsteady combustion phenomena, FFT

analysis is carried out and plotted in Fig. 9. For the

cases of divergent nozzle type combustor, two

dominant frequencies are observed at around 2 kHz and

10 kHz for the cases without cavity, and one more

dominant frequency at 5 kHz for the cases with cavity.

The frequency of 5 kHz is considered as the second

mode of cavity oscillation, as predicted by Rossiters

semi empirical formula in the previous works. For the

cases of constant area combustor, very low frequencies

are dominant. Major oscillation frequencies are spread

over 2 to 10 kHz bad, but they are mixed-up and are

not easily identified, due to its transient nature of

continuous pressure build-up.

Conclusion

The reacting flow dynamics in a scramjet combustor

was studied by means of a comprehensive numerical

analysis. The present results show a wide range of

phenomena resulting from the combustor area effects,

the interactions among the injector flows, shock waves,shear layers, and oscillating cavity flows. The overall

results were also discussed by means of time averaging

the unsteady flow fields. Further analyses are expected

from these results to clarify and quantify the overall

combustion characteristics.

Acknowledgements

For the present study, the first author was

sponsored by Agency for Defense Development and

National Research Laboratory program of Korea

Science and Engineering Foundation. The supports areacknowledged greatly.

References

[1] Centre for Hypersonics - HyShot Scramjet TestProgramme,

http://www.mech.uq.edu.au/hyper/hyshot/

[2] McClinton, C., Roudakov, A., Semenov, V. and V.Kopehenov, Comparative flow path analysis and

design assessment of an axisymmetric hydrogen

fueled scramjet flight test engine at a Mach number

of 6.5, AIAA Paper 96-4571, VA, Nov. 1996[3] Mathur, T., Gruber, M. Jackson, K., Donbar, J.,Donaldson, W., Jackson, T and Billig, F.,

Supersonic Combustion Experiments with a

Cavity-Based Fuel Injector, Journal of

Propulsion and Power, Vol.17, No.6, 2001,

pp.1305-1312.

[4] Papamoschou, D., and Hubbard, D.G., "VisualObservations of Supersonic Transverse Jets,"

Experiments in Fluids, Vol. 14, May 1993, pp.

468-471.,

http://supersonic.eng.uci.edu/scramjet.htm

[5] Ben-Yakar, A., Kamel, M .R., Morris, C. I. andHanson, R. K., "Experimental Investigation of H

2

Transverse Jet Combustion in Hypervelocity

Flows," AIAA Paper 97-3019, 1997.

[6] Von Lavante, E., Zeitz, D. And Kallenberg, M.,Numerical Simulation of Supersonic Airflow with

Transverse Hydrogen Injection, Journal of

Propulsion and Power, Vol.17 No.6, 2001,

pp.1319-1326.

[7] Choi, J.-Y., Yang, V. and Ma., F., "CombustionOscillations in a Scramjet Engine Combustor with

Transverse Fuel Injection," Proceedings of the

Combustion Institute, Vol. 30/2, Dec. 2004, pp.

2851-2858, or AIAA Paper 2003-4515.


9/9

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[8] Papamoschou, D., and Roshko, A., "The TurbulentCompressible Shear Layer: An Experimental

Study," Journal of Fluid Mechanics, Vol. 197,

1988, pp. 453-477.[9] Ben-Yakar, A. and Hanson, R. K., "Cavity Flame-Holders for Ignition and Flame Stabilization in

Scramjets: An Overview," Journal of Propulsion

and Power, Vol. 17, No. 4, 2001, pp.869-877.

[10]Choi, J. Y., Jeung, I. S. and Yoon, Y., NumericalStudy of SCRam-Accelerator Starting

Characteristics, AIAA Journal, Vol. 36, No. 6,

1998, pp. 1029-1038.

[11]Choi, J.-Y., Jeung, I.-S. and Yoon, Y.,"Computational Fluid Dynamics Algorithms for

Unsteady Shock-Induced Combustion, Part 1:

Validation," AIAA Journal, Vol. 38, No. 7, July

2000, pp.1179-1187.[12]Smith, G. P., Golden, D. M., Frenklach, M.,Moriarty, N. W., Eiteneer, B., Goldenberg, M.,

Bowman, C.T., Hanson, R.K., Song, S., Gardiner

Jr., W.C., Lissianski, V.V., and Qin, Z., GRI-

Mech, http://www.me.berkeley.edu/gri_mech/

[13]Menter, F. R., Two-Equation Eddy-ViscosityTurbulence Models for Engineering Application,

AIAA Journal, Vol. 32, No. 8, 1994, pp.1598-

1605.

[14]Wilcox, D. C., Turbulence Modeling for CFD,DCW Industries, La Caada, CA, 1993.

[15]Bardina, J. E., Huang, P. G., and Coakly, T. J.,Turbulence Modeling Validation, AIAA 97-

2121, 1997.

[16]Choi, J.-Y., Jeung, I.-S. and Yoon, Y., "Unsteady-State Simulation of Model Ram Accelerator in

Expansion Tube," AIAA Journal, Vol. 37, No. 5,1999, pp.537-543.

[17]Choi, J.-Y., Jeung, I.-S. and Yoon, Y., "ScalingEffect of the Combustion Induced by Shock Wave/

Boundary Layer in Premixed Gas,"Proceedings of

the Combustion Institute, Vol. 27, 1998, pp. 2181-

2188.

[18]Aso, S., Okuyama, K., Kawai, S. and Ando, Y.,Experimental Study on Mixing Phenomena in

Supersonic Flows with Slot Injection, AIAA

Paper 1991-0016, 1991.

[19]Rizzetta, D., Numerical Simulation of SlotInjection into a Turbulent Supersonic Stream,

AIAA Paper 1992-0827, 1992.[20]Chenault, C.F. and Beran, P.S., Kand ReynoldsStress Turbulence Model Comparisons for Two-

Dimensional Injection Flows, AIAA Journal.

Vol.36, No. 8, 1998, pp.1401-1412.

[21]Mbus, M., Gerlinger, P. and Brggermann,Scalar and Joint scalar-Velocity-Frequency

Monte Carlo PDF simulation of Supersonic

Combustion, Combustion and Flame, Vol. 132,

2003, pp.3-24.

[22]Norris, J. W. and Edwards, J. R., Large-EddySimulation of High-Speed Turbulent Diffusion

Flames with Detailed Chemistry, AIAA Paper 97-

0370, 1997.

Jeong-Yeol Choi, Fuhua Ma and Vigor Yang- Dynamics Combustion Characteristics in Scramjet Combustors with Transverse Fuel Injection

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