The Role of Density Gradient in
Liquid Rocket Engine Combustion Instability
Amardip Ghosh
Aerospace Engineering Department
University of Maryland
College Park, MD 20742
Advisor - Kenneth Yu
Sponsors- NASA CUIP (Claudia Meyer)
NASA/DOD
Liquid Rocket Engine (LRE)
Combustion Chamber
With Shear Coax Shower Head
Shear Coaxial Injector
SSME – LOX / LH2
Arianne 5 – LOX / Kerosene
Soyuz – LOX / Kerosene
Ghosh, 2008 PhD
2
Combustion Instability
Onset of Instability
Stable Combustion
Large
amplitude pressure oscillations (Reardon, 1961)
Increased
heat transfer rates to the combustor walls (Male, 1954)
Increased
mechanical loading on the thrust chamber assembly
Off
Design operation of entire engine
Catastrophic
Ghosh, 2008 PhD
Combustion Instability
Failures
3
Scope of present work
Correlations Exist
 Recognized as a key element
 Injector Geometry
controlling
LRE stability margins
 Outer Jet Momentum
 Outer
Jet Temperature
 Rich
Physics
 Reacting
Recess
Interface
 Hydrodynamic
Hydrocarbon Fuel
Instabilities
p

p

Kelvin Helmholtz
Lacking
Rayleigh Taylor
 Physics
Based Mechanisms
Richtmyer
Meshkov
 Chamber
Predictive Capability
Acoustics
Baroclinic Interactions
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4
Recent Work
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5
Technical objectives
 To better understand the physical mechanisms that play key role
during the onset of combustion instability in liquid rocket engines
(LRE).
What leads pressure perturbations (p’) to couple with heat release oscillations (q’)
 Hydrodynamic Modes
 Jet and Wake Modes
 Chamber Acoustics
 Heat Release
 Coupling between two or more of the above
 To model the relative importance of various flow-field parameters
affecting flame acoustic interaction in LREs
Fuel-Oxidizer Density Ratio
Fuel-Oxidizer Velocity Ratio
Fuel-Oxidizer Momentum Ratio
Fuel composition
 To build experimental database for CFD code validation
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6
Experimental Apparatus and Techniques
Two-Dimensional Slice of Shear-Coax
Injector Configuration
 Turbulent Diffusion Flames




Central O2 Jet
Outer H2 Jet
Inert Wall Jet at Boundary
Transverse Acoustic Forcing
Flow Visualization
 Phase-Locked OH* Chemiluminescence
 Phase-Locked Schlieren/Shadowgraphy
 High Speed Cinematographic Imaging
Measurement Devices
 Static Pressure Sensors (Setra)
 Dynamic Pressure Sensors (Kistler)
 ICCD Camera (DicamPro)
Photomultiplier Tube
Hotwire
High Speed Camera
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Experimental Apparatus and Techniques
Instrumentation
 Signal Generator
 Amplifier
 Oscilloscope
 LabView based VIs
Firing Sequence (Reacting Flow Cases)





H2-O2-H2 tests
O2/N2-H2-O2/N2 test
H2/Ar-O2/He-H2/Ar tests
H2/Ar/He-O2-H2/Ar/He tests
H2/CH4-O2-H2/CH4 tests
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Preliminary Flame-Acoustic Interaction Tests
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Acoustic Characterization using Broadband Forcing
 Acoustically excited response
using band-limited (< 5000Hz)
white noise
 Dynamic pressure
 Spectral analysis using FFT
(400 spectra averaged).
 Non-reacting and reacting
environments.
Tap#
1
2
3
4
x (in)
- 1.625
- 0.500
0.500
1.625
y (in)
0.500
0.500
0.500
0.500
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Acoustic Characterization using Broadband Forcing
Flow Conditions
A
B
C
D
Density Ratio (ρo/ ρf)
14.5
11
7
3
O2 flow rate
(g/s)
1.06
1.06
1.06
1.06
Velocity (m/s)
4.5
4.5
4.5
4.5
Reynolds
number
5500
5500
5500
5500
H2 flowrate (g/s)
0.125
0.104
0.070
0.018
CH4 flowrate
(g/s)
0.015
0.058
0.126
0.231
H2 mole
fraction
99%
94%
82%
37%
CH4 mole
fraction
1%
6%
18%
63%
Velocity (m/s)
13.0
11.3
8.7
4.6
Velocity Ratio (uf/uo)
2.9
2.5
1.9
1.0
Rate of Heat Release (kW)
15.9
15.5
14.9
13.8
Oxygen
Fuel
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Non-reacting Flow Experimental Results
f1
0.002
0.001
f3
f0
0
0
500
Quarter-wave mode of the
chamber (transverse)
 Sensitive to the density ratio
 Insensitive to the sensor
location
1500
2000
2500
Frequency (Hz)
3000
3500
4000
Tap #1
f2
Tap #2
Tap #3
f3
0.002
Tap #4
f0
0.001
0
0
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1000
0.003
f1
B

Three-quarter-wave mode of the
chamber (longitudinal)
 Sensitive to the density ratio
 Relatively insensitive to the
sensor location
Density Ratio = 14.5
Density Ratio = 11
Density Ratio = 7
f2
Spectral Amplitude (psi)

0.003
Spectral Amplitude (psi)
Quarter-wave mode of the
oxidizer post (longitudinal)
 Insensitive to the density ratio
 Insensitive to the sensor
locations
B

500
1000
1500
2000
2500
Frequency (Hz)
3000
3500
4000
12
Modeling Resonance in Variable Density Flowfields
 Complete Reaction Model
 Consider variation in speed of sound through heterogeneous media consisting of
fuel, oxidizer, and equilibrium products
f / 4 
1
 W   W
4 o   2 f
 ao   a f
  W  Wo  2W f

 
ap
 
 f m f

o m o



 
 Jet-Core Mixing-Length Model
 Assign two different length scales in the streamwise direction -- incompletelymixed near-field region defined by jet-core length (Ln~6D) and fully-mixed far-field
region consisting of the equilibrium products
 Near-field mixture fraction determined by velocity ratio
 f m f
 f m f Vo m f m o

Far-Field:


Near-Field:
o

m
 V

m
V V
o
o
o
f
f
o
 Transverse Entrainment Model
 Oxidizer entrainment depends on cross-flow momentum ratio (i.e., ratio between
transverse pressure force and total injection momentum)
 Average mixture fraction depends on the momentum ratio
p' DL
f
f
m
 entrained 
m

 oVo  m
 fV f
m
o m o  m entrained
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Comparison of Isothermal Case Data
 Resonance at f1
 Longitudinal first-quarter wave mode
of the oxidizer post
 Well predicted
 Resonance at f2
 Longitudinal three-quarter wave
mode of the chamber
 Adequately predicted by various
models
 Resonance at f3
 Transverse first-quarter wave mode
of the chamber
 Under-predicted by complete
reaction model (implies the fuel
content is actually higher than the
equilibrium approximation)
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f3
T/4
f2
L/4
f1
O/4
14
Acoustic Excitation of Density Stratified Non-Reacting
Flows
Symbol
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Frequency
(Hz)
f1
234
f2
458
f3
750
f4
1016
f5
1433
f6
1608
f7
2100
f8
2466
15
Acoustic Excitation of Density Stratified Non-Reacting Flows Schlieren Results for Helium Jet
He (18m/s)
Air
He (18m/s)
6m/s
Baseline
ReAir (Center Jet)~ 7000
234 Hz
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Phase = 0o
90o
180o
270o
16
Acoustic Excitation of Density Stratified Non-Reacting Flows Schlieren Results for Helium Jet
He (18m/s)
Air
He (18m/s)
6m/s
400 Hz
ReAir (Center Jet)~ 7000
625 Hz
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Phase = 0o
90o
180o
17
Acoustic Excitation of Density Stratified Non-Reacting Flows Schlieren Results for Helium Jet
He (18m/s)
Air
He (18m/s)
6m/s
771 Hz
ReAir (Center Jet)~ 7000
1094 Hz
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Phase = 0o
270o
90o
180o
18
Hydrodynamic Modes - Hot Wire Experiments
 Jet Preferred Mode
Wake Mode Instability
Wake Mode Frequencies
F1 = 1134 Hz
F2 = 756 Hz
F3 = 378 Hz
Jet Preferred Mode Frequencies
St 
St 
Dh 
fD
U
St 
fD
U
fDh
U
4 Area
WettedPeri meter
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Hydrodynamic Modes - Hot Wire Experiments
Probe
Air
He
18m/s
6m/s
He
18m/s
ReAir (Center Jet)~ 7000
 Low Quality Resonant Response
f1 = 429.7 Hz, f2 = 869.4 Hz,f3=1289.3 Hz
 Forced Response Closely Follows
Natural Response.
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Hydrodynamic Modes– Excitation of Wake Mode
He (18m/s)
Air
He (18m/s)
6m/s
429.7 Hz (Wake Mode Excitation)
Phase = 0o
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90o
180o
ReAir (Center Jet)~ 7000
270o
21
Reacting Flow Experiments
Characteristic Flame-Acoustic Interactions
H2 O H2
2
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Reacting Flow Experiments
Characteristic Flame-Acoustic Interactions
300 Hz
1150 Hz
Phase = 0o
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90o
180o
270o
23
Asymmetric Excitation for the H2-O2-H2 flame
Baroclinic Vorticity as a potential mechanism
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Effect of Density Gradient Reversal
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Effect of Density Ratio Variations
 Fix velocity ratio constant at 3 and at stoichiometric H2-O2 ratio
 Vary density ratio by mixing inert gas

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Effect of Density Ratio Variations
Instantaneous OH* Chemiluminescence
(Acoustic Forcing Characteristics Held Constant at 1150Hz;12.5W)
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Chapter 5 - Effect of Density Ratio Variations
Ensemble Averaged OH* Chemiluminescence
(Acoustic Forcing Characteristics Held Constant at 1150Hz;12.5W)
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Measurements of Flame Wrinkling Amplitude
 Quantifying the special extent of flame wrinkling from time-averaged
OH*-chemiluminescence data
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Effect of Density Gradient on Flame-Acoustic Interaction
 Time-Averaged Measurement of Flame Wrinkling Thickness
 Fixed OH Ratio, Velocity Ratio, Acoustic Forcing Amplitude
 Variable Density by Ar or He Dilution
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Effect of Heat Release Variations
 Use noble gas to dilute
fuel and oxidizer streams
while keeping the
velocities constant
 Gradual change in heat
release with dilution
 O2/He and H2/Ar
combination
 Exponential change in
density ratio
 Ideal for isolating the
density effect
 O2/Ar and H2/He
combination
 Little change in density
ratio
 Ideal for studying the effect
on chemistry
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Effect of Heat Release Variations
Under Constant Forcing, Constant Heat Release, Different Density Ratios
 Unforced
 Heat Release: 15 kW
 6% Dilution by Mole
 Density Ratio: 7.0 or 15.2
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 Acoustically Forced
 Heat Release: 15 kW
 6% Dilution by Mole
 Density Ratio: 7.0 (left) and 15.2 (right)
32
Effect of Jet Momentum Variations
 Use noble gas to dilute
fuel and oxidizer streams
while keeping the
velocities constant
 Exponential change in
Density Ratio with
dilution
 O2/He and H2/Ar
combination
 Exponential change in
density ratio
 Linear increase in outer jet
momentum
 Linear Increase in total jet
momentum
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Effect of Jet Momentum Variations
Acoustic Excitation – 1150 Hz, 15.8 Watts
 Case 1
 Outer Jet Momentum :0.0055
 Inner Jet Momentum : 0.0047 kg.m/s2
 Density Ratio: 8
kg.m/s2
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 Case 2
 Outer Jet Momentum :0.0055 kg.m/s2
 Inner Jet Momentum : 0.0036 kg.m/s2
 Density Ratio: 2
34
Rayleigh Taylor Growth Rate
Richtmyer-Meshkov Instability
Rayleigh-Taylor Instability
g
Rayleigh-Taylor Instability Youngs (1984)
Richtmyer-Meshkov Instability
Sunhara et al. (1996)
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Rayleigh Taylor Growth Rate
 Classical Rayleigh-Taylor mode instability
analysis yields wavelength-dependent
growth rate
 Intermittent fluid acceleration by pressure
waves is used instead of gravitational
acceleration
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Parametric Studies. Dimensional Analysis for the
Shear-Coax Injector Problem
δ(x)=|ro- ri |,
where I(x,r) satisfies
Imax(x)-I(x,ro)=Imax(x)-I(x,ri)=0.9[Imax(x)-Ibackground(x)]
  f ( x, o ,  f , uo , u f , Y1,...Yn ,...)
  f ( x, o ,  f , uo , u f , chem )

x

D


 f(
 f(
o u f  chem
, ,
)
 f uo x /uo
o u f
YCH 4
, ,
)
 f uo YCH 4  YH 2
o   f u f  uo
YCH 4
 f(
,
,
)
D
o   f u f  uo YCH 4  YH 2

 /D
 (o   f ) /(o   f )
Ghosh, 2008 PhD
  / D
 (u f  u o ) /(u f  u o )
 /D
YCH 4 /(YCH 4  YH 2 )
37
Parametric Studies. Effect of Density Ratio
 Time-Averaged Measurement of Flame Wrinkling Thickness
 Fixed OH Ratio, Velocity Ratio, Acoustic Forcing Amplitude
 Variable Density by Ar or He Dilution
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Parametric Studies. Effect of Velocity Ratio.
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Parametric Studies. Effect of Velocity Ratio
 OH* Chemiluminescence Imaging
 Uf/Uo : 3.02, 3.36, 3.64, 4.01,4.51, 5.03, 5.27
 Density Ratio: 8
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Parametric Studies. Effect of Velocity Ratio
 Time-Averaged Measurement of Flame Wrinkling Thickness
 Fixed OH Ratio, Density Ratio, Acoustic Forcing Amplitude
 Variable Velocity Ratio by He Addition to outer Jet
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Parametric Studies. Effect of Momentum Change
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Parametric Studies. Effect of Momentum Change
 Case A
 Case B
Increase in Outer Jet Momentum
Increase in Outer Jet Momentum
Densities Fixed (Density Ratio ~ 8)
Velocities fixed (Velocity Ratio ~ 3)
Increase in Fuel Oxidizer Velocity Ratio (3 - 5.3)
Decrease in Oxidizer Fuel Density Ratio (6 - 2)
Jf
2.2
2.6
3.2
4.0
5.5
Jf
2.2
2.6
3.2
4.0
5.5
Dr
8
8
8
8
8
Dr
6
5
4
3
2
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Parametric Studies. Effect of Momentum Change
 Case A
 Fixed Densities
 Outer Jet Velocity is Increased
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 Case B
 Fixed Velocities
 Density Ratio is Decreased
44
Parametric Studies. Effect of Chemical Composition.
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Parametric Studies. Effect of Chemical Composition
Lifted flame using only methane as fuel (a) OH* average (b) CH* average
(c) OH* instantaneous (d) CH* instantaneous
50% methane and 50% hydrogen flame subjected to acoustic excitation.
(a) OH* average (b) CH* average (c) OH* instantaneous (d) CH* instantaneous
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Parametric Studies. Effect of Chemical Composition.
 Time-Averaged Measurement of Flame Wrinkling Thickness
 Fixed Density Ratio ~ 6
 Fixed Velocity Ratio ~ 3
 Fuel Composition is varied.
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Chapter 5- Parametric Studies .Dependence of Flame-Acoustic Interaction
on Density Ratio, Velocity Ratio, HC Mole Fraction
Density ratio
y = 0.022 exp(5.1 x)
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Velocity ratio
Fuel mixture ratio
y = -3.5 x + 3.6
y = -0.87 x + 2.3
(methane mole fraction)
48
Simultaneous Measurement of Pressure and
Heat Release Oscillations
Pressure Oscillation
Density Ratio = 14.5
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OH* Oscillation
Density Ratio = 3
49
OH* Chemiluminescence Oscillations
 Photomultiplier Measurements
 Forcing Frequency = 1150 Hz
f = 1150 Hz
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OH* Chemiluminescence Oscillations
 Photomultiplier Measurements
 Forcing Frequency = 1150 Hz
f = 1150 Hz
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OH* Chemiluminescence Oscillations
 Photomultiplier Measurements
 Forcing Frequency = 1150 Hz
Low Frequency Response
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OH* Chemiluminescence Oscillations
 Photomultiplier Measurements
 Forcing Frequency = 1150 Hz
Low Frequency Response
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Vortex Pairing and Excitation of Secondary Frequencies
 High-Speed Imaging Results
 Framing Rate – 1000 fps
 Density Gradient
 Vorticity Generation at Forcing Frequency
 Velocity Gradient
 Vortex Pairing and Merging
 Deviation from Forcing Frequency
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 Dynamic Interactions
 Amplification of small disturbance by flameacoustic coupling
54
Secondary Evidence of RT instability
RT unstable
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Density Tailoring for Reduction of Flame Acoustic
Interaction - Possible Control Strategy
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Summary and Conclusions
 Model shear-coaxial injector flames were acoustically forced from
transverse direction to characterize the flame-acoustic interaction
during the onset of combustion instability. Qualitative characterization
of flame response under acoustic excitations revealed :




Flame response depends on frequency and amplitude of forcing
Acoustic Modes Setup in the Combustor
Interactions differ if responding to travelling waves or standing waves
Depends on the nature and orientation of acoustic media in the volume of
interest.
 Density Ratio between fuel and Oxidizer was identified as a critical
parameter affecting flame Acoustic Interactions.
 It was shown that small acoustic disturbances could be amplified by flame-acoustic
coupling, leading to substantial modulation in spatial heat release fluctuation for
flame fronts with large density ratios.
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Summary and Conclusions
 A New Physical Mechanism (Intermittent Baroclinic Vorticity) based on
density ratio between fuel and Oxidizer was identified as a key mechanism
in LRE Combustion Instability.
 This kind of mechanism involving intermittent baroclinic torque arising from the
interactions between misaligned pressure and density gradients has never been
reported in liquid rocket engine instability studies.
 Parametric Studies were conducted. Effects of density ratio, velocity ratio,
and fuel mixture fraction on flame-acoustic interaction were studied by
systematically changing each parameter while holding others constant.
 The amount of flame-acoustic interaction was most sensitive to changes in density
ratio. Similar changes in velocity ratio and fuel mixture ratio produced relatively smaller
effects.
 Density ratio affected flame-acoustic interaction by changing the amplitude of
periodically applied baroclinic torque on the mixture interface. The observed
dependence on density ratio was exponential.
 Increasing the outer jet velocity reduced the amount of interaction almost linearly. This
effect was attributed to the decrease in acoustic energy per mass flow rate.
 Increasing the methane mole fraction also reduced the amount of interaction linearly.
This effect was attributed to the reduction in total heat release rate which affected the
amplification mechanism.
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Summary and Conclusions
 Non-linear response in flame-acoustic interaction.
 Flame forced at 1550 Hz responded not only at 1150 Hz but also at a substantially lower
frequency.
 Model development.
 Well-stirred reactor based Model.
 Jet mixing length based Model.
 Acoustically driven entrainment Model.
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Significance of this Work
 The possible existence of a new mechanism in the initiation of Combustion
instabilities in liquid rocket engines has been identified.
 This kind of mechanism involving intermittent baroclinic torque arising from the
interactions between misaligned pressure and density gradients has never been
reported in liquid rocket engine instability studies.
 Instead of modifying the acoustic boundary conditions to control the amplitude
of acoustic oscillations, new control strategies based on tailoring the density
field inside the combustor can now be attempted.
 Improve the stability margin of the combustor
 Decrease the growth rate of instabilities even when initiated.
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