the role of density gradient in liquid rocket engine combustion instability amardip ghosh aerospace...

The Role of Density Gradient in Liquid Rocket Engine Combustion Instability

Amardip GhoshAerospace Engineering Department

University of MarylandCollege Park, MD 20742

Advisor - Kenneth YuSponsors- NASA CUIP (Claudia Meyer)

NASA/DOD

2Ghosh, 2008 PhD

Liquid Rocket Engine (LRE)

Combustion Chamber

With Shear Coax Shower Head

Shear Coaxial Injector

SSME – LOX / LH2

Arianne 5 – LOX / Kerosene

Soyuz – LOX / Kerosene

3Ghosh, 2008 PhD

Combustion Instability

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 Failures

Stable Combustion Combustion Instability

Onset of Instability

4Ghosh, 2008 PhD

Scope of present work

Correlations Exist

Injector Geometry

Outer Jet Momentum

Outer Jet Temperature

Recess

Hydrocarbon Fuel

Lacking

Physics Based Mechanisms

Predictive Capability

p

p

Recognized as a key element controlling LRE stability margins

Rich Physics

Reacting Interface

Hydrodynamic Instabilities

Kelvin Helmholtz

Rayleigh Taylor

Richtmyer Meshkov

Chamber Acoustics

Baroclinic Interactions

5Ghosh, 2008 PhD

Recent Work

6Ghosh, 2008 PhD

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 ModesChamber AcousticsHeat ReleaseCoupling between two or more of the above

To model the relative importance of various flow-field parameters affecting flame acoustic interaction in LREsFuel-Oxidizer Density RatioFuel-Oxidizer Velocity RatioFuel-Oxidizer Momentum RatioFuel composition

To build experimental database for CFD code validation

7Ghosh, 2008 PhD

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 TubeHotwireHigh Speed Camera

8Ghosh, 2008 PhD

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

9Ghosh, 2008 PhD

Preliminary Flame-Acoustic Interaction Tests

10Ghosh, 2008 PhD

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

11Ghosh, 2008 PhD

Acoustic Characterization using Broadband Forcing

Flow Conditions A B C D

Density Ratio (ρo/ρf) 14.5 11 7 3

Oxygen 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

Fuel 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

12Ghosh, 2008 PhD

Non-reacting Flow Experimental Results

0

0.001

0.002

0.003

0 500 1000 1500 2000 2500 3000 3500 4000

Density Ratio = 14.5Density Ratio = 11Density Ratio = 7

B

Frequency (Hz)

Spe

ctra

l Am

plitu

de (

psi)

f2

f1

f0

f3

0

0.001

0.002

0.003

0 500 1000 1500 2000 2500 3000 3500 4000

Tap #1

Tap #2

Tap #3

Tap #4

B

Frequency (Hz)

Spe

ctra

l Am

plit

ude

(p

si)

f1 f2

f3

Quarter-wave mode of the oxidizer post (longitudinal) Insensitive to the density ratio Insensitive to the sensor

locations

Three-quarter-wave mode of the chamber (longitudinal) Sensitive to the density ratio Relatively insensitive to the

sensor location

Quarter-wave mode of the chamber (transverse) Sensitive to the density ratio Insensitive to the sensor

location

f0

13Ghosh, 2008 PhD

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

Jet-Core Mixing-Length Model Assign two different length scales in the streamwise direction -- incompletely-

mixed 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

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

fo

f

f

o

o

a

WWW

a

W

aW

f2

24

14/

of

of

f

o

o

f

o

f

VV

mm

V

V

m

m

ffooentrained VmVm

DLpm

'

entrainedo

f

o

f

mm

m

o

f

o

f

m

m

o

f

o

f

m

m

Near-Field: Far-Field:

14Ghosh, 2008 PhD

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)

f1

f2

f3

T/4

L/4

O/4

15Ghosh, 2008 PhD

Acoustic Excitation of Density Stratified Non-Reacting Flows

Symbol Frequency(Hz)

f1 234

f2 458

f3 750

f4 1016

f5 1433

f6 1608

f7 2100

f8 2466

16Ghosh, 2008 PhD

Acoustic Excitation of Density Stratified Non-Reacting Flows - Schlieren Results for Helium Jet

Air

6m/s

He (18m/s)He (18m/s)

Phase = 0o 90o 180o 270o

ReAir (Center Jet)~ 7000

Baseline

234 Hz

17Ghosh, 2008 PhD



Phase = 0o 90o 180o 270o

Air

6m/s


400 Hz

625 Hz

18Ghosh, 2008 PhD



Phase = 0o 90o 180o 270o

Air

6m/s


771 Hz

1094 Hz

19Ghosh, 2008 PhD

Hydrodynamic Modes - Hot Wire Experiments

Jet Preferred Mode

U

fDSt

U

fDSt h

meterWettedPeri

AreaDh

4

Wake Mode Frequencies

F1 = 1134 Hz

F2 = 756 Hz

F3 = 378 Hz

Wake Mode Instability

U

fDSt

Jet Preferred Mode Frequencies

20Ghosh, 2008 PhD

Hydrodynamic Modes - Hot Wire Experiments

Air

6m/sHe

18m/s

He

18m/s

Probe

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.

21Ghosh, 2008 PhD

Hydrodynamic Modes– Excitation of Wake Mode


Phase = 0o 90o 180o 270o

Air

6m/s

ReAir (Center Jet)~ 7000429.7 Hz (Wake Mode Excitation)

22Ghosh, 2008 PhD

Reacting Flow Experiments Characteristic Flame-Acoustic Interactions

O2H2 H2

23Ghosh, 2008 PhD

Reacting Flow Experiments Characteristic Flame-Acoustic Interactions

300 Hz

1150 Hz

Phase = 0o 90o 180o 270o

24Ghosh, 2008 PhD

Asymmetric Excitation for the H2-O2-H2 flame Baroclinic Vorticity as a potential mechanism

25Ghosh, 2008 PhD

Effect of Density Gradient Reversal

26Ghosh, 2008 PhD

Effect of Density Ratio Variations

Fix velocity ratio constant at 3 and at stoichiometric H2-O2 ratio

Vary density ratio by mixing inert gas

27Ghosh, 2008 PhD

Effect of Density Ratio Variations Instantaneous OH* Chemiluminescence

(Acoustic Forcing Characteristics Held Constant at 1150Hz;12.5W)

28Ghosh, 2008 PhD

Chapter 5 - Effect of Density Ratio Variations Ensemble Averaged OH* Chemiluminescence

(Acoustic Forcing Characteristics Held Constant at 1150Hz;12.5W)

29Ghosh, 2008 PhD

Measurements of Flame Wrinkling Amplitude

Quantifying the special extent of flame wrinkling from time-averaged OH*-chemiluminescence data

30Ghosh, 2008 PhD

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

31Ghosh, 2008 PhD

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

32Ghosh, 2008 PhD

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

Acoustically Forced Heat Release: 15 kW 6% Dilution by Mole Density Ratio: 7.0 (left) and 15.2 (right)

33Ghosh, 2008 PhD

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

34Ghosh, 2008 PhD

Effect of Jet Momentum VariationsAcoustic Excitation – 1150 Hz, 15.8 Watts

Case 1 Outer Jet Momentum :0.0055 kg.m/s2

Inner Jet Momentum : 0.0047 kg.m/s2

Density Ratio: 8

Case 2 Outer Jet Momentum :0.0055 kg.m/s2

Inner Jet Momentum : 0.0036 kg.m/s2

Density Ratio: 2

35Ghosh, 2008 PhD

Rayleigh Taylor Growth Rate

Rayleigh-Taylor Instability Richtmyer-Meshkov Instability

Richtmyer-Meshkov Instability Sunhara et al. (1996)

Rayleigh-Taylor Instability Youngs (1984)

g

36Ghosh, 2008 PhD

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

37Ghosh, 2008 PhD

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)]

,...),...,,,,,( 1 nfofo YYuuxf

),,,,,( chemfofo uuxf

x

f (o f

,u fuo

, chemx /uo

)

D

f (o f

,u fuo

,YCH 4

YCH 4 YH 2

)

D

f (o f

o f

,u f uou f uo

,YCH 4

YCH 4 YH 2

)

/D (o f ) /(o f )

)/()(

/

ofof uuuu

D

/D YCH 4 /(YCH 4 YH 2)

38Ghosh, 2008 PhD

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

39Ghosh, 2008 PhD

Parametric Studies. Effect of Velocity Ratio.

40Ghosh, 2008 PhD

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

41Ghosh, 2008 PhD

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

42Ghosh, 2008 PhD

Parametric Studies. Effect of Momentum Change

43Ghosh, 2008 PhD


Increase in Outer Jet Momentum

Densities Fixed (Density Ratio ~ 8)

Increase in Fuel Oxidizer Velocity Ratio (3 - 5.3)

Increase in Outer Jet Momentum

Velocities fixed (Velocity Ratio ~ 3)

Decrease in Oxidizer Fuel Density Ratio (6 - 2)

Case A Case B

Jf 2.2 2.6 3.2 4.0 5.5

Dr 8 8 8 8 8

Jf 2.2 2.6 3.2 4.0 5.5

Dr 6 5 4 3 2

44Ghosh, 2008 PhD


Case A Fixed Densities Outer Jet Velocity is Increased

Case B Fixed Velocities Density Ratio is Decreased

45Ghosh, 2008 PhD

Parametric Studies. Effect of Chemical Composition.

46Ghosh, 2008 PhD

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

47Ghosh, 2008 PhD

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.

48Ghosh, 2008 PhD

Chapter 5- Parametric Studies .Dependence of Flame-Acoustic Interactionon Density Ratio, Velocity Ratio, HC Mole Fraction

y = 0.022 exp(5.1 x) y = -3.5 x + 3.6 y = -0.87 x + 2.3

Density ratio Velocity ratio Fuel mixture ratio (methane mole fraction)

49Ghosh, 2008 PhD

Simultaneous Measurement of Pressure and Heat Release Oscillations

Density Ratio = 14.5 Density Ratio = 3

Pressure Oscillation

OH* Oscillation

50Ghosh, 2008 PhD

OH* Chemiluminescence Oscillations

Photomultiplier Measurements Forcing Frequency = 1150 Hz

f = 1150 Hz

51Ghosh, 2008 PhD



f = 1150 Hz

52Ghosh, 2008 PhD



Low Frequency Response

53Ghosh, 2008 PhD



Low Frequency Response

54Ghosh, 2008 PhD

Vortex Pairing and Excitation of Secondary Frequencies

Density Gradient Vorticity Generation at Forcing Frequency

Velocity Gradient Vortex Pairing and Merging Deviation from Forcing Frequency

High-Speed Imaging Results Framing Rate – 1000 fps

Dynamic Interactions Amplification of small disturbance by flame-acoustic coupling

55Ghosh, 2008 PhD

Secondary Evidence of RT instability

RT unstable

56Ghosh, 2008 PhD

Density Tailoring for Reduction of Flame Acoustic Interaction - Possible Control Strategy

57Ghosh, 2008 PhD

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.

58Ghosh, 2008 PhD


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.

59Ghosh, 2008 PhD


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.

60Ghosh, 2008 PhD

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.

the role of density gradient in liquid rocket engine combustion instability amardip ghosh aerospace...

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