1. 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

2. 2 Ghosh, 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

3. 3 Ghosh, 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 CombustionCombustion Instability Onset of Instability

4. 4 Ghosh, 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  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

5. 5 Ghosh, 2008 PhD Recent Work

6. 6 Ghosh, 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 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

7. 7 Ghosh, 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 Tube  Hotwire  High Speed Camera

8. 8 Ghosh, 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

9. 9 Ghosh, 2008 PhD Preliminary Flame-Acoustic Interaction Tests

10. 10 Ghosh, 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#1234 x (in)- 1.625- 0.5000.5001.625 y (in)0.500

11. 11 Ghosh, 2008 PhD Acoustic Characterization using Broadband Forcing Flow ConditionsABCD Density Ratio (ρo/ρf)14.51173 OxygenO2 flow rate (g/s) 1.06 Velocity (m/s)4.5 Reynolds number 5500 FuelH2 flowrate (g/s)0.1250.1040.0700.018 CH4 flowrate (g/s) 0.0150.0580.1260.231 H2 mole fraction 99%94%82%37% CH4 mole fraction 1%6%18%63% Velocity (m/s)13.011.38.74.6 Velocity Ratio (uf/uo)2.92.51.91.0 Rate of Heat Release (kW)15.915.514.913.8

12. 12 Ghosh, 2008 PhD Non-reacting Flow Experimental Results f2 f1 f0 f3 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

13. 13 Ghosh, 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 (L n ~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 Near-Field: Far-Field:

14. 14 Ghosh, 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

15. 15 Ghosh, 2008 PhD Acoustic Excitation of Density Stratified Non-Reacting Flows SymbolFrequency (Hz) f1f1 234 f2f2 458 f3f3 750 f4f4 1016 f5f5 1433 f6f6 1608 f7f7 2100 f8f8 2466

16. 16 Ghosh, 2008 PhD Acoustic Excitation of Density Stratified Non-Reacting Flows - Schlieren Results for Helium Jet Air 6m/s He (18m/s) Phase = 0 o 90 o 180 o 270 o Re Air (Center Jet)~ 7000 Baseline 234 Hz

17. 17 Ghosh, 2008 PhD Acoustic Excitation of Density Stratified Non-Reacting Flows - Schlieren Results for Helium Jet He (18m/s) Phase = 0 o 90 o 180 o 270 o Air 6m/s Re Air (Center Jet)~ 7000 400 Hz 625 Hz

18. 18 Ghosh, 2008 PhD Acoustic Excitation of Density Stratified Non-Reacting Flows - Schlieren Results for Helium Jet He (18m/s) Phase = 0 o 90 o 180 o 270 o Air 6m/s Re Air (Center Jet)~ 7000 771 Hz 1094 Hz

19. 19 Ghosh, 2008 PhD Hydrodynamic Modes - Hot Wire Experiments  Jet Preferred Mode Wake Mode Frequencies F1 = 1134 Hz F2 = 756 Hz F3 = 378 Hz  Wake Mode Instability Jet Preferred Mode Frequencies

20. 20 Ghosh, 2008 PhD Hydrodynamic Modes - Hot Wire Experiments Air 6m/s He 18m/s He 18m/s Probe Re Air (Center Jet)~ 7000  Low Quality Resonant Response f 1 = 429.7 Hz, f 2 = 869.4 Hz,f 3 =1289.3 Hz  Forced Response Closely Follows Natural Response.

21. 21 Ghosh, 2008 PhD Hydrodynamic Modes– Excitation of Wake Mode He (18m/s) Phase = 0 o 90 o 180 o 270 o Air 6m/s Re Air (Center Jet)~ 7000 429.7 Hz (Wake Mode Excitation)

22. 22 Ghosh, 2008 PhD Reacting Flow Experiments Characteristic Flame-Acoustic Interactions O2O2 H2H2 H2H2

23. 23 Ghosh, 2008 PhD Reacting Flow Experiments Characteristic Flame-Acoustic Interactions 300 Hz 1150 Hz Phase = 0 o 90 o 180 o 270 o

24. 24 Ghosh, 2008 PhD Asymmetric Excitation for the H2-O2-H2 flame Baroclinic Vorticity as a potential mechanism

25. 25 Ghosh, 2008 PhD Effect of Density Gradient Reversal

26. 26 Ghosh, 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

27. 27 Ghosh, 2008 PhD Effect of Density Ratio Variations Instantaneous OH* Chemiluminescence (Acoustic Forcing Characteristics Held Constant at 1150Hz;12.5W)

28. 28 Ghosh, 2008 PhD Chapter 5 - Effect of Density Ratio Variations Ensemble Averaged OH* Chemiluminescence (Acoustic Forcing Characteristics Held Constant at 1150Hz;12.5W)

29. 29 Ghosh, 2008 PhD Measurements of Flame Wrinkling Amplitude  Quantifying the special extent of flame wrinkling from time-averaged OH*-chemiluminescence data

30. 30 Ghosh, 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

31. 31 Ghosh, 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  O 2 /He and H 2 /Ar combination  Exponential change in density ratio  Ideal for isolating the density effect  O 2 /Ar and H 2 /He combination  Little change in density ratio  Ideal for studying the effect on chemistry

32. 32 Ghosh, 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)

33. 33 Ghosh, 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  O 2 /He and H 2 /Ar combination  Exponential change in density ratio  Linear increase in outer jet momentum  Linear Increase in total jet momentum

34. 34 Ghosh, 2008 PhD Effect of Jet Momentum Variations Acoustic Excitation – 1150 Hz, 15.8 Watts  Case 1  Outer Jet Momentum :0.0055 kg.m/s 2  Inner Jet Momentum : 0.0047 kg.m/s 2  Density Ratio: 8  Case 2  Outer Jet Momentum :0.0055 kg.m/s 2  Inner Jet Momentum : 0.0036 kg.m/s 2  Density Ratio: 2

35. 35 Ghosh, 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

36. 36 Ghosh, 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

37. 37 Ghosh, 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)]

38. 38 Ghosh, 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

39. 39 Ghosh, 2008 PhD Parametric Studies. Effect of Velocity Ratio.

40. 40 Ghosh, 2008 PhD Parametric Studies. Effect of Velocity Ratio  OH* Chemiluminescence Imaging  U f /U o : 3.02, 3.36, 3.64, 4.01,4.51, 5.03, 5.27  Density Ratio: 8

41. 41 Ghosh, 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

42. 42 Ghosh, 2008 PhD Parametric Studies. Effect of Momentum Change

43. 43 Ghosh, 2008 PhD Parametric Studies. Effect of Momentum Change 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 JfJf 2.22.63.24.05.5 Dr88888 JfJf 2.22.63.24.05.5 Dr65432

44. 44 Ghosh, 2008 PhD Parametric Studies. Effect of Momentum Change  Case A  Fixed Densities  Outer Jet Velocity is Increased  Case B  Fixed Velocities  Density Ratio is Decreased

45. 45 Ghosh, 2008 PhD Parametric Studies. Effect of Chemical Composition.

46. 46 Ghosh, 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

47. 47 Ghosh, 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.

48. 48 Ghosh, 2008 PhD Chapter 5- Parametric Studies.Dependence of Flame-Acoustic Interaction on Density Ratio, Velocity Ratio, HC Mole Fraction y = 0.022 exp(5.1 x) y = -3.5 x + 3.6y = -0.87 x + 2.3 Density ratio Velocity ratio Fuel mixture ratio (methane mole fraction)

49. 49 Ghosh, 2008 PhD Simultaneous Measurement of Pressure and Heat Release Oscillations Density Ratio = 14.5 Density Ratio = 3 Pressure Oscillation OH* Oscillation

50. 50 Ghosh, 2008 PhD OH* Chemiluminescence Oscillations  Photomultiplier Measurements  Forcing Frequency = 1150 Hz f = 1150 Hz

51. 51 Ghosh, 2008 PhD OH* Chemiluminescence Oscillations  Photomultiplier Measurements  Forcing Frequency = 1150 Hz f = 1150 Hz

52. 52 Ghosh, 2008 PhD OH* Chemiluminescence Oscillations  Photomultiplier Measurements  Forcing Frequency = 1150 Hz Low Frequency Response

53. 53 Ghosh, 2008 PhD OH* Chemiluminescence Oscillations  Photomultiplier Measurements  Forcing Frequency = 1150 Hz Low Frequency Response

54. 54 Ghosh, 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

55. 55 Ghosh, 2008 PhD Secondary Evidence of RT instability RT unstable

56. 56 Ghosh, 2008 PhD Density Tailoring for Reduction of Flame Acoustic Interaction - Possible Control Strategy

57. 57 Ghosh, 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.

58. 58 Ghosh, 2008 PhD 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.

59. 59 Ghosh, 2008 PhD 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.

60. 60 Ghosh, 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.