Modern tools for thermoacoustic study of annular combustion chambers

1. Application of modern tools for the thermo-acoustic study of annular combustion chambers Franck Nicoud University Montpellier II I3M CNRS UMR 5149 and CERFACS

2. November, 2010 VKI Lecture 2 Introduction A swirler (one per sector)

3. November, 2010 VKI Lecture 3 Introduction O ne swirler per sector 10-24 sectors

4. November, 2010 VKI Lecture 4 Y. Sommerer & M. Boileau CERFACS Introduction

5. November, 2010 VKI Lecture 5 Introduction

6. November, 2010 VKI Lecture 6 SOME KEY INGREDIENTS Flow physics turbulence , partial mixing, chemistry, two-phase flow , combustion modeling , heat loss, wall treatment, radiative transfer, Acoustics complex impedance , mean flow effects, acoustics/flame coupling, non-linearity, limit cycle, non-normality, mode interactions, Numerics Low dispersive low dissipative schemes, non linear stability, scalability, non-linear eigen value problems ,

7. November, 2010 VKI Lecture 7 PARALLEL COMPUTING www.top500.org june 2010 # Site Computer 1 Oak Ridge National Laboratory USA Cray XT5-HE Opteron Six Core 2.6 GHz 2 National Supercomputing Centre in Shenzhen (NSCS) China Dawning TC3600 Blade, Intel X5650, NVidia Tesla C2050 GPU 3 DOE/NNSA/LANL USA BladeCenter QS22/LS21 Cluster, PowerXCell 8i 3.2 Ghz / Opteron DC 1.8 GHz, Voltaire Infiniband 4 National Institute for Computational Sciences USA Cray XT5-HE Opteron Six Core 2.6 GHz 5 Forschungszentrum Juelich (FZJ) Germany Blue Gene/P Solution 6 NASA/Ames Research Center/NAS USA SGI Altix ICE 8200EX/8400EX, Xeon HT QC 3.0/Xeon Westmere 2.93 Ghz, Infiniband 7 National SuperComputer Center in Tianjin/NUDT China NUDT TH-1 Cluster, Xeon E5540/E5450, ATI Radeon HD 4870 2, Infiniband 8 DOE/NNSA/LLNL USA eServer Blue Gene Solution 9 Argonne National Laboratory USA Blue Gene/P Solution 10 Sandia National Laboratories USA Cray XT5-HE Opteron Six Core 2.6 GHz 70 000 processors or more 458 Tflops or more

8. November, 2010 VKI Lecture 8 PARALLEL COMPUTING www.top500.org june 2007 (3 years ago ) # Site Computer 1 DOE/NNSA/LLNL United States BlueGene/L - eServer Blue Gene Solution IBM 2 Oak Ridge National Laboratory United States Jaguar - Cray XT4/XT3 Cray Inc. 3 NNSA/Sandia National Laboratories United States Red Storm - Sandia/ Cray Red Storm, Cray Inc. 4 IBM Thomas J. Watson Research Center United States BGW - eServer Blue Gene Solution IBM 5 Stony Brook/BNL, New York Center for Computional United States New York Blue - eServer Blue Gene Solution IBM 6 DOE/NNSA/LLNL United States ASC Purple - eServer pSeries p5 575 1.9 GHz IBM 7 Rensselaer Polytechnic Institute, Computional Center United States eServer Blue Gene Solution IBM 8 NCSA United States Abe - PowerEdge 1955, 2.33 GHz, Infiniband Dell 9 Barcelona Supercomputing Center Spain MareNostrum - BladeCenter JS21 Cluster, IBM 10 Leibniz Rechenzentrum Germany HLRB-II - Altix 4700 1.6 GHz SGI 10 000 processors or more (factor 7) 56 Tflops or more (factor 8)

9. November, 2010 VKI Lecture 9 PARALLEL COMPUTING Large scale unsteady computations require huge computing resources, an efficient codes processors Speed up

10. November, 2010 VKI Lecture 10 PARALLEL COMPUTING

11. November, 2010 VKI Lecture 11 Thermo-acoustic instabilities The Berkeley backward facing step experiment. The Berkeley backward facing step experiment. Premixed gas

12. November, 2010 VKI Lecture 12 Thermo-acoustic instabilities Self-sustained oscillations arising from the coupling between a source of heat and the acoustic waves of the system Known since a very long time (Rijke, 1859; Rayleigh, 1878) Not fully understood yet but surely not desirable

13. November, 2010 VKI Lecture 13 Better avoid them LPP SNECMA AIR FUEL LPP injector (SNECMA)

14. November, 2010 VKI Lecture 14 Flame/acoustics coupling COMBUSTION ACOUSTICS Modeling problem Wave equation Rayleigh criterion: Flame/acoustics coupling promotes instability if pressure and heat release fluctuations are in phase

15. November, 2010 VKI Lecture 15 A tractable 1D problem BURNT GAS IMPOSED VELOCITY IMPOSED PRESSURE FLAME n , FRESH GAS 0 L L/2 Kaufmann, Nicoud & Poinsot, Comb. Flame , 2002

16. November, 2010 VKI Lecture 16 Equations 0 L CLASSICAL ACOUSTICS 2 wave amplitudes CLASSICAL ACOUSTICS 2 wave amplitudes

17. November, 2010 VKI Lecture 17 Dispersion relation Solve the 4x4 homogeneous linear system to find out the 4 wave amplitudes Consider Fourier modes Condition for non-trivial (zero) solutions to exist Coupled modes Uncoupled modes

18. November, 2010 VKI Lecture 18 Stability of the coupled modes Eigen frequencies Steady flame n=0 : Asymptotic development for n<<1 : Kaufmann, Nicoud & Poinsot, Comb. Flame , 2002

19. November, 2010 VKI Lecture 19 Time lag effect The imaginary part of the frequency is Steady flame modes such that The unsteady HR destabilizes the flame if unstable unstable unstable unstable

20. November, 2010 VKI Lecture 20 Time lag effect The imaginary part of the frequency is Steady flame modes such that The unsteady HR destabilizes the flame if unstable unstable unstable

21. November, 2010 VKI Lecture 21 Numerical example Steady flame STABLE UNSTABLE Real frequency (Hz) Imaginary frequency (Hz) Unteady flame n=0.01 =0.1 ms STABLE UNSTABLE Real frequency (Hz) Imaginary frequency (Hz) Uncoupled modes

22. November, 2010 VKI Lecture 22 OUTLINE 1. Computing the whole flow 2. Computing the fluctuations 3. Boundary conditions 4. Analysis of an annular combustor

23. November, 2010 VKI Lecture 23 OUTLINE 1. Computing the whole flow 2. Computing the fluctuations 3. Boundary conditions 4. Analysis of an annular combustor

24. November, 2010 VKI Lecture 24 BASIC EQUATIONS reacting, multi-species gaseous mixture

25. November, 2010 VKI Lecture 25 BASIC EQUATIONS energy / enthalpy forms Sensible enthalpy of species k Specific enthalpy of species k Sensible enthalpy of the mixture Specific enthalpy of the mixture Total enthalpy of the mixture Total non chemical enthalpy of the mixture E = H p/ Total non chemical energy of the mixture

26. November, 2010 VKI Lecture 26 BASIC EQUATIONS Diffusion velocity / mass flux Practical model Satisfies mass conservation Exact form

27. November, 2010 VKI Lecture 27 BASIC EQUATIONS Stress and heat flux

28. November, 2010 VKI Lecture 28 Turbulence 1. Turbulence is contained in the NS equations 2. The flow regime (laminar vs turbulent) depends on the Reynolds number : Velocity Length scale viscosity Re laminar turbulent

29. November, 2010 VKI Lecture 29 RANS LES - DNS time Streamwise velocity RANS DNS LES

30. November, 2010 VKI Lecture 30 About the RANS approach Averages are not always enough (instabilities, growth rate, vortex shedding) Averages are even not always meaningful T1 T2>T1 T time Oscillating flame T2 T1 Prob(T=T mean )=0 !!

31. The basic idea of LES Modeled scales Resolved scales November, 2010 VKI Lecture 31

32. November, 2010 VKI Lecture 32 LES equations Assumes small commutation errors Filtered version of the flow equations :

33. November, 2010 VKI Lecture 33 Laminar contributions Assumes negligible cross correlation between gradient and diffusion coefficients:

34. November, 2010 VKI Lecture 34 Sub-grid scale contributions Sub-grid scale stress tensor to be modeled Sub-grid scale mass flux to be modeled Sub-grid scale heat flux to be modeled

35. November, 2010 VKI Lecture 35 The Smagorinsky model From dimensional consideration , simply assume: The Smagorinsky constant is fixed so that the proper dissipation rate is produced, C s = 0.18

36. November, 2010 VKI Lecture 36 The Smagorinsky model The sgs dissipation is always positive Very simple to implement , no extra CPU time Any mean gradient induces sub-grid scale activity and dissipation, even in 2D !! Strong limitation due to its lack of universality. Eg.: in a channel flow, Cs=0.1 should be used Solid wall No laminar-to-turbulent transition possible

37. November, 2010 VKI Lecture 37 The Dynamic procedure (constant ) By performing , the following sgs contribution appears Lets apply another filter to these equations By performing , one obtains the following equations From A and B one obtains A B

38. November, 2010 VKI Lecture 38 The dynamic Smagorinsky model Assume the Smagorinsky model is applied twice Assume the same constant can be used and write the Germano identity C s dynamically obtained from the solution itself

39. November, 2010 VKI Lecture 39 The dynamic procedure The dynamic procedure can be applied locally : the constant depends on both space and time good for complex geometries, but requires clipping (no warranty that the constant is positive)

40. How often should we accept to clip ? November, 2010 40 VKI Lecture 1. Example of a simple turbulent channel flow 2. Not very satisfactory , and may degrade the results Y + % clipping Y + U + Local Dynamic Smagorinsky Plane-wise Dynamic Smagorinsky DNS

41. The global dynamic procedure The dynamic procedure can also be applied globally : The constant depends only on time no clipping required, just as good as the static model it is based on Requires an improved time scale estimate November, 2010 41 VKI Lecture

42. Sub-grid scale model 1. Practically, eddy-viscosity models are often preferred 2. The gold standard today is the Dynamic Smagorinsky model 3. Looking for an improved model for the time scale November, 2010 42 VKI Lecture Dynamically Computed From the grid or filter width

43. Null for isotropic Null for axi-symmetic flows Null for 2D or 2C flows Null only if g ij = 0 Description of the - model Eddy-viscosity based: Start to compute the singular values of the velocity gradient tensor (neither difficult nor expensive) AND near-wall behavior is O(y 3 ) !! November, 2010 43 VKI Lecture

44. November, 2010 VKI Lecture 44 Sub-grid scale contributions Sub-grid scale stress tensor Sub-grid scale mass flux of species k and heat flux In practice, constant SGS Schmidt and Prandtl numbers

45. November, 2010 VKI Lecture 45 Sub-grid scale heat release The chemical source terms are highly non-linear (Arrhenius type of terms) The flame thickness is usually very small (0.1 - 1 mm), smaller than the typical grid size Poinsot & Veynante, 2001

46. November, 2010 VKI Lecture 46 The G-equation approach The flame is identified as a given surface of a G field The G-field is smooth and computed from Universal turbulent flame speed model not available Poinsot & Veynante, 2001

47. November, 2010 VKI Lecture 47 The thickened flame approach From laminar premixed flames theory : Multiplying a and dividing A by the same thickening factor F Poinsot & Veynante, 2001

48. November, 2010 VKI Lecture 48 The thickened flame approach The thickened flame propagates at the proper laminar speed but it is less wrinkled than the original flame: Total reaction rate R 1 Total reaction rate R 2 This leads to a decrease of the total consumption:

49. November, 2010 VKI Lecture 49 The thickened flame approach An efficiency function is used to represent the sub-grid scale wrinkling of the thickened flame This leads to a thickened flame (resolvable) with increased velocity (SGS wrinkling) with the proper total rate of consumption The efficiency function is a function of characteristic velocity and length scales ratios

50. November, 2010 VKI Lecture 50 The thickened flame approach Advantages 1. finite rate chemistry (ignition / extinction) 2. fully resolved flame front avoiding numerical problems 3. easily implemented and validated 4. degenerates towards DNS: does laminar flames But the mixing process is not computed accurately outside the reaction zone 1. Extension required for diffusion or partially premixed flames 2. Introduction of a sensor to detect the flame zone and switch the F and E terms off in the non-reacting zones

51. November, 2010 VKI Lecture 51 In the near wall region, the total shear stress is constant. Thus the proper velocity and length scales are based on the wall shear stress w : In the case of attached boundary layers, there is an inertial zone where the following universal velocity law is followed About solid walls y u

52. November, 2010 VKI Lecture 52 A specific wall treatment is required to avoid huge mesh refinement or large errors, Use a coarse grid and the log law to impose the proper fluxes at the wall Wall modeling u v 12 model 32 model y u Exact velocity gradient at wall Velocity gradient at wall assessed from a coarse grid

53. November, 2010 VKI Lecture 53 Close to solid walls, the largest scales are small About solid walls Boundary layer along the x-direction: Vorticity x - steep velocity profile, L t ~ y - Resolution requirement: y + = O(1), z + and x + = O(10) !! - Number of grid points: O(R 2 ) for wall resolved LES z y x u

54. Not even the most energetic scales are resolved when the first off-wall point is in the log layer No reliable model available yet Modeled scales Resolved scales November, 2010 VKI Lecture 54 Wall modeling in LES

55. November, 2010 VKI Lecture 55 OUTLINE 1. Computing the whole flow 2. Computing the fluctuations 3. Boundary conditions 4. Analysis of an annular combustor

56. November, 2010 VKI Lecture 56 Considering only perturbations

57. November, 2010 VKI Lecture 57 Linearized Euler Equations assume homogeneous mixture neglect viscosity decompose each variable into its mean and fluctuation assume small amplitude fluctuations

58. November, 2010 VKI Lecture 58 Linearized Euler Equations the unknown are the small amplitude fluctuations , the mean flow quantities must be provided requires a model for the heat release fluctuation q 1 contain all what is needed, and more : acoustics + vorticity + entropy

59. November, 2010 VKI Lecture 59 Zero Mach number assumption No mean flow or Zero-Mach number assumption Probably well justified below 0.01

60. November, 2010 VKI Lecture 60 Linear equations - The unknowns are the fluctuating quantities - The mean density, temperature, fields must be provided - A model for the unsteady HR q 1 is required to close the system

61. November, 2010 VKI Lecture 61 Flame Transfer Functions Relate the global HR to upstream velocity fluctuations General form in the frequency space n- model (Crocco, 1956) , low-pass filter/saturation (Dowling, 1997) , laminar conic or V-flames (Schuller et al, 2003) , entropy waves (Dowling, 1995; Polifke, 2001) , Justified for acoustically compact flames

62. November, 2010 VKI Lecture 62 Local FTF model Flame not necessarily compact Local FTF model The scalar fields must be defined in order to match the actual flame response LES is the most appropriate tool assessing these fields Two scalar fields

63. VKI Lecture 63 November, 2010 ACOUSTIC VELOCITY AT THE REFERENCE POSITION FLUCTUATING HEAT RELEASE + ACOUSTIC WAVE Giauque et al., AIAA paper 2008-2943 Local FTF model

64. November, 2010 VKI Lecture 64 Back to the linear equations Let us suppose that we have a reasonable model for the flame response The set of linear equations still needs to be solved

65. November, 2010 VKI Lecture 65 Time domain integration Use a finite element mesh of the geometry Prescribe boundary conditions Initialize with random fields Compute its evolution over time This is the usual approach in LES/CFD !

66. November, 2010 VKI Lecture 66 A simple annular combustor (TUM) Pankiewitz and Sattelmayer, J. Eng. Gas Turbines and Power, 2003 Unsteady flame model with characteristic time delay

67. November, 2010 VKI Lecture 67 Example of time domain integration Pankiewitz and Sattelmayer, J. Eng. Gas Turbines and Power, 2003 TIME DELAY LEADS TO STABLE CONDITIONS TIME DELAY LEADS TO UNSTABLE CONDITIONS ORGANIZED ACOUSTIC FIELD

68. November, 2010 VKI Lecture 68 The Helmholtz equation Since periodic fluctuations are expected, lets work in the frequency space From the set of linear equations for 1 , u 1 , p 1 , T 1 , the following wave equation can be derived

69. November, 2010 VKI Lecture 69 3D acoustic codes Let us first consider the simple steady flame case With simple boundary conditions Use the Finite Element framework to handle complex geometries

70. November, 2010 VKI Lecture 70 Discrete problem If m is the number of nodes in the mesh, the unknown is now Applying the FE method , one obtains Linear Eigenvalue Problem of size N

71. November, 2010 VKI Lecture 71 Solving the eigenvalue problem The QR algorithm is the method of choice for small/medium scale problems Shur decomposition: AQ=QT, Q unitary, T upper triangular Krylov-based algorithms are more appropriate when only a few modes needs to be computed Partial Shur decomposition: AQ n =Q n H n +E n with n << m A possible choice: the Arnoldi method implemented in the P- ARPACK library (Lehoucq et al., 1996)

72. November, 2010 VKI Lecture 72 Solving the eigenvalue problem

73. November, 2010 VKI Lecture 73 Computing the TUM annular combustor FE mesh of the plenum + 12 injectors + swirlers + combustor + 12 nozzles Mean temperature field prescribed from experimental observations Experimentally, the first 2 modes are : 150 Hz (1L) 300 Hz (1C plenum) PLENUM COMBUSTOR SWIRLED INJECTORS

74. November, 2010 VKI Lecture 74 TUM combustor: first seven modes f (Hz) type 143 1L 286 1C plenum 503 2C plenum 594 2L 713 3C plenum 754 1C combustor 769 3L

75. November, 2010 VKI Lecture 75 An industrial gas turbine burner Industrial burner (Siemens) mounted on a square cross section combustion chamber Studied experimentally (Schildmacher et al., 2000) , by LES (Selle et al., 2004) , and acoustically (Selle et al., 2005)

76. November, 2010 VKI Lecture 76 Instability in the LES From the LES, an instability develops at 1198 Hz

77. November, 2010 VKI Lecture 77 Acoustic modes f (Hz) type 159 1L 750 2L 1192 1L1T 1192 1L1T 1350 3L 1448 2L1T 1448 2L1T

78. November, 2010 VKI Lecture 78 Turning mode The turning mode observed in LES can be recovered by adding the two 1192 Hz modes with a 90 phase shift LES SOLVER: 1198 Hz ACOUSTIC SOLVER: 1192 Hz PRESSURE FLUCTUATIONS FOR 8 PHASES Selle et al., 2005

79. November, 2010 VKI Lecture 79 Realistic boundary conditions Complex, reduced boundary impedance Reflection coefficient Using the momentum equation, the most general BC is

80. November, 2010 VKI Lecture 80 Discrete problem with realistic BCs In general, the reduced impedance depends on and the discrete EV problem becomes non-linear: Quadratic Eigenvalue Problem of size N Assuming

81. November, 2010 VKI Lecture 81 From quadratic to linear EVP Given a quadratic EVP of size N : Add the variable: Rewrite : Obtain an equivalent Linear EVP of size 2N and use your favorite method !!

82. November, 2010 VKI Lecture 82 Academic validation ACOUSTIC SOLVER EXACT UNSTABLE STABLE UNSTABLE STABLE UNSTABLE STABLE

83. November, 2010 VKI Lecture 83 Multiperforated liners Multiperforated plate (????) Solid plate ( Neumann ) Dilution hole (resolved) Example of a TURBOMECA burner

84. November, 2010 VKI Lecture 84 Multiperforated liners Designed for cooling purpose but has also an acoustic effect. Cold gas (casing) Burnt gas (combustion chamber)

85. November, 2010 85 Multiperforated liners 400 Hz 250 Hz Absorption coefficient

86. November, 2010 86 VKI Lecture The Rayleigh conductivity (Howe 1979) Under the M=0 assumption: (impedance-like condition) Multiperforated liners

87. November, 2010 VKI Lecture 87 Academic validation

88. November, 2010 VKI Lecture 88 Question There are many modes in the low-frequency regime They can be predicted in complex geometries Boundary conditions and multiperforated liners have first order effect and they can be accounted for properly All these modes are potentially dangerous Which of these modes are made unstable by the flame ?

89. November, 2010 VKI Lecture 89 Accounting for the unsteady flame Need to solve the thermo-acoustic problem In discrete form Non-Linear Eigenvalue Problem of size N

90. November, 2010 VKI Lecture 90 Iterative method 1. Solve the Quadratic EVP 2. At iteration k , solve the Quadratic EVP 3. Iterate until convergence is solution of the thermo-acoustic problem

91. November, 2010 VKI Lecture 91 Comparison with analytic results INLET OUTLET n=5 , =0.1 ms exact modes Iteration 0 Iteration 1 Iteration 2 Iteration 3

92. November, 2010 VKI Lecture 92 About the iterative method No general prove of convergence can be given except for academic cases If it converges, the procedure gives the exact solution of the discrete thermo-acoustic problem The number of iterations must be kept small for efficiency (one Quadratic EVP at each step and for each mode) Following our experience , the method does converge in a few iterations in most cases !!

93. November, 2010 VKI Lecture 93 OUTLINE 1. Computing the whole flow 2. Computing the fluctuations 3. Boundary conditions 4. Analysis of an annular combustor

94. November, 2010 VKI Lecture 94 BC essential for thermo-acoustics u=0 p=0 Acoustic analysis of a Turbomeca combustor including the swirler, the casing and the combustion chamber C. Sensiau (CERFACS/UM2) AVSP code

95. LES or Helmholtz solver Boundary Conditions November, 2010 95 VKI Lecture Acoustic boundary conditions

96. Assumption of quasi-steady flow (low frequency) Theories (e.g.: Marble & Candle, 1977; Cumpsty & Marble, 1977; Stow et al., 2002; ) Conservation of Total temperature, Mass flow, Entropy fluctuations M 2 M 1 Nozzle Inlet Nozzle Outlet Analytical approach November, 2010 96 VKI Lecture

97. Mass Momentum Energy 1 2 Analytical approach November, 2010 97 VKI Lecture

98. Analytical approach Compact systems Entropy wave Acoustic wave Acoustic wave November, 2010 98 VKI Lecture

99. Example: Compact nozzle 1. Compact choked nozzle: 2. Compact unchoked nozzle: November, 2010 99 VKI Lecture

100. Non compact elements The proper equations in the acoustic element are the quasi 1D LEE : Due to compressor or turbine November, 2010 100 VKI Lecture

101. Principle of the method 1. The boundary condition is well known at x out (e.g.: p=0) 2. Impose a non zero incoming wave at x in 3. Solve the LEE in the frequency space 4. Compute the outgoing wave at x in 5. Deduce the effective reflection coefficient at x in November, 2010 101 VKI Lecture

102. Example of a ideal compressor Imposed boundary condition p = 0 November, 2010 102 VKI Lecture

103. Example of a ideal compressor Compressor Imposed boundary condition p = 0 November, 2010 103 VKI Lecture

104. Example of a ideal compressor Compact theory November, 2010 104 VKI Lecture

105. Realistic air intake November, 2010 105 VKI Lecture Acoustic modes of the combustion chamber Need to be computed Outlet Choked B.C. ( taken as an acoustic wall ) Intlet B.C

106. Realistic air intake November, 2010 106 VKI Lecture BP Compressor HP Compressor Intake Air-Intake duct Outlet Choked B.C. ( taken as an acoustic wall ) Intlet B.C

107. Realistic air intake November, 2010 107 VKI Lecture BP Compressor HP Compressor Intake

108. Realistic air intake November, 2010 108 VKI Lecture BP Compressor HP Compressor Intake

109. November, 2010 VKI Lecture 109 OUTLINE 1. Computing the whole flow 2. Computing the fluctuations 3. Boundary conditions 4. Analysis of an annular combustor

110. VKI Lecture 110 November, 2010 Helicopter engine 15 burners From experiment: 1A mode may run unstable Overview of the configuration

111. VKI Lecture 111 November, 2010 700 Hz 500 Hz 575 Hz 609 Hz When dealing with actual geometries, defining the computational domain may be an issue Turbomachinery are present upstream/downstream The combustor involves many details : combustion chamber, swirler, casing, primary holes, multi-perforated liner Dass et al., AIAA 2008- 3007 , Combustion Chamber Casing CC + casing + swirler CC + casing + swirler + primary holes About the computational domain Frequency of the first azimuthal mode 1A

112. VKI Lecture 112 November, 2010 The acoustic mode found at 609 Hz has a strong azimuthal component, like the experimentally observed instability Its stability can be assessed by solving the thermo-acoustic problem which includes the flame response In this annular combustor, there are 15 turbulent flames Do they share the same response ? Is this mode stable ?

113. VKI Lecture 113 November, 2010 Large Eddy Simulation of the full annular combustion chamber Staffelbach et al., 2008 The first azimuthal mode is found unstable from LES, at 740 Hz Same mode found unstable experimentally Using the brute force

114. VKI Lecture 114 November, 2010 In annular geometries, two modes may share the same frequency These two modes may be of two types Consider the pressure as a function of the angular position Mode structure A+ A- From simple acoustics:

115. VKI Lecture 115 November, 2010 Turning mode. A+ = 1 & A- = 0 Companion mode turns counter clockwise

116. VKI Lecture 116 November, 2010 Standing mode. A+ = A- = 1 Companion mode has its nodes/antinodes at opposite locations

117. VKI Lecture 117 November, 2010 LES mode. A+ = 1 A- = 0.3 A simple analytical model can explain the global mode shape obtained from LES Can we predict the stability by using the Helmholtz solver ?

118. VKI Lecture 118 November, 2010 Because of the self-sustained instability, the pressure in front of each burner oscillates This causes flow rate oscillations Because the unstable mode is turning, the flow rate through the different burners are not in phase Responses of the burners

119. VKI Lecture 119 November, 2010 Global Response of the flames From the full LES, the global flame transfer of the 15 burners can be computed from the global HR and the flow rate signals For all the 15 sectors , the amplitude of the response is close to 0.8 and the time lag is close to 0.6 ms This result support the ISSAC assumption

120. VKI Lecture 120 November, 2010 As far as the flame response is concerned I ndependent S ector A ssumption for A nnular C ombustor Allows performing a single sector LES with better resolution to obtain more accurate FTF Of course the1A mode cannot appear is such computation FTF deduced from single sector LES pulsated at 600 Hz ISAAC Assumption

121. VKI Lecture 121 November, 2010 Typical field of interaction index Local flame transfer function

122. VKI Lecture 122 November, 2010 Define one point of reference upstream of each of the 15 burners Use the ISAAC Assumption the interaction index , and the time delay are the same in all sectors Extended the local n- model

123. VKI Lecture 123 November, 2010 Assume ISAAC and duplicate the flame transfer function over the whole domain In the same way, duplicate the field of speed of sound Extended the local n- model

124. VKI Lecture 124 November, 2010 The 1A mode from the Helmholtz solver looks like the 1A from the full LES BUT 1. Its frequency remains close to 600 Hz instead of 740 Hz 2. It is found stable ! Stability of the 1A mode Helmholtz LES

125. VKI Lecture 125 November, 2010 The 1A modes from the Helmholtz and LES solvers resemble But a closer look reveals important differences What is observed in the Helmholtz solver 1. Pressure amplitude (slightly) larger upstream of the burners 2. No phase shift between upstream and downstream Shape of the 1A mode Helmholtz solver angle (rad) angle (rad) LES solver

126. VKI Lecture 126 November, 2010 The Helmholtz solver is not as CPU demanding and allows parametric studies The 1A mode is found stable for time delays smaller than T/2 At 600 Hz, this corresponds to < 0.83 ms The time delay of the FTF (0.65ms) is below this critical value Whats wrong ?

127. VKI Lecture 127 November, 2010 Remember the full LES oscillates at 740 Hz This corresponds to a critical value for the time delay of T/2 = 0.67 ms , very close to the prescribed time delay From the LES , the flow rate and HR are indeed in phase opposition Whats wrong ?

128. VKI Lecture 128 November, 2010 Instead of imposing the time delay in the FTF, lets impose the phase shift between flow rate and HR The 1A mode is now found unstable by the Helmholtz solver, consistently with LES and experiment The mode shape is also in better agreement Back to the s tability of the 1A mode Helmholtz solver LES solver

129. November, 2010 VKI Lecture 129 THANK YOU More details, slides, papers, http://www.math.univ-montp2.fr/~nicoud/