Analysis and Modelling of Entropy Modes in a Realistic Aeronautical Gas Turbine

A combustion instability in a combustor typical of aero-engines is analyzed and modeled thanks to a low order Helmholtz solver. A Dynamic Mode Decomposition (DMD) is first applied to the Large Eddy Simulation (LES) database. The mode with the highest amplitude shares the same frequency of oscillation as the experiment (approx. 350 Hz) and it shows the presence of large entropy spots generated within the combustion chamber and convected down to the exit nozzle. The lowest purely acoustic mode being in the range 650–700 Hz, it is postulated that the instability observed around 350 Hz stems from a mixed entropy/acoustic mode where the acoustic generation associated with the entropy spots being convected throughout the choked nozzle plays a key role. A Delayed Entropy Coupled Boundary Condition is then derived in order to account for this interaction in the framework of a Helmholtz solver where the baseline flow is assumed at rest. When fed with appropriate transfer functions to model the entropy generation and convection from the flame to the exit, the Helmholtz solver proves able to predict the presence of an unstable mode around 350 Hz, in agreement with both the LES and the experiments. This finding supports the idea that the instability observed in the combustor is indeed driven by the entropy/acoustic coupling.© 2013 ASME

[1]  Thierry Poinsot,et al.  Acoustic and Large Eddy Simulation studies of azimuthal modes in annular combustion chambers , 2012 .

[2]  Franck Nicoud,et al.  Using Boundary Conditions to Account for Mean Flow Effects in a Zero Mach Number Acoustic Solver , 2012 .

[3]  Matthieu Leyko,et al.  Comparison of Direct and Indirect Combustion Noise Mechanisms in a Model Combustor , 2009 .

[4]  Michael J. Brear,et al.  Thermoacoustic limit cycles in a premixed laboratory combustor with open and choked exits , 2009 .

[5]  Franck Nicoud,et al.  About the Zero Mach Number Assumption in the Calculation of Thermoacoustic Instabilities , 2009 .

[6]  P. Schmid,et al.  Dynamic mode decomposition of numerical and experimental data , 2008, Journal of Fluid Mechanics.

[7]  F. Nicoud,et al.  Acoustic modes in combustors with complex impedances and multidimensional active flames , 2007 .

[8]  T. Poinsot,et al.  Large-eddy simulation and experimental study of heat transfer, nitric oxide emissions and combustion instability in a swirled turbulent high-pressure burner , 2007, Journal of Fluid Mechanics.

[9]  Paul Kuentzmann,et al.  Unsteady Motions in Combustion Chambers for Propulsion Systems , 2006 .

[10]  V. Yang,et al.  Bifurcation of flame structure in a lean-premixed swirl-stabilized combustor: transition from stable to unstable flame , 2004 .

[11]  F. Ducros,et al.  A thickened flame model for large eddy simulations of turbulent premixed combustion , 2000 .

[12]  A. Dowling.  THE CALCULATION OF THERMOACOUSTIC OSCILLATIONS , 1995 .

[13]  T. Poinsot.  Boundary conditions for direct simulations of compressible viscous flows , 1992 .

[14]  F. E. Marble,et al.  Acoustic disturbance from gas non-uniformities convected through a nozzle , 1977 .

[15]  Tim Lieuwen,et al.  Combustion Instabilities In Gas Turbine Engines: Operational Experience, Fundamental Mechanisms, and Modeling , 2006 .

[16]  T. Poinsot,et al.  Theoretical and numerical combustion , 2001 .