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Compressible Large Eddy Simulation (LES) of turbulent reacting flows potentially offers a realistic representation of the physical phenomena involved in thermoacoustic instabilities. Coupled with Helmholtz solvers, which provide a mean of identification of the acoustic eigenmodes in complex geometries, that advanced numerical tool may provide a very powerful environment to assess the stability of real engine combustion chambers. The common use of a fully unstructured LES solver and a Helmholtz eigenmode tool proves here to be very useful to diagnose two operating points of a real gas turbine chamber which are known to operate with self-sustained oscillations. If the geometrical complexity and proper computational domain are chosen, the two thermo-acoustic instabilities are very well reproduced. Predictibility of LES for such problems is thus demonstrated provided that issues pertaining to the inlet acoustic impedances can be properly answered.

Free-stream turbulence preceding high-pressure turbine blades has a crucial impact on blade fields including the heat transfer on the wall. Many parameters characterize this turbulence; its intensity, length scales and physical spectrum are addressed in the study of various operating points of the LS89 configuration. Usually, operating points where weak turbulence is injected are well predicted for all fields by Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES). The MUR235 operating point however, with an experimentally injected turbulence level of 6%, remains incorrectly predicted when imposing the experimental values in the simulations. Such difficulties raise many questions amongst which mesh size and turbulent kinetic energy spectrum are of specific importance for LES. Going away from synthetic turbulence injection by imposing a physical energy spectrum can help improving the prediction of heat transfer. From the present study, it seems that turbulent spots developing in a pre-transition region for higher levels of turbulence on the suction side are important features to capture for proper predictions. In parallel, typical structures of boundary layers such as streamwise oriented vortices have been observed and their existence conditions the heat transfer field on the blade wall. From this specific study, all of these physical processes are seen to be highly dependent on the turbulent specification and turbulent transition observed for the MUR235 case. Depending on these inflow specifications, a transitional boundary layer may be encountered upstream of the shock thus modifying the heat transfer profile.

Indirect combustion noise, generated by the acceleration and distortion of entropy waves through the turbine stages, has been shown to be the dominant noise source of gas turbines at low-frequencies and to impact the thermoacoustic behavior of the combustor. In the present work, indirect combustion noise generation is evaluated in the realistic, fully 3D transonic high-pressure turbine stage MT1 using Large-Eddy Simulations (LES). An analysis of the basic flow and the different turbine noise generation mechanisms is performed for two configurations: one with a steady inflow and a second with a pulsed inlet, where a plane entropy wave train at a given frequency is injected before propagating across the stage generating indirect noise. The noise is evaluated through the Dynamic Mode Decomposition of the flow field. It is compared with previous 2D simulations of a similar stator/rotor configuration, as well as with the compact theory of Cumpsty and Marble. Results show that the upstream propagating entropy noise is reduced due to the choked turbine nozzle guide vane. Downstream acoustic waves are found to be of similar strength to the 2D case, highlighting the potential impact of indirect combustion noise on the overall noise signature of the engine.

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