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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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This paper aims at evaluating Large Eddy Simulations (LES) for the prediction of the performance line and flow at off-design conditions in a multistage high-pressure compressor. A coarse and an intermediate grid are specifically investigated, since their associated computational cost appears affordable in an industrial context. Several operating conditions of the 3.5 stages high-pressure compressor CREATE are simulated, then results are compared to experimental data and to an existing URANS simulation. Both grids yield iso-speed performance lines close to experimental measurements, but only the intermediate one is able to correctly predict the experimental point at lowest mass flow rate. The unstable regime is specifically investigated in the last stage of the intermediate grid, showing the presence of rotating instabilities. Their amount and spinning velocity are similar to experimental observations and previous URANS results. Hence coarse LES appears as an interesting tradeoff for off-design predictions of flow in a multistage compressor.

High-fidelity Large Eddy Simulations (LES) are performed to study the effect of hydrogen injection on a lean turbulent CH 4 /Air premixed flame. An Analytically Reduced Chemistry (ARC) mechanism is used to achieve a detailed description of CH 4 /Air-H 2 chemistry. First, a validation of this kinetic scheme against the detailed GRI-Mech 3.0 mechanism is presented considering both simplified and complex transport properties. When hydrogen is added to the mixture, large variations of the mixture Prandtl and of the N 2 Schmidt numbers are observed depending on the local species concentration, features that are missed by simplified models. LES is then applied to study the structure and stabilization mechanisms of a lean (φ = 0.8) premixed CH 4 /Air swirled flame enriched with hydrogen by using different transport modeling strategies. First, the fully premixed CH 4 /Air case is considered and results are found to validate the LES approach. In agreement with experiments, a classical V-shape flame is stabilized in the low-velocity region near the flame holder created by a central recirculation zone (CRZ). Then, hydrogen enrichment is achieved injecting 2% of the CH 4 thermal power with a central fuel injection lance. Both premixed and diffusion flame branches are present in this case, impacting flame stabilization and angle. The flame root the main premixed flame stabilized by a diffusion flame kernel created by the injected hydrogen reacting with the oxygen in excess of the premixed stream. Moreover, the H 2 consumed with the remaining oxygen in burnt gases leads to the formation of a second flame branch inside the CRZ which is responsible of an increase of the flame angle. Given the high concentration of hydrogen, an impact of the molecular transport models is observed on the flame lift-off height highlighting the importance of using complex transport properties in any LES involving hydrogen combustion.

Abstract. Recent advances in computer science and highly parallel algorithms make Large Eddy