Lean premixed combustion technology became state-of-the-art in modern gas turbines for power generation to reduce NOx emissions. In these systems, thermo-acoustic oscillations are easily excited in the combustion chamber. Due to the high heat release density, extreme amplitudes can occur which reduce component life or may even cause damage to the engine. Knowledge of the acoustic behavior is required in order to understand and predict these instabilities. This study of the combustor-turbine interaction is focused on the reflection coefficient analysis. The interface between the combustion system and the first turbine stage is the focus area of this study. The rotating components need to be included as outlook of this work. Compressible Large Eddy Simulation (LES) resolving acoustics is applied based on the open source CFD code OpenFOAM. Five cases of increasing complexity are presented. The main idea is to begin the study based on simple geometries such as a convergent-divergent nozzle and two nozzles respectively convergent and divergent, to proceed with increased complexity by adding a vane section, and finally to investigate the behavior of a realistic turbine design. The real engine case consists of an authentic geometry including a can annular combustion chamber and turbine vane section. These cases are studied as basic generic tests in order to validate analytical formulae and to test the CFD methods applied. Calculations with acoustic excitation and non reflecting boundary conditions (NRBC) at the computational inlet and outlet domains are carried out to verify the plausibility of the acoustic set up. The forced response approach is applied provoking a wave excitation at the inlet of the combustion chamber. Multi-harmonic excitation with small amplitudes is used to stay in the linear range. The post-processing for all cases is performed using the two-microphone method in order to calculate the reflection coefficient and the acoustic impedance taking into account the effects of the mean flow.

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