Thermally accurate LES of the stability-emission performance of staged gas-turbine combustion

Modern gas turbines use turbulent lean partially premixed combustion in order to minimise nitrous oxide (NOX) emissions while ensuring flashback safety. The Large-Eddy Simulation (LES) of such a device is the goal of this work. Focus is laid on correctly predicting the NOX emissions, which are influenced by four factors: heat transfer, mixing quality, combustion modelling and thermo-acoustic stability. As NOX reaction rates are strongly influenced by temperature, heat transfer by radiation and convection is included. Radiation is predicted by a model, which assumes that the gases are optically thin. Convective heat transfer is included via a newly developed and validated wall-function approach based on the logarithmic law of the wall for temperature. An optimised 2-step reduced chemical reaction scheme for lean methane combustion is presented. This scheme is used for the LES in conjunction with an additional third reaction, fitted to produce the same NOX reaction rates as in the complete reaction mechanism. Turbulence is accounted for with the thickened flame model in a form, which is optimised for changing equivalence ratios and mesh-resolutions. Mixing is essential not only for predicting flame stabilisation, but also for pollutant emissions as NOX reaction rates depend exponentially on equivalence ratio. Therefore the full burner geometry, including 16 fuel injections is resolved in LES. Additionally, effusion cooling and film cooling is accounted for in a simplified manner. The non-reacting flow is extensively validated with experimental results. As mixture-fraction fluctuations do not only arise from turbulence, but also from thermo-acoustic instabilities, care was taken to provide acoustic boundary conditions that come close to reality. The resulting LES shows a strong thermoacoustic instability, comparing well with experimental observations. By making the boundaries completely anechoic it is shown that when the instability disappears, the NOX levels are reduced by 75%. Additionally, neglecting all heat transfer, effusion and film cooling, the NOX levels are increased again by one order of magnitude