Flame lift-off dynamics in a thermoacoustically self-excited swirl flame: A sensitivity analysis based on LES

Kurzfassung

In this work, compressible Large-Eddy Simulations are used to investigate a well-established swirl flame. This swirl flame is known for its strong self-excited thermoacoustic oscillations. Due to the availability of an extensive experimental database, the flame is frequently studied numerically, although a limited agreement is observed between experimental and numerical data. It is found that this limited agreement can be attributed to the investigated operational point which is close to an unstable region where the flame switches between attached and lifted state. This results in a configuration that is highly sensitive to small differences between the numerical and experimental boundary conditions. Targeted investigations are used to demonstrate that parameters such as the amplitude of the pressure oscillation as well as minor deviations in the operational point significantly influence the computational results. Taking these details into account, a very good agreement with experimental data is achieved. Subsequently, it is demonstrated that the processes inside the swirler, where fuel and oxidizer mix, are highly unsteady and must be modeled with caution. Moreover, coherent flow structures are found to be present during sporadic events of flame lift-off. Although this behavior is reported in literature at other operational points, this is shown for the first time for the operational point considered here.