Saturation mechanism of the heat release response of a premixed swirl flame using LES

The nonlinear heat release response of a premixed swirl flame to velocity perturbations is investigated using Large Eddy Simulation. The nonlinear heat release response is required for the prediction of thermoacoustic limit cycle pressure amplitudes and is represented here by the Flame Describing Function (FDF). As test case a premixed atmospheric swirl flame, for which experimental data on the FDF is available, was used. First a steady reacting LES solution was obtained and compared to experimental data. The simulation was then excited by superimposing a mono-frequency harmonic wave on the inlet boundary condition. Both the frequency and amplitude of the acoustic wave were varied to obtain the FDF. The calculated FDF was in good agreement with experimental data. At a frequency of 115 Hz, the flame was found to saturate for larger excitation amplitudes. A detailed analysis of the LES results revealed that the mechanism causing the saturation was a nonlinear evolution of the area of the flame surface with increasing perturbation amplitudes. Furthermore it was shown that for the present swirl flame, the heat release and flame surface fluctuations were linearly dependent on the tangential component of velocity fluctuations upstream of the flame, while they increased nonlinearly with the axial component of velocity fluctuations.