Response of non-premixed swirl-stabilized flames to acoustic excitation and jet in cross-flow perturbations

An investigation into the response of non-premixed swirl-stabilized flames to acoustic excitation and jet in cross-flow (JICF) perturbations was carried out. The fluctuations of OH∗ chemiluminescence emission measured simultaneously with the pressure were used to determine flame describing function (FDF). It could be seen that the three specific conditions exhibited stronger heat release oscillations coupled with pressure fluctuations under acoustic forcing and JICF perturbations, which were expressed by the in-phase delay and positive flame response indices originated from Rayleigh criterion to conduct the stability analysis. Low acoustic excitation amplitude triggered more significant response in the flame heat release rate than high acoustic excitation amplitude at the particular forcing frequency, indicating that nonlinear saturation characteristic of the FDF is also emphasized in JICF. Poincaré map was adopted to analyze the lock-in amplitudes of the dynamics system and JICF effects on combustion instabilities under acoustic forcing frequencies nearby/far from the natural frequency. It was found that lock-in occurred more easily when the forcing frequency was greater than the natural frequency, showing that JICF did not affect the system dynamics due to their little impact on the vortex/flame interactions in this work studied.