60216 - Experimental Investigation on Nonlinear Response of a Low-Swirl Flame to Acoustic Excitation with Large Amplitude 

Flame dynamic response at large amplitude excitation is a key element in predicting non-linear thermoacoustic instabilities in gas turbine combustors, such as limit cycle oscillation and mode switching. In this present study, the unsteady responses of a low-swirl flame to acoustic excitation at large excitation levels are experimentally studied. Emphases are focused on the nonlinear behavior of the flame. The low-swirl flame is excited using a loudspeaker with a perturbation level ranging from 5% to 80% of inlet velocity at two frequencies, 75 Hz and 195 Hz. The bulk inlet velocity is fixed at 5 m/s. A photomultiplier is used to measure global flame heat release rate and a high-speed ICCD camera is employed to capture unsteady flame behavior. Proper orthogonal decomposition is used to identify the coherent structures in the low-swirl flame. The nonlinearity of flame response is examined and flame describing function (FDF) is concluded. Results show that the low-swirl flame becomes wider under acoustic excitation than that for stable condition. Flame response in terms of gain and phase shows similar trends at different excitation amplitudes. Flame begins to show nonlinearity at 25% perturbation level. Blow off and flashback occur when the perturbation level excesses 40%. The nonlinearity is attributed to the fuel-air mixing and rolling-up structures at the tail of the flame.