Nonlinear response of a premixed low-swirl flame to acoustic excitation with large amplitude

This paper describes an experimental investigation of the nonlinear response of a premixed methane/air low-swirl flame to imposed acoustic excitation with perturbation amplitude up to 95% of the bulk velocity. Amplitude dependence of flame dynamic at a low frequency,75 Hz, and a high frequency, 195 Hz, over a wide range of excitation level is analyzed in detail. Proper orthogonal decomposition is used to identify the dominant structures in the low-swirl flame. Experimental results show the gain of FDF of the lowswirl flame has a peak value at 65 Hz and a local minimum at 105 Hz which is caused by the destructive (out of phase) and constructive (in phase) interference of the axial and azimuthal velocity fluctuations. The low-swirl flame shows complex nonlinear amplitude dependence which depends on the excitation frequency. Flame harmonic response has a close relation with non-linearity behavior. Vortices are formed and convected downstream along the flame approximately at bulk flow velocity, inducing flame rollup and shear layer instabilities. Fluctuation of heat release rate in the flame tail region and shear layer due to the combined impact of flame rollup and shear layer instabilities govern the flame nonlinear response. Further POD analysis shows the most energetic modes (mode 1 and mode 2) feature symmetric wave-like structures. As the perturbation level increases, the higher anti-symmetric and helical modes are inhibited. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.