Nonlinear dynamic responses of a self-excited thermoacoustic system subjected to acoustic forcing

On the combustion experimental platform of high temperature and pressure, the thermoacoustic system with a concentric stratified swirl-stabilized partially premixed-prevaporized flame with self-excited response was excited by external-excited signal with frequency ff of 78-716Hz and amplitude A of 0.026-0.629, and its nonlinear dynamic responses were studied. It was found that the system self-excited frequency fn was 366.85Hz; when the ratio of external-excited frequency to self-excited frequency was 1.022 and amplitude A was 0.629; the flame was dominated by the harmonic frequency and its state trajectory was dense in a very narrow closed zone. When the ratio of external-excited frequency to self-excited frequency and A was in other ranges, the flame responded not only at ff and fn, but also at the harmonic, combining and divided frequencies. It showed that when amplitude A was large enough and external-excited frequency was close to the self-excited frequency, the flame was phase-locked. When the flame was not phase-locked, the nonlinear responses of the harmonic, combining and divided frequencies appeared, and the flame oscillated quasi-periodically. Forced van Der Pol oscillator model can predict phase-locked, odd harmonic frequencies and combining frequencies 2fn±ff.