Physical Mechanisms of a Lifted NonPremixed Flame Stabilized in an Acoustic Field

Main acoustic effects on a nonpremixed lifted flame in the hysteresis zone are analyzed by means of high speed tomography and LDA techniques. A burner with a convergent profiled tube is designed to obtain a jet with organized eddy structures, even with no excitation. A chart summarizes the different flame responses to a sine-shaped forcing. Depending on ranges of amplitudes and frequencies, acoustics either can prevent reattachment to the burner and enhance combustion, or weaken the flame stability (large fluctuations of the liftoff height and/or reattachment). In all the cases, the flame behavior is driven by the organization of jet vortices, which differs from one case to another: Without acoustics or with low amplitudes of excitation, the streamwise vortices due to secondary instabilities are of paramount importance in the stabilization of the flame; for high amplitudes, it is rather the oscillating jet column that acts on the flame.