Characterization of acoustically forced swirl flame dynamics

Abstract Lean premixed combustors are highly susceptible to combustion instabilities, caused by the coupling between heat release fluctuations and combustor acoustics. In order to predict the conditions under which these instabilities occur and their limit cycle amplitudes, understanding of the amplitude dependent response of the flame to acoustic excitation is required. This study presents an analysis of phase-locked OH PLIF images of acoustically excited swirl flames, to identify the key controlling physical processes and qualitatively discuss their characteristics. This analysis suggests that the flame dynamics are controlled by a superposition of the following processes: (1) annular jet fluctuations, (2) oscillatory turbulent flame brush development, (3) flame stabilization, and (4) fluid mechanical instabilities of the backward facing step, jet column, swirl, and shear layer. These results illustrate that the flame response is not controlled by any single physical process but, rather, by several simultaneously occurring processes which are potentially competing, and whose relative significance depends upon forcing frequency, amplitude of excitation, and flame stabilization dynamics.