A model for the thermoacoustic response of a premixed swirl burner, Part II: The flame response

A comprehensive model for the response of a premixed flame to acoustic velocity fluctuations is developed and applied to the same small laboratory swirl burner as was treated in Part I, using the validated model for the acoustics of the system. A number of possible mechanisms of acoustic interactions with the flame are reviewed, and the three judged to be the most important for this application are quantified on a consistent basis. The three mechanisms are those due to the direct influence of the velocity and turbulence, to coherent vortices shed from the burner exit, and to equivalence ratio fluctuations. The time-averaged position of the flame front at which most of the excitation takes place is identified from the observations, and the direction of the incident flow is deduced. The overall response to planar acoustic waves through the burner is then worked out. Comparison with experimental data for a wide range of conditions shows that the major trends in frequency of self-excitation are correctly predicted.