Appearance and stability of a laminar conical premixed flame subjected to an acoustic perturbation

The paper describes an experimental study of quasi-conical premixed air/methane flames vibrating under periodic fluctuations of the upstream flow. The flow field and the flame were characterized by an LDV system and image processing. The flow velocity fluctuations were created by an acoustic perturbation with a frequency between 20 and 1000 Hz and a perturbation intensity, defined as the ratio of the rms amplitude of the flow velocity fluctuations to the mean velocity at the exit of the burner, ranging from 0 to 150%. In the perturbation plane (frequency and intensity), a chart is established that shows the configurations assumed by the flame in response to the acoustic perturbation. Three configurations were studied. The "wrinkled flame" configuration exhibits a flame with undulations symmetrical about the vertical axis and vibrating at the perturbation frequency. This work completes previous studies. In particular, a critical value of the frequency is found. Above this. acoustic amplitudes of deformation are damped at the front, which is unaffected by the wrinkling. The "subharmonic flame" configuration exhibits a flame whose front oscillates with half the perturbation frequency, symmetrical or not, about the burner centreline. The asymmetrical flame is composed of two half-fronts moving in opposition. This instability on Bunsen burners has never previously been observed or predicted. The "chaotic flame" configuration characterizes a flame not necessarily attached to the burner that exhibits a disordered geometry that leads to aperiodicity of its motion. A comparison of the kinematics of the fresh gas flow and of the flame shows a strong coupling between the nature of the perturbation and the response of the front. The flame can be considered as a semi-autonomous system: with the forced jet, its specific response depends nor only on the imposed experimental conditions but also on its intrinsic nature. (C) 1998 by The Combustion Institute.