Inverted conical methane/air flame shape transformation under acoustic excitation

Abstract

An inverted conical plane-symmetrical premixed methane–air flame under acoustic excitation was investigated experimentally. The flame was a conical one stabilized by a thin transverse rod. The Reynolds numbers were varied within 400–3000, and the fuel equivalence ratios were 0.8–1.4. Such a flame can take the shape of two fundamentally different types: M—when the flame front attaches both to the nozzle edge and stabilization rod; V—only to the stabilization rod. The transition conditions from one front configuration to another (M–V and V–M) were studied under various excitation parameters. Acoustic disturbances with a frequency of up to 420 Hz and an amplitude of longitudinal velocity pulsations up to 10% of the mean flow velocity excited the flame. New experimental data were obtained on the conditions corresponding to the M–V and V–M transitions. Qualitative and quantitative parameters of the hysteresis effect at which M–V and V–M transitions are realized, similar to the case of swirling flames, were revealed. The frequencies at which the acoustics most strongly affects the M–V and V–M transitions conditions were found. The hysteresis deterioration for M–V and V–M transitions was shown for the first time. It was noted that the conditions for the hysteresis deterioration are close to the theoretical frequency of the descent of vortices from the nozzle edge. The local maxima of the characteristic Strouhal numbers corresponding to the features of the minimum divergence of the flow velocities at the M–V and V–M transitions were determined.