Response of a laminar premixed V-flame to a high-frequency transverse acoustic field

摘要

This work presents new results describing the action of an acoustic transverse standing wave on a methane-air premixed V-flame located at a pressure antinode. Investigations of the jet highlight a mode conversion from a transverse cavity mode to a longitudinal mode, involving a "plugging" flow-rate modulation. This essential mechanism generates vortical structures convected toward the flame stabilized on a vertical rod introduced inside the burner and aligned with its axis. Depending on acoustic conditions, they develop through one of the following patterns: a pairing process, a multiple-vortex interaction in the jet outer layer, or a helical mode in the inner layer behind the rod. The flame responses are arranged in the physical space defined by the acoustic pressure amplitude (P_(ref)) and the Strouhal numbers, St characteristic of the dominant (outer or inner) shear-layer. A harmonic flame response at the forcing frequency, f_0, is not the only possible behavior. The other behaviors observed are an asymmetrical "elongated wrinkled flame" due to the helical mode; an "aperiodic fluctuating flame"; and a "sub-harmonic rolled-up flame" due to the pairing process, which induces strong CH~* emission fluctuations at f_0/2. Several flame dynamics evolutions are noted when P_(ref) increases. The heat release rate (h.r.r.) always begins to fluctuate at f_0. Then, depending on St, it can undergo the nonlinear frequency bifurcation; and next, it may fluctuate at f_0 again in addition to f_0/2. Another scenario shows the h.r.r. losing any kind of ordered modulation before it eventually fluctuates at f_0. The flame stabilization is described at blowout via the leading edge behavior. Foot-displacement amplitude, and foot-width thinning and straightening are complementary dynamics and morphology features that characterize flame destabiliza-tion. The different patterns, observed as P_(ref) increases, of successive ordered flame responses, sometimes alternating with aperiodic responses, appear as the main elements in understanding the processes involved in thermoacoustic instabilities.