Vortex zone dynamics in premixed flame under complex gravity and acoustic impact

Abstract

An inverted conical, plane-symmetrical premixed methane–air flame stabilized by a bluff body under acoustic excitation and various gravity conditions was experimentally investigated. Recirculation zone characteristics were found by means of the phase-resolved particle image velocimetry method. An increase in the size of the longitudinal vortex zone was shown with an increase in both fuel concentration and flow velocity under normal and reverse gravity. The longitudinal size of the vortex zone is independent of frequency, regardless of the direction of gravity at low flow velocity (≤5 m/s) in a stoichiometric flame under the considered excitation frequency range (40–420 Hz). With a flow velocity increase, the size of the vortex zone becomes sensitive to the excitation frequency. An increase in the excitation frequency results in a length decrease in the vortex zone. In rich flames, an inverse relation of the longitudinal vortex zone size to the excitation frequency is observed at lower velocities (5 m/s) for normal gravity conditions. Whereas, under conditions of inverted gravity, the fuel air ratio increase does not lead to such a relation; the vortex zone has a constant length under various excitation frequencies. An external acoustic excitation causes a periodic change in the vortex zone longitudinal size, and for a stoichiometric mixture, the amplitude does not depend on the disturbance frequency. For a rich mixture, a frequency increase results in an amplitude decrease. For selected frequencies and flow velocities, desynchronization of the vortex zone oscillations with external disturbances is observed.