Flickering Buoyant Diffusion Flames in Weakly Rotatory Flows

Abstract

Flickering buoyant diffusion methane flames in weakly rotatory flows were computationally and theoretically investigated. The prominent computational finding is that the flicker frequency nonlinearly increases with the rotational intensity number R (up to 0.24), which measures the relative importance of the rotational speed compared with the methane jet speed. This finding is consistent with the previous experimental observations that flame flicker is enhanced by rotatory flows within a certain extent. Based on the vortex-dynamical understanding of flickering flames that the flame flicker is caused by the periodic shedding of buoyancy-induced toroidal vortices, we formulated a scaling theory for flickering buoyant diffusion flames in weakly rotatory flows. The theory predicts that, with respect to the flicker frequency f_0 at R=0, the increase of the flicker frequency f at nonzero R obeys the scaling relation f-f_0~R^2, which agrees very well with the present computational results. In physics, the externally rotatory flow enhances the radial pressure gradient around the flame, and the significant baroclinic effect contributes an additional source for the growth of toroidal vortices so that their periodic shedding is faster.