Liftoff and blowout of the Emmons flame: Analysis of the triple flame

The steady burning and stabilization of the boundary layer diffusion flame over a gasifying condensed fuel surface, commonly called the Emmons flame, is an important problem in the study of boundary layer combustion. We investigate herein, theoretically and numerically, the liftoff distance and blowout limit of the Emmons flame, through the corresponding response of the controlling triple flame in the leading edge of the bulk flame. An explicit solution of the flame liftoff distance and the critical blowout limit is derived, with the theoretical results agreeing well with the numerical simulation for an extensive range of the system parameters. In particular, it is shown that the transversal velocity gradient (TVG) ahead of the triple flame renders the flame harder to liftoff and blowout, with this effect increasing for increasing TVG and decreasing triple flame curvature, which is related to the mixture fraction gradient. Furthermore, the Spalding mass transfer number, B_v, for the surface segment ahead of the flame front affects the flame stabilization and blowout limit by modifying the similarity structure of the flow and the location of stoichiometry. Thermal expansion of the flow around the triple flame together with the surface viscous drag also significantly promotes flame stabilization.