On the outward propagation of ascending cylindrical flame

Abstract

We have investigated the characteristics of outwardly propagating flame (OPF) in a gravitational field that is directed downward. By using a numerical approach tracking the motion of a flame front, the long-term evolution can be simulated, which accounts for the interactions between the flame and the flow-field dynamics, leading to such critical mechanism as Darrieus–Landau (DL) hydrodynamic instability. If the Froude number is sufficiently large, the flame evolution is further complicated by increasing effect of buoyant force, yielding significant influence of Rayleigh–Taylor (RT) instability. Specifically, the dome of flame surface was largely wrinkled due to RT instability which enhanced the destabilization that was initiated by DL instability. The interplays between these flame-front instabilities and the buoyancy-induced convection generated extraordinary vorticities and circulation, which would lead to self-turbulized flow field. Furthermore, as a consequence of large increase in the flame surface area, the total burning rate was substantially increased. Although the resulted cellular structure was analogous to that performed in zero gravity which showed self-similar cascading of cells, the flame surface was largely deformed by the ambient flow due to natural convection. The convective flow brought the cells downward along the flame surface, whereby the flame stretch suppressed their further formation, while cells continued to appear on the upper part during the expansion of the flame bubble. As a result, due to continuous topological variation of the expanding flame, the center of mass did not follow a t-square law as that expected for a floating up burning sphere, but was characterized by an ascending motion with a smaller exponent of time.