Wrinkling of Large-Scale Flame in Lean Propane-Air Mixture Due to Cellular Instabilities

摘要

In accidental gas explosions, flame acceleration owing to cellular instabilities such as diffusional-thermal instability and Darrieus-Landau instability can cause considerable damages, for example, the formation of a strong blast wave. In particular, as the flame scale increases, Darrieus-Landau instability, caused by a density jump, progressively dominates in the flame acceleration. In this study, we experimentally investigated the growth and wrinkling owing to Darrieus-Landau instability of a spherically expanding flame in a large-scale experiment, in which a propane-air mixture of the equivalence ratio phi = 0.8 was filled and ignited in a plastic tent of 27 m(3). Experimental images of large-scale flames of the lean propane-air mixture, for which the flame is diffusional-thermally stable, were analyzed. The edge of flame was detected and rearranged in polar coordinates. The results show that small-scale cells merge and form a bigger cell. The generated bigger cell grows by the instability mechanism and eventually forms a large single cusp. In addition, the peak-to-peak amplitude of the wrinkled flame was evaluated. The value of peak-to-peak amplitude increased as time progressed. Such a cellular flame gives rise to a fractal-like structure and acceleration of its propagation speed. The fractal dimension of the wrinkled flame surface was evaluated by logarithmically plotting the flame speed versus its radius and also by a box-counting method. The results demonstrated that the wrinkled structure of a large-scale flame can be characterized by its fractal dimension and that a transition period into a well-developed self-similar regime exists.