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

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 ϕ = 0.8 was filled and ignited in a plastic tent of 27 m3. 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.