Flame front disturbance induced by a weak pressure wave

Abstract

An experimental study has been conducted on the effect of unburned mixture properties on flame front disturbance induced by acceleration. Experiments were performed using a rectangular combustion chamber of 80×80×440 mm. The flame front disturbance was observed in two different directions by high-speed schlieren photography. Mixtures used are of three different concentrations (Φ=0.8, 1.0, and 1.25) of methane/air and two different concentrations (Φ=1.0 and 1.5) of propane/air. For the methane/air mixture of Φ=1.0, experiments were performed at three different initial pressures ( P i =50, 70, and 101 kPa). The observed disturbance was of a very fine structure of circular spikes, which penetrated into the burned gas. The scales of disturbance were measured and indicated to be in the range of 1.7–4.0 mm. The circular spike shape is a typical structure induced by accelerating the flame front where the density changes steeply. However, the shape of the disturbance observed for a rich propane/air mixture was not of circular spikes but of a net of ridges. For the rich propane/air mixture, the effect by preferential diffusion was remarkable and the disturbance of a different structure was generated. For the methane/air mixtures, the scale was the smallest at Φ=1.0 and larger at Φ=0.8 and 1.25. The scale for the propane/air mixture of Φ=1.0 was slightly larger than that in the methane/air mixture of Φ=1.0. The scale became larger with the decrease of the intial pressure. The measured scales were compared with the preheat zone thicknesses of corresponding flames. It was shown that the scale is closely related with the flame thickness. The scale of disturbance is found to be about 15 times as large as the preheat zone thickness.