Experimental and theoretical analysis of acceleration of a gas flame propagating over a dust deposit

Abstract

In the present work, we studied experimentally and theoretically the propagation of a methane-air flameand its interaction with a coal dust deposit. The experiments were conducted in a horizontal flame-acceleration tube, 1.48 m long and 89×89 mm square cross section. Photodiodes and a digital video camera were used to detect flame propagation. Flame propagation was analyzed with a one-dimensional flow model and a turbulent-burning-rate model. The results indicate that there are three distinguishable stages and an interesting propagation pattern of three transient fronts when a gaseous flame propagates over a dust deposit. In the first stage, the gas flame accelerates gradually until the dust deposits are dispersed into the air and heated by the gaseous products so that the dust ignites. In the second stage, a flame of three transient fronts—a leading gas flame, a forward dust flame, and a backward dust flame—was observed. At the last stage, a complex gas-dust flame forms when the dust flame is integrated into the gaseous flame. The results also show that, under the present conditions, the flame is accelerated slightly by the dust deposits but dramatically by obstacles. The terminal flame speed with obstacles A (blockage ratio, 0.05) or obstacle B (blockage ratio, 0.10) is about 4 to 16 times of that without obstacles. Because the dispersion of the dust is dominated by gas flow, the dust generally flies in the same direction as the flame propagation when the flame propagates from the closed end of the tube. However, the dust flies in the opposite direction when the flame travels from the open end of the tube.