Mechanisms of flame propagation through combustible particle clouds

Abstract

Abstract An experimental study has been conducted on the mechanisms of flame propagation through combustible solid particle clouds of 1-octadecanol. The combustible particle cloud is ignited in its centre by an electric spark, and the growth of flame kernel is observed with a CCD video camera. The direct light emission and schlieren images of propagating flame and the laser light scattering images of particles have been simultaneously recorded. After ignition, a flame kernel is observed to grow with a yellow luminous zone whose outline is of an irregular shape. At the same time, a smooth shaped schlieren front is observed to propagate at 4–8 mm ahead of the outline of the yellow luminous zone. Inside the schlieren front, dispersed blue flame spots appear but no smaller particles can be seen, and only bigger particles are observed in the border region near the schlieren front. Across the schlieren front, smaller particles (most of them are about 10–20 μm in diameter) rapidly gasify just behind the schlieren front, while the gasification of particles with a diameter larger than 80 μm is delayed and the vapour lumps formed behind the schlieren front ignite to form circular dispersed blue flames. It has also been revealed that the average propagation velocity of the schlieren front increases with the number density of smaller particles, while it is scarcely affected by the mean diameter of combustible particle clouds. This fact implies that flame propagation is mainly supported by the combustion of smaller particles gasifying across the schlieren front.