Flame propagation through different sizes of metal mesh and mitigation using fine water sprays
Explosion is one of the major problems faced in dealing with flammable hydrocarbon gases.Following the recent tragic incident resulting from gas and vapour cloud explosion aroundthe globe, there has been a great interest in the use of fine water spray to suppress theexplosion. The present work is focused on the mitigation of slow-moving deflagrationsflames flow through various mesh sizes and water spray, with resulting speeds of ≤30m/s.Thus, the mesh sizes thickness and droplets within the spray must be small enough to extractheat in the short finite moments that the flame, mesh and droplets interact (approximately0.03ms for a representative 1mm thick flame front, 0.94mm, 1.31mm, and 6mm diametermeshes). A novel technique, of woven wire steel mesh and perforated steel mesh combinedwith a high-pressure atomiser known as a Spill Return Atomiser (SRA), was selected, whichcontained a unique swirl chamber. The investigation was conducted in three stages including configurations with dry, dry plusmesh and wet plus mesh trials. At the initial stage, the hot trials of homogeneous methane-airmixtures throughout the whole flammable range of 6, 7 and 9% was conducted and the flamespeeds observed were 26.32, 27.01 and 30 m/s respectively. The second stage involved flameflow through the mesh. The flame speed observed for the trial was 20.36, 22.75 and 23.31m/s for 6, 7 and 9% methane-air mixture respectively, for 0.94mm mesh. Mesh insertion intothe system reduces the flame speed, and also a decrease in temperature was observed due tothe heat loss to the mesh. Similar trend were observed for 1.31 and 6mm meshes. Finally, theflame flow through both mesh and water sprays was investigated, with an average flamespeed within the range of 4 – 30 m/s. Whereby a configuration consisting of a steel mesh anda cross flow (X/F) of 4 spill return atomizers at a separation distance of 1000 mm from themesh in the direction of the flame propagation. The spill return atomizers were configured at105 mm and 120˚ apart and opposed to each other, thereby providing a total spray region of315 mm. The flame speed observed during this trial was 6 and 11.99 m/s for 6 and 7%methane-air mixture respectively at downstream 0.94mm mesh and upstream water spray,and they are fully mitigated. For 1.31mm mesh and water spray, the flame speed observedwas 4.49, 5 and 12.42 m/s for 6, 7 and 9% methane-air mixture respectively, and were fullymitigated. This is evidence that as the mesh thickness increases, mitigation of the flamepropagation was achieved easily. Conclusively, the effect of the steel mesh was investigated and shows a good characteristic ininfluencing the flame propagation and mitigating behaviour, though found to be better whilecombined with fine water spray.
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