Ahmadu, AA
2018,
Flame propagation through different sizes of metal mesh and mitigation using fine water sprays
, MRes thesis, University of Salford.
Abstract
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 around
the globe, there has been a great interest in the use of fine water spray to suppress the
explosion. The present work is focused on the mitigation of slow-moving deflagrations
flames 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 extract
heat in the short finite moments that the flame, mesh and droplets interact (approximately
0.03ms for a representative 1mm thick flame front, 0.94mm, 1.31mm, and 6mm diameter
meshes). A novel technique, of woven wire steel mesh and perforated steel mesh combined
with a high-pressure atomiser known as a Spill Return Atomiser (SRA), was selected, which
contained a unique swirl chamber.
The investigation was conducted in three stages including configurations with dry, dry plus
mesh and wet plus mesh trials. At the initial stage, the hot trials of homogeneous methane-air
mixtures throughout the whole flammable range of 6, 7 and 9% was conducted and the flame
speeds observed were 26.32, 27.01 and 30 m/s respectively. The second stage involved flame
flow through the mesh. The flame speed observed for the trial was 20.36, 22.75 and 23.31
m/s for 6, 7 and 9% methane-air mixture respectively, for 0.94mm mesh. Mesh insertion into
the system reduces the flame speed, and also a decrease in temperature was observed due to
the heat loss to the mesh. Similar trend were observed for 1.31 and 6mm meshes. Finally, the
flame flow through both mesh and water sprays was investigated, with an average flame
speed within the range of 4 – 30 m/s. Whereby a configuration consisting of a steel mesh and
a cross flow (X/F) of 4 spill return atomizers at a separation distance of 1000 mm from the
mesh in the direction of the flame propagation. The spill return atomizers were configured at
105 mm and 120˚ apart and opposed to each other, thereby providing a total spray region of
315 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 observed
was 4.49, 5 and 12.42 m/s for 6, 7 and 9% methane-air mixture respectively, and were fully
mitigated. This is evidence that as the mesh thickness increases, mitigation of the flame
propagation was achieved easily.
Conclusively, the effect of the steel mesh was investigated and shows a good characteristic in
influencing the flame propagation and mitigating behaviour, though found to be better while
combined with fine water spray.
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