A modern perspective on the problem of vapor cloud and explosions is given. Particular emphasis is placed on the important practical problem of detonation hazards. Significant advances in our fundmental understanding of detonation and flame acceleration have occurred in the last decade. We review these developments in large scale exerimentation and numerical modelling of reacting flows. Results of recent large scale experiments on flame acceleration and deflagration to detonation transition are discussed. It has been found that the important factors determining the possibility of detoniation include the intrinsic chemical sensitivity of the fuel-air system, the extent of confinement, the location of obstacles within the cloud, and the presence of jet ignition sources. Direct initation of detonation is unlikely for most fuel-air systems in a typical accidental release situation. Deflagration to detoniation transition is considered to be the most likely mean of detonation initiation. This has been observed only for the most sensitive fuel in truly unconfined mixtures lacking flame interaction with nearby walls or obstacles. A technique for estimation detonation hazards in gaseous releases is proposed. This technique is based on evaluating the intrinsic sensitivity of an explosive gas through a combination of lab experiments and numerical simulation of the detailed chemical kinetics. Examples are given for a variety of fuel-air systems.
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