Fuel/air cloud

Accidental vapor cloud explosions do not occur under controlled conditions. Various experimental programs have been carried out simulating real accidents. Quantities of fuel were spilled, dispersed by natural mechanisms, and ignited. Full-scale experiments on flame propagation in fuel-air clouds are extremely laborious and expensive, so only a few such experiments have been conducted. 

Experiments to Study Deflagration of Fuel-Air Clouds after a Dispersion Process... 

TABLE 4.2. Overview of Test Results on Deflagrative Combustion of Fuel-Air Clouds under Uncontrolled Conditions... 

Conclusions from experiments on deflagrative combustion of fuel-air clouds under uncontrolled conditions follow ... 

Flame acceleration was minimal after ignition of dispersed fuel-air clouds under unconhned conditions in the absence of obstacles. 

Fishbum et al. (1981) used the HEMP-code of Giroux (1971) to simulate gas dynamics resulting from a large cylindrical detonation in a large, flat, fuel-air cloud containing 5000 kg of kerosene. Blast effects were compared with those produced by a 100,000-kg TNT charge detonated on the ground. 

Fireball A burning fuel-air cloud whose energy is emitted primarily in the form of radiant heat. The inner core of the cloud consists almost completely of fuel, whereas the outer layer (where ignition first occurs) consists of a flammable fuel-air mixture. As the buoyancy forces of hot gases increase, the burning cloud tends to rise, expand, and assume a spherical shape. 

Fireball - Is a rapid turbulent combustion of a fuel-air cloud whose energy is emitted primarily in the form of radiant heat, usually rising as a ball of flame. 

FAE is presently a two-step process an expl burster disperses the fuel into a suitable fuel/air cloud, then a second-event expl, activated within an easily-detonable region of the cloud, initiates detonation of the entire fuel/air cloud. A single-event FAE can be produced by the simultaneous dispersal and detonation of the fuel/air cloud as it is being formed. Alternatively, a spontaneous FAE can be achieved by dispersing a pyrophoric fuel into an appropriate fuel/air cloud within the ignition induction period. Such a single-event system has decided advantages over the two-event FAE... 

Most important are the mechanisms of release, the release position relative to the surroundings, and atm conditions. These determine how the fuel mixes with air and its movement in the atm once a fuel/air cloud is formed. The conditions under which the fuel had been contained and its pressure-temp characteristics frequently determine if the cloud is predominantly a mist and whether it is dense or light compared to the atm... 

Explosions A release of energy that causes a pressure discontinuity or blast wave. Fireball The atmospheric burning of a fuel-air cloud in which the energy is mostly emitted in the form of radiant heat. The iimer core of the fuel release consists of... 

Detonations of unconflned fuel-air clouds require significant ignition energy (Fishbum et al., 1981 CCPS, 1994), thus requiring, perhaps, the prior gas explosion (Harris, 1983) of the main office and/or the works office bailing to act as a precursor or trigger for the detonation. 

Moen, I.O., Sulmistras, A., Hjertager, B.H. and Bakke, J.R., "Turbulent Flame Propagation and Transition to Detonation in Large Fuel-Air-Clouds," Proceedings of the Twenty-First Symposium fInternational on Cumbustion. The Combustion Institute,... 

In contrast the speed of travel of the flame front during a detonation Is supersonic. In a fuel/air cloud a detonation wave will move at speeds of between 1500 and 2000 m/s and the peak pressure in front of the flame can reach 15 to 20 bar. This pressure front when enclosed can cause substantial damage including the collapse of structures. 

In an explosion of a fuel/air cloud Ignited by a spark, the flame will normally start out with a velocity of the order of 3-4 m/s. If the cloud is truly unconfined and unobstructed (i.e. no equipment or other structures are engulfed by the cloud) the flame is unlikely to accelerate to velocities of more than 20-25 m/s, and the overpressure will be negligible if the cloud is not confined. 

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