The second charge then detonates the cloud and creates a massive blast wave. The cloud of fuel flows around objects and into structures. After the munition is dropped or fired, the first explosive charge bursts open the container at a predetermined height and disperses the fuel (and possibly ionizes it, depending on whether a fused quartz dispersal charge container was employed) in a cloud that mixes with atmospheric oxygen (the size of the cloud varies with the size of the munition). Fuel–air explosive Ī fuel–air explosive (FAE) device consists of a container of fuel and two separate explosive charges. Piston-type afterburning is also believed to occur in such structures, as flame-fronts accelerate through it. This rarefaction effect has given rise to the misnomer "vacuum bomb". Further damage can result as the gases cool and pressure drops sharply, leading to a partial vacuum. In confinement, a series of reflective shock waves is generated, which maintain the fireball and can extend its duration to between 10 and 50 ms as exothermic recombination reactions occur. The continual combustion of the outer layer of fuel molecules, as they come into contact with the air, generates added heat which maintains the temperature of the interior of the fireball, and thus sustains the detonation. That weakness may be eliminated by designs in which the fuel is preheated well above its ignition temperature so that its cooling during its dispersion still results in a minimal ignition delay on mixing. The upper limit has been demonstrated to influence the ignition of fogs above pools of oil strongly. Close in, blast from the dispersal charge, compressing and heating the surrounding atmosphere, has some influence on the lower limit. Ĭonventional upper and lower limits of flammability apply to such weapons. In some designs, strong munitions cases allow the blast pressure to be contained long enough for the fuel to be heated well above its autoignition temperature so that once the container bursts, the superheated fuel autoignites progressively as it comes into contact with atmospheric oxygen. Ī thermobaric bomb's effective yield depends on a combination of a number of factors such as how well the fuel is dispersed, how rapidly it mixes with the surrounding atmosphere and the initiation of the igniter and its position relative to the container of fuel. The most recent development involves the use of nanofuels. Fuels are chosen on the basis of the exothermicity of their oxidation, ranging from powdered metals, such as aluminum or magnesium, to organic materials, possibly with a self-contained partial oxidant. Ī typical weapon consists of a container packed with a fuel substance, the center of which has a small conventional-explosive "scatter charge". Accidental unconfined vapor cloud explosions now happen most often in partially or completely empty oil tankers, refinery tanks, and vessels, such as the Buncefield fire in the United Kingdom in 2005, where the blast wave woke people 150 kilometres (93 mi) from its center. Such dust explosions happened most often in flour mills and their storage containers, and later in coal mines, prior to the 20th century. Thermobaric explosives apply the principles underlying accidental unconfined vapor cloud explosions, which include those from dispersions of flammable dusts and droplets. In contrast to an explosive that uses oxidation in a confined region to produce a blast front emanating from a single source, a thermobaric flame front accelerates to a large volume, which produces pressure fronts within the mixture of fuel and oxidant and then also in the surrounding air. The typical blast wave of a thermobaric weapon lasts significantly longer than that of a conventional explosive. The initial explosive charge detonates as it hits its target, opening the container and dispersing the fuel mixture as a cloud. They are, however, considerably more effective when used in enclosed spaces such as tunnels, buildings, and non-hermetically sealed field fortifications ( foxholes, covered slit trenches, bunkers). Their reliance on atmospheric oxygen makes them unsuitable for use under water, at high altitude, and in adverse weather. Most conventional explosives consist of a fuel– oxidizer premix, but thermobaric weapons consist only of fuel and as a result are significantly more energetic than conventional explosives of equal weight. Aftermath of explosion, with unburned flour on the ground
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