Ignition temperature of solid explosives exposed to a fire.

摘要

When a system containing solid explosive is engulfed in a fire it receives a heat flux that causes the temperature of the system to rise monotonically. The temperature rise can often be approximated by a linear rise for extended periods of time. When some portion of the explosive, usually near the surface, reaches its ignition temperature it will begin to burn. If the explosive is unconfined, or can breach its confinement at low pressure, it will burn, not explode. Typically the burn front will propagate through a slab or shell at speeds on the order of a centimeter a minute. If the explosive is confined, the gas resulting from its burning will generate pressures high enough to rupture the confinement, but the peak pressure will generally be only a fraction of the pressure from a true detonation. When a system is not engulfed in the fire, but is close enough to be heated slowly by the fire, the behavior will be different. If the explosive is heated slowly it will have a nearly uniform temperature and ignition will occur inside the explosive. This almost always causes an explosion, even when the explosive as a whole is unconfined. The reason for this behavior is not well understood but slow heating of an explosive generally results in a more violent explosion than fast heating. These two situations are recognized by fast and slow cookoff tests used with munitions. Many munitions pass the fast cookoff test with heating rates around 2 K/min. Slow cookoff tests with heating rates around 4 K/hr generally result in an explosion. (The equations in this paper assume absolute temperatures in Kelvins, equal to Celsius + 273.16.) Mathematical models predicting the time to explosion are usually based on the assumption that the explosive has a uniform initial temperature and that the outer surface is suddenly raised to some temperature and held there. The earliest such models where those of Semenov and Frank-Kamenetskii.