Dynamic Parameters of Gaseous Detonations

Abstract

In addition to gases, flammable liquids and solids in the form of fine droplets and dust particles also form explosive mixtures with air. An explosive mixture can, in general, support two modes of combustion. The slow laminar deflagration mode is at one extreme; here the flame propagates at typical velocities of the order 1 m s -1 relative to the unburned gases and there is negligible overpressure development when the explosion is unconfined. At the other extreme is the detonation mode, in which the detonation wave propagates at about 2000 m s -1 accompanied by an overpressure rise of about 20 bars across the wave. The propagation of laminar defiagrations is governed by the molecular diffusion of heat and mass from the reaction zone to the unburned mixture. The propagation of detonations depends on the adiabatic shock compression of the unburned mixtures to elevated temperatures to bring about autoignition. The very strong exponential temperature dependence of chemical reaction rates in general makes possible the rapid combustion in the detonation mode. Two­ phase liquid droplets or dust-air mixtures are similar, but they require more physical processes (e.g. droplet break-up, phase change, mixing, etc.) prior to combustion. Thus, characteristic time or length scales associated with the combustion front are usually much larger than those of homogeneous gaseous fuel-air mixtures. The essential mechanisms of propagation of the combustion waves, however, are similar. In between the two extremes of laminar detlagration and detonation, there is an almost continuous spectrum of burning rates where turbulence plays the dominant role in the combustion process. Due to space limitations, only homogeneous gaseous fuel-air detonations are considered in this article.