A study of the role of radiative ignition in the propagation of large explosions

Abstract

We consider the hypothesis that the anomalously high rates of propagation of unconfined vapour cloud explosions are caused, or aggravated, by radiation-induced multi-point ignition due to small particles ahead of the main flame front. The mechanism by which loose agglomerates of fine fibres ignite at very low radiation fluxes is examined and the measured dependence of ignition lag on the level of radiation provides the basis for calculating maximum flame speed enhancement. It is shown that, for compact particles, there is an absolute minimum size for radiative ignition due to vaporization below the minimum critical energy and that fibrous aggregates are more likely to be raised and dispersed by convection than any solid particle capable of inducing ignition. Modelling the growth of such flames for various densities of igniting particles displays the development in time of a convoluted flame front with greatly increased surface area and rate of burning, resembling in appearance the consequences of large-scale turbulence. The effective flame speeds are of the order required to account for observed levels of damage, even when no other enhancement is considered; in fact the mechanism suggests that interaction with other proposed causes of flame speed enhancement is highly probable