Further studies of the role of radiative ignition in the propagation of large explosions

Abstract

Following previous work demonstrating that the anomalously high rates of propagation of unconfined vapour cloud explosions could be caused, or aggravated, by radiation-induced multi-point ignition due to small fibrous particles ahead of the main flame front, the investigation is extended towards more practical criteria. Ignition lag measurements are carried out in various hydrocarbons, at different relative velocities between particle and mixture, in a radiation flux that is varied in magnitude, direction and range of wavelengths. An image furnace with a tungsten filament as source is used as an alternative to a CO 2 laser, to obtain a closer approximation to radiation from combustion products. Measurements on particles of clean insulating and cotton wool are complemented by more ‘practical’ dusts collected from, for example, vacuum cleaners and shelves. The spectral absorption of various particles is compared with the radiance distribution due to various path lengths of combustion products, and the effect of coating clean particles with lamp black (to simulate the effect of dirt) is assessed. The basic hypothesis is reassessed in terms of a more detailed theoretical model that allows for slip between particle and gas velocity, radiation from secondary ignition centres and a definition of flame speed based on the rate of product gas volume generation. It is found that the susceptibility of various hydrocarbons and mixtures to this type of ignition accords with other combustion properties such as ignition temperatures and burning velocities. While the absorp­tive properties of certain particles and the propensity of others to emit inert blanketing vapour renders them less effective in producing ignition in the laboratory, all particles become hazardous when blackened by dirt and particularly, when subjected to previous exposure to radiation while not surrounded by a mixture of explosive composition. Theoretical modelling confirms that flame speeds of the order required to account for the damage observed in practice are generated by this mechanism and that this result applies for surprisingly low concentrations of hazardous particles. Thus, given the facility with which agglomerates of fibres are raised in suspension and the likelihood of their presence in numbers sufficient for this mechanism, the hazard warning implied by the previous work is reinforced.