Ignition of flammable atmospheres by radiation-heated fibrous agglomerates

Abstract

The conditions under which laser irradiation of loose agglomerates of fine fibres or particles leads to the ignition of a surrounding flammable gas mixture are studied in relation to the hazard associated with the use of optical sensing in explosive atmospheres. Results for stoichiometric mixtures in air of a range of hydrocarbons as well as of diethyl ether, carbon disulphide and hydrogen, are presented in the form of minimum igniting radiation flux as a function of the time to ignition. The minimum igniting fluxes at long induction times prove to be surprisingly low, down to 22 kW m –2 for carbon disulphide/air mixtures. Traverses with small thermocouples, along with equilibrium temperature measurements during irradiation in air and cooling curves obtained following switch off, show that the system acts as a two-dimensional slab, though its heat transfer properties differ somewhat from bodies with well-defined boundaries. Irradiation with lasers of widely differing wavelengths and the use of different target surface coatings leads to widely varying equilibrium temperatures. While the minimum igniting flux cannot be correlated simply with any of the obvious ignition criteria, the corresponding equilibrium temperature reached in an inert atmosphere points to a critical ignition temperature for each particular flammable mixture. This is used to propose a tentative hazard assessment based on the concept of black body radiation interacting with a ‘black ’ particle which, although not the worst case for monochromatic radiation, proves to be more hazardous than the worst case yet encountered in practice. The variation of ignition lag with radiant power flux is such that the total energy flux varies linearly with time; effective activation energies for various fuels are deduced from the variation of induction time with temperature. A simplified theoretical model is proposed, based on classical thermal explosion theory and the large activation energy idealization. It is shown that the radiant power flux is the sole ignition criterion so long as the irradiated area is large by comparison with the quenching dimensions of the mixture. Measurements on smaller beam diameters carried out by refocusing the beam emerging from an optical fibre indicate that, below these dimensions, minimum igniting power fluxes rise, while hazardous laser powers fall to values of the order of 100 mW.