TIME-DEPENDENT LASER IGNITION OF A COMBUSTIBLE STAGNANT BOUNDARY-LAYER

A spatially one-dimensional model for the ignition of a combustible ga s layer adjacent to a plane solid surface is considered. The effect of an incident laser beam from the gas side is to raise the surface temp erature; the other boundary is taken to be a poor thermal conductor in the form of an inert gaseous medium. It is assumed that the exothermi c chemical reaction within the layer has a negligible reactant consump tion. Three examples for the effect of the laser on the solid surface are considered: (a) a large instantaneous temperature rise, (b) a temp erature jump at t = 0 which is maintained for t > 0, (c) a surface tem perature variation which is a linear function of time. For (a) and (b) conditions for criticality are obtained. For appropriate states of th e system, times to ignition for (a), (b), and (c) have been determined . It is shown that the theoretical results are in reasonably good agre ement with experiment. The kind of physical situation envisaged might occur in a mineshaft when a stagnant combustible layer is subject to i ntense transient light. Although an idealization; our analysis determi nes conditions under which thermal runaway and subsequent explosion co uld take place.