On propagation limits in porous media combustion

Subsonic combustion in inert porous beds saturated by an explosive premixture is known to occur either as a slow nearly isobaric reaction wave (0.1-10 m s(-1)) or as a fast wave (100-300 m s(-1)) with a smooth rise in pressure. Whereas the slow wave is basically a corrugated deflagration driven by molecular transport, the fast wave is sustained by the drag induced diffusion of pressure. In general, the frictional drag, which is rather prominent in porous systems, promotes spontaneous transition from deflagrative to pressure-driven combustion. Practically, however, the transition does not always take place, and the deflagration may well endure without provoking the fast wave. Employing a reduced model for subsonic combustion in inert porous beds accounting for the combined effect of hydraulic resistance and heat losses, the transition free deflagration is explained in terms of, the disparity between the respective propagation limits. The model predicts a jumpwise (hysteretic) transition from the deflagrative to fast-subsonic-regime under gradual elevation of the initial pressure. This effect is likely to be related to an abrupt change-over from conductive to convective burning, known to occur in combustion of granular explosives.