Multiphase combustion of porous materials gives rise to significant two-phase-flow effects associated with the relative motion of gaseous and condensed phases. For example, deflagrations in confined porous energetic materials are characterized by an overpressure in the burned gas region that reverses the gas flow and leads to permeation of the hot gases into the unburned porous material. This results in a superadiabatic effect that increases the combustion temperature and hence the burning rate. Under the assumption of gas-phase quasi-steadiness, an asymptotic model, one that is amenable to a perturbation analysis of the basic solution and its stability, is derived. This basic solution, corresponding to a steadily-propagating planar combustion wave, is shown to be susceptible to a pulsating form of instability that collapses to previous results in the limit of zero porosity. For nonzero porosity, the effect of increasing over pressure, which increases the combustion temperature, is shown to be generally stabilizing, analogous to the effects of decreasing heat losses on combustion temperature and stability in single-phase deflagrations. Additional effects attributable to porosity and two-phase flow will also be discussed.
This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000.