BUOYANCY-DRIVEN FILTRATION COMBUSTION

A theoretical study of combustion in porous media driven by a gravity induced gas flux is conducted. Filtration of the oxidizer carrying gas arises in response to heating of the gas due to exothermic conversion of the solid fuel. Specifically, we consider a reaction front propaga ting through a porous matrix consisting of reactive (fuel) and inert c omponents. Gas, consisting of both oxidizer and inert components, filt ers through the matrix and reacts with the solid fuel. The hot gases i n the medium rise due to buoyancy, thus drawing in Fresh gas from belo w. Pie employ approximate analytical methods and numerical simulations to analyze all the basic combustion phenomena, including self ignitio n, external ignition, both upward and downward as well as adiabatic an d nonadiabatic propagating combustion waves. Our simulations also desc ribe the dynamics of buoyancy driven combustion waves. In conventional combustion systems the combustion waves are traveling waves, whose wa ve characteristics, e.g., propagation velocity and shape, are constant , and the time and length scales for the ignition period are independe nt of the length L of the sample. In contrast, here the waves are not traveling waves. Rather, they are quasisteady waves, whose characteris tics do depend on L. Thus, knowledge of the combustion characteristics determined From experiments on a specific sample of a given size can not be generalized to samples of larger size as is the case in convent ional combustion. We derive estimates for the dependence of various co mbustion characteristics on L. Finally, suggestions for the experiment al verification of some of the qualitative results of our analysis are presented.