Selective diffusion during flame propagation and quenching in a porous medium

The propagation of propane-air flames in an inert high-porosity medium with nitrogen dilution and oxygen enrichment of the mixture was studied experimentally. It is shown that variation in the nitrogen or oxygen concentration (in the gas phase) leads to a more significant variation in the flame propagation velocity than in the laminar burning velocity; with the addition of nitrogen, the rate of increase in the flame velocity with the initial pressure becomes lower and the concentration range of flame propagation becomes narrower. At the flame propagation limit, the Peclet number obtained from the laminar burning velocity of the initial mixture is not constant but depends on the fuel-to-oxidizer ratio and the nitrogen content in the mixture. The results are interpreted from a physical point of view based on the hypothesis of selective diffusion. It is shown that accounting for the effects of the Lewis numbers of the fuel and oxidizer allows flame propagation in inert porous media to be described quantitatively over wide parameter ranges using a unified relation. At the flame propagation limit, the Peclet number constructed from the laminar burning velocity taking into account these effects is a constant.