Heat recirculation and heat transfer in porous burners

Flames stabilized within porous media differ from conventional flames primarily due to the heat recirculation provided by the solid matrix. Heat is recirculated through solid conduction and solid-to-solid radiation from the matrix downstream of the flame to the matrix upstream of the flame. Solid-to-gas convection upstream of the flame preheats the incoming reactants, resulting in enhanced flame speeds and local temperatures above the adiabatic flame temperature. In this paper, the heat recirculation in a porous burner is analyzed using a one-dimensional time-dependent formulation with complete chemistry. The enhancement of flame speed is presented in terms of a ratio between the effective flame speed of a flame in a porous medium and the laminar flame speed. A heat recirculation efficiency is defined as a percentage of the firm,, rate transferred into the preheat zone. The importance of solid conduction and solid-to-solid radiation is discussed for various,,table operating conditions. The radiant output efficiency is also presented. In addition, discussions of peak and exit gas temperature trends are included. Results indicate that with increasing equivalence ratio, heat recirculation efficiency decreases. Both solid conduction and radiation play important roles in the heat transfer process. In addition, the results indicate that the observed trends are valid for burners of various lengths. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.