Fully-resolved 3D premixed H2/air flames in a micro-combustor partially filled with porous media: Effects of detailed pore structures

Micro-combustors filled with porous media allow dual pathways for heat recirculation, one from the solid matrix and the other the combustor wall, thus representing a potential solution for micro-combustion applications. In the case of porous micro-combustors, the pore size is nearly comparable to the characteristic diameter/height of the combustor, and therefore pore-scale flame characteristics need to be fully resolved, which however cannot be realized in the conventional volume-averaged models (VAMs). A 3D pore-scale model (PSM) is developed in the present study to simulate the premixed H2/air combustion in a planar channel (H = 1 mm) partially filled with porous media. Conjugate heat transfer between the combustor wall and reacting gases is considered. Two types of porous media are geometrically modelled, they are, in-line arranged alumina spheres (1-mm-diameter) and SiC foam, with the resulting porosities to be 0.48 and 0.95, respectively. The comparison of flame spatial features indicates that the 2D VAM fails to delineate both the flame position and the flame thickness (volume) in such small combustors. For the two types of porous media considered, the results of the 3D PSMs clearly explain the difference in flame stabilization mechanism inside the porous zone. Heat recirculation (through the solid matrix and the combustor wall) and heat losses in the flame zone are quantified through numerical integrations. It is found that under the specific conditions considered, the alumina spheres exhibit a higher blow-off limit, a lower flame temperature, a lower fraction of heat recirculation and a lower fraction of heat losses than the SiC foam does. The present study confirms the necessity of considering detailed pore structures in modelling porous micro-combustors, and in future studies a wider range of flow velocity will be modelled for more generalized conclusions.