Simulations of premixed combustion in porous media

A numerical model for planar premixed flames of methane in ceramic porous media has been developed to improve the understanding of the structure of such flames. The model successfully reproduces experimental data for both single- and two-layer surface flames. The success is attributed to the detail given to the boundary conditions and the radiation modelling, which was done by solving the radiation transfer equation inside the porous medium without any simplifying models. Surface-stabilized flames yielded S_L/S_LO < 1 (S_LO being the free-flame laminar burning velocity) and had energy balances where convection in the gas phase was balanced by heat transfer from the solid for most of the length of the burner before the reaction zone, while the heat release was mostly balanced by heat exchange with the solid. In contrast, submerged flames in foams with large pores yielded S_L/S_LO > 1 and their energy balance was similar to that of a free flame, which implies that the burning velocity acceleration is due to the reactant preheat. The flame solutions were further analysed with concepts from the computational singular perturbation method to construct reduced mechanisms. For all types of combustion (surface or submerged), an almost identical ordering of chemistry timescales to free flames was found and previously developed reduced mechanisms for free flames were accurate also for the flames inside the porous medium. The results suggest that the thermal exchange between the two phases that is responsible for the flame behaviour remains decoupled from the fast part of the chemistry.