Effects of heat transfer on flame stability limits in a planar micro-combustor partially filled with porous medium

Abstract

Inserting porous medium into micro-combustors is able to further enhance heat recirculation, thus favoring the extension of flame stability limits. Both experimental and numerical studies have shown that extended standing-wave combustion regimes exist in micro-combustors filled with porous medium. However, the underlying mechanism that dictates the critical flame stability limits is not well understood. As such, numerical simulations are conducted for a planar micro-combustor partially filled with porous medium in order to quantify heat transfer and to analyze its effects on the critical conditions under which flames will break the upper or lower boundaries of the porous medium. Based on the proposed definitions of the preheat zone and the heat loss zone, preheating and heat loss occurring inside the porous micro-combustor are quantified. It is shown that neither preheating nor heat loss alone is sufficient to determine the flame stability limits. Therefore, their relative importance is considered by a ratio (Rp-hl) which measures the net heat gain inside the porous zone. Making use of the correlations of Rp-hl with the flow velocity and the equivalence ratio, a flow velocity range is obtained beyond which the flame cannot be stabilized within the porous medium, regardless of the equivalence ratio used. A parametric study is subsequently carried out to examine the effects of two important parameters, they are, the thermal conductivity and the porosity of the porous medium, with the results briefly analyzed. In principal, the approach presented in this paper could be readily applied to other configurations of micro-combustors as well. By incorporating pore-scale flow, heat and mass transfer details into the numerical model, the results are expected to be more quantitatively accurate.