Effect of external surface emissivity on flame-splitting limit in a micro cavity-combustor

Heat transfer processes are of significance for combustion in microscale channel. In the present paper, three different values (i.e., ε = 0.1, 0.5 and 0.92) are selected to numerically investigate the effect of external surface emissivity on the flame-splitting limit (the critical velocity when the combustion efficiency drops to 80%) using Fluent 6.3. The results demonstrate that the flame-splitting limit has a negatively monotonic variation trend with the external surface emissivity. This feature is interpreted from viewpoints of heat-loss from the exterior wall and heat recirculation through the upstream wall. The analyses show that the heat-loss ratio decreases with the reduction of external surface emissivity, which gives rise to a more intense reaction in the cavity and a higher wall temperature level. Therefore, more heat is recirculated to upstream wall and a better preheating of the fresh mixture can be achieved. Consequently, the chain reactions of H2/air mixture start earlier and the reaction rates are much faster for a smaller surface emissivity. The combined actions of heat loss and heat recirculation lead to a faster consumption of the hydrogen fuel at the flame tip, and a larger flame-splitting limit can be obtained for a smaller surface emissivity.