A non-monotonic variation of blow-off limit of premixed CH4/air flames in mesoscale cavity-combustors with different thermal conductivities

Recently, we developed micro- and meso-scale combustors with wall cavities, which have been verified to be a simple and effective way for flame anchoring. In the present paper, the effect of wall thermal conductivity on flame blow-off limit was investigated numerically. It is very interesting to find that the flame blow-off limit is a non-monotonic function of the wall thermal conductivity, i.e., it achieves smaller values at moderate thermal conductivities. Our previous study has demonstrated that the occurrence of flame blow-off is due to the flame splitting phenomenon at the transition point between the ramped cavity wall and downstream channel wall. This is because the heat loss rate and strain rate at this point is very large which lead to local extinction and flame blow-off. Therefore, if the flame front approaches the transition point earlier, the flame blow-off limit will be smaller. For the present work, the differences in wall thermal conductivity result in different heat recirculation effect, which has a significant influence on the shape of flame front. Three typical thermal conductivities (λs = 0.5, 1.05 and 50 W/m K) were taken to analyze the non-monotonic trend of the flame blow-off limit. It is revealed that, under the same condition, the distance between flame front and transition point is the shortest at λs = 1.05, which leads to the smallest flame blow-off limit among the three cases. Therefore, it can be concluded that the heat recirculation effect is of crucial importance to the flame blow-off limit of the mesoscale cavity-combustor.