An analysis of sub-limit flame dynamics using opposite propagating flames in mesoscale channels

Abstract

The excess enthalpy flames and their dynamics below the flammability limit are studied by considering two flames that propagate in opposite directions in parallel channels. The model enables the coupling between the external heat loss, convection preheating, diffusion transport and finite rate chemistry. Analytical expressions for the flame temperature, separation distance, and extinction limit are obtained. The results show that flame extinction can be caused by the external heat loss without heat conduction of inner wall in the streamwise direction. The heat recirculation across the separating wall dramatically increases the flame speed and extends the flammability limit. It is shown that the maximum and minimum flame speeds corresponding respectively to the fast and slow flame modes exist at all separation distances between the two flames. It is found that the flame can adjust its separation distance to adapt to the variation of heat loss, heat recirculation and fuel concentration. There exists a maximum flame separation distance beyond which sub-limit flame does not exist. The results also showed that heat recirculation significantly extends the flammability limit. Furthermore, at low fuel concentrations, the flame can be stabilized in a narrow range of separation distance. The present study not only generalized the previous analyses of the heat recirculation flames but also provided a model for the study and control of sub-limit flames in micro power devices and reactors.