Effect of heat recirculation on the combustion stability of methane-air mixtures in catalytic micro-combustors

Abstract

The combustion stability of premixed methane-air mixtures in catalytic single-channel and heat-recirculating micro-combustors were studied numerically, using a two-dimensional computational fluid dynamics (CFD) model with detailed chemistry and transport. The effects of wall thermal conductivity, feed composition, inner and outer walls, and inlet velocity on stability were investigated to understand the underlying mechanisms of heat recirculation. Stability diagrams were constructed to guide the design of heat-recirculating systems. Additionally, the roles of preheating and heat loss in stability were also studied. It was shown that heat recirculation can result in a substantial improvement in stability, but this excess enthalpy effect only occurs for highly insulating materials. In contrast, comparisons with the single-channel system revealed that the stability of the two systems is practically equivalent for highly conductive materials. Heat recirculation strongly affects blowout, but has minimal effect on extinction. The combustion chamber walls affect stability more strongly than the other walls, with the most stable system having highly insulating walls. Furthermore, the optimal placement of catalyst was also explored; an effective thermal management strategy is a combination of preheating and heat recirculation. Finally, the choice of the wall thermal conductivity is crucial, when designing heat-recirculating combustors.