Catalytically stabilized combustion characteristics of methane-air mixtures in micro-scale heat-recirculating systems

Abstract

In applying catalysis to real combustion systems, the modeling of chemistry and transport interactions may be the key to understanding combustion characteristics. The catalytically stabilized combustion characteristics of a methane-air mixture in a micro-scale heat-recirculating system were investigated to gain a greater understanding of the mechanisms of flame stabilization and to gain new insights into how to design the system with improved robustness and stability. The system comprised catalytically-active and catalytically-inactive channels in a counterflow heat exchange relationship. The essential factors for design considerations were determined with improved flame stability and combustion characteristics. Two primary mechanisms responsible for the loss of flame stability were discussed. The results indicated that both chemical and thermal environments are improved with the catalytically stabilized combustion method and the heat-recirculating structure. The design incorporates the best features of both catalytic combustion and thermal flame methods. The system is essentially free of mass transfer limitations. Excess enthalpy combustion can occur in an efficient and rapid manner, resulting from the injection of free radicals and heat produced by the catalytic reaction. The flow velocity, wall thermal conductivity, equivalence ratio, exterior heat losses are important factors in determining the performance of the system. Stable operation of the system is limited to a relatively wide flow regime, and the flow velocity is critical to achieving flame stability. There is an optimum wall thermal conductivity in terms of flame stability. The system with a moderate wall thermal conductivity will be most robust against the surrounding conditions. Blowout shifts homogeneous combustion downstream significantly without substantially reducing the reaction rate. Finally, engineering maps denoting flame stability were constructed, and design recommendations were made.