Hetero-/homogeneous combustion and flame stability of fuel-lean propane–air mixtures over platinum in catalytic micro-combustors

Abstract

The hetero-/homogeneous combustion characteristics and flame stability of lean premixed propane–air mixtures over platinum in micro-combustors were numerically studied. For catalytic micro-combustors in particular, numerical studies focused on key thermal management issues, including impact of combustor materials and heat recirculation through the combustor walls. A global heterogeneous reaction step validated over a wide range of parameters has been established, and was further coupled to a detailed homogeneous reaction scheme. Parametric studies were performed with a two-dimensional numerical model to elucidate the role of key operating conditions (wall thermal conductivity, combustor dimension, inlet velocity, flow rate, and equivalence ratio) in determining flame stability and combustion efficiency. Based on these insights, the roles of heat loss and heat recirculation on the mechanisms of flame stability, blowout and extinction were studied. Simulations indicated that the combustor dimension (which includes gap distance and wall thickness) and wall material are important design variables. An optimum thermal conductivity exists for best flame stability, which is lower than that for the homogeneous counterpart. High conductivity materials should be preferred when high-power systems are desired; contrarily, low-power systems would favor more insulating materials to minimize heat losses. Extinction occurs at lower values of the external heat transfer coefficient. Operation diagram denoting flame stability (stability maps) was constructed, and design recommendations for catalytic micro-combustors were made. Comparisons to homogeneous micro-combustors were also presented.