Flame Stability and Combustion Characteristics in Catalytic Micro-combustors

Abstract

The flame stability and combustion characteristics in catalytic micro-combustors were studied using an elliptic two-dimensional computational fluid dynamics model that includes detailed homogeneous and heterogeneous chemical reaction schemes, heat conduction in the solid wall, surface radiation heat transfer, and external heat losses. Simulations were carried out to investigate the effects of wall thermal conductivity, wall thickness, inlet velocity, and operating conditions on combustion characteristics and the steady-state, self-sustained flame stability of hydrogen-air mixtures. Simulation results reveal that the reaction is limited by heat transfer near the entrance and by mass transfer further downstream, despite the small scales of this system. Large transverse and axial gradients are observed even at these small scales under certain conditions. Wall thermal conductivity and thickness are very important as they determine the upstream heat transfer, which is necessary for micro-flame ignition and stability, and the material's integrity by controlling the existence of hot spots. Wall thermal conductivity is vital in determining the flame stability of the system, as the walls are responsible for the majority of the upstream heat transfer as well as the external heat losses. Thin walls exhibit large axial temperature gradients, resulting in hot spots. Thicker walls have a large cross-sectional area, which allows for greater heat transfer and more uniform, lower temperatures. Inlet velocity plays a competing role in flame stability. Low flow velocities result in reduced power generation, and high flow velocities decrease the convective timescale below that of the upstream heat transfer through the walls. There exists a range of flow velocities that allow stabilized combustion in catalytic micro-combustors.