Effects of catalytic walls on hydrogen/air combustion inside a micro-tube

Numerical simulations with detailed heterogeneous and homogeneous chemistries of hydrogen/air mixture reactions inside a catalytic micro-tube were performed. The characteristics of heterogeneous and homogeneous interaction are delineated in terms of flow velocity, tube diameter and wall thermal conductivity. With the catalytic wall, the homogeneous combustion is obviously weakened. The heterogeneous reactions will consume part of the fuel near the entrance and will make the homogeneous combustion region shift downstream. The micro-tube can be divided into two regions. The upstream region is dominated by the heterogeneous reaction and the downstream region is dominated by the homogeneous combustion. With increasing inlet velocity, the region dominated by heterogeneous reactions expanded downstream and finally occupied the whole tube. Decreasing the tube diameter enhanced the heterogeneous reactions. However, the increased heat released at the wall will be beneficial for homogeneous combustion. The maximum allowed inlet velocity for homogeneous combustion first increased and then decreased to almost zero at 0.2 mm tube. This means that homogeneous combustion can not be sustained inside such a small catalytic micro-tube. Finally, three different wall materials are simulated. Higher wall temperature gradient for lower wall thermal conductivity will promote the homogeneous combustion shift upstream and will have a wider temperature distribution. Since the homogeneous combustion is ignited and sustained by the heat from the heterogeneous reaction, the effect of wall thermal conductivity is not as obvious as it is in the micro-tube without catalyst walls.