STUDY ON CATALYTIC COMBUSTION OF HYDROGEN-AIR INSIDE MICROTUBE

The catalytic combustion characteristics of a hydrogen-air mixture inside a microtube were investigated numerically and experimentally. Numerical simulations with detailed gas-phase and surface catalytic reaction mechanisms of hydrogen-air combustion were investigated. Combustion characteristics for different reaction models and the influence of wall thermal conductivity, inlet velocity, and tube diameter, on surface catalytic combustion reactions are discussed. The computational results indicate that surface catalytic combustion restrains gas-phase combustion. The higher wall temperature gradient for low wall thermal conductivity promotes gas-phase combustion upstream and results in a higher temperature distribution. The microtube can be divided into two regions. The upstream region is dominated by the surface catalytic reaction and the downstream region is dominated by gas-phase combustion. With increasing inlet velocity, the region dominated by surface catalytic reactions expanded downstream and finally occupied the whole tube. The temperature of the flame core decreased with a decrease in tube diameter. Decreasing the tube diameter enhanced the surface catalytic reactions. Catalytic combustion of premixed hydrogen-air inside a microtube with a channel width of 0.8 mm was experimentally studied. The experimental results indicate that the existence of a catalytic wall helped to achieve a complete reaction inside the microtube. In the comparison of numerical and experimental studies, the average outlet temperature and hydrogen conversion ratio at the outlet for different inlet velocities were investigated. The numerical and experimental results indicate that the outlet temperature and hydrogen conversion ratio at the tube exit obviously decreased when the inlet velocity exceeded 8 m/s. Some theoretical evidence is provided for the application of catalytic combustion to microelectromechanical systems (MEMS) and extension of the combustion limits.