Investigation on H-atom Recombination over Silica, Alumina and Magnesia Surfaces Using a Wall Stagnation Flame

抄録

In order to examine wall chemical effects caused by radical adsorption and recombination on wall surfaces under flame-wall interaction, H-atom recombination on different walls are examined by using a methane (CH₄) -air premixed flame formed in a stagnation flow near a heated wall. Quartz, alumina, magnesia and platinum are chosen as the wall materials. To ensure the identical thermal boundary condition with the different wall materials, 300 nm-thick alumina, magnesia and platinum thin films are deposited on the same quartz substrates. Gas-chromatography analysis and numerical simulation with detailed gas/surface chemistry are carried out to evaluate the near-wall H₂ concentration, which could be increased by the H₂ desorption following the H recombination on the wall. Firstly, the wall stagnation flame near the platinum, of which surface reaction mechanism is well known, are examined to verify the numerical scheme. The simulation results of the flame position and the near-wall H₂ concentrations are in accordance with the measurement data. Then, the measured H₂ concentrations near the quartz, alumina and magnesia walls with different wall temperatures T_w are compared to the simulation results with various H-atom recombination rate constants k_r. It is found that, with increase in T_w, k_r on the quartz is increased and the H-atom recombination of the quartz shifts from adsorption-recombination competition regime to adsorption-limited regime. The H₂ concentrations near the alumina are in good agreement with the numerical results without the surface reaction, indicating that the alumina is a chemically-inert material. In contrast, as for the magnesia, the near-wall H₂ concentration is decreased with increase in T_w and becomes lower than that near the inert wall, implying that H₂ decomposition rather than H recombination occurs at high T_w.