Effect of the Gas-Phase Reaction on Hydrogen Micro-Combustion in a Pt/Γ-Al2O3 Catalytic Plane Channel with Detailed Chemical Kinetic Mechanisms

Abstract

Hetero-/homogeneous micro-combustion of fuel-lean hydrogen–air mixtures in the plane channel containing a Pt/γ-Al2O3 catalyst washcoat was investigated numerically with detailed chemical kinetic mechanisms. The main theme of the paper is assessing the relative significance of the H2 gas-phase reaction as compared with catalytic reaction at different plane channel heights H, inlet mass fluxes J, and equivalence ratios ϕ in plane micro-channels. The numerical model of microcombustion was employed, which included detailed gas-phase and surface catalytic reaction mechanisms, heat transfer mechanisms, and diffusion of multicomponent species. In order to sustain micro-combustion of fuel-lean hydrogen–air mixtures over Pt/γ-Al2O3 in the range of operating conditions, the solid wall temperature of the micro-combustors was maintained at 1400 K. In plane micro-channels, as the channel dimension is decreased, the micro-combustion characteristics of hydrogen–air mixtures are significantly impacted due to radical and effective heat losses to the walls. However, the catalytic walls contribute to sustain the H2 gas-phase reaction in the micro-channels by decreasing heat losses to the walls due to the exothermic surface catalytic reaction, which also restrains the H2 gas-phase reaction by extracting radicals owing to typically high absorption rates of the above-mentioned species at the walls. Therefore, the detailed radical chain reaction mechanisms can be significantly changed by the presence of surface catalytic reaction (wall reaction), and the radical accumulation in the gas-phase can be restrained. In the present work, the effects of the above-mentioned three key parameters on the interaction between the surface catalytic and gas-phase reactions are discussed. For fuel-lean hydrogen–air mixtures, in each case, the limiting values of the plane channel height, inlet mass flux, and equivalence ratio beyond which the gas-phase reaction become negligible as compared with the surface catalytic reaction are explored. The computational results indicate that variation of the hydrogen–air inlet mass flux at constant inlet equivalence ratio and plane channel height alters the balance between the diffusive and convective mass fluxes. Finally, the equivalence ratio of the hydrogen–air mixtures significantly effects the contribution of the H2 gas-phase reactions to the entire H2 conversion.