Rarefaction effects on the catalytic oxidation of hydrogen in microchannels

Catalytic oxidation of H2/air mixtures in the slip regime is numerically simulated in order to assess the slip/jump effects in platinum-coated planar microchannels. A colocated finite-volume method is used to solve the governing equations. A concentration jump model derived from the kinetic theory of gases is employed to account for the concentration discontinuity at the reactive walls. A detailed surface reaction mechanism for hydrogen oxidation on Pt along with a multi-component species diffusion model are used to study the effects of concentration jump coupled with velocity slip and temperature jump on the walls. The slip/jump effects are studied under different operating conditions: wall temperature, channel height, inlet mass flux and surface accommodation coefficient are varied to examine their individual effects. The results suggest that the temperature discontinuity at the wall is the dominant term in the concentration jump boundary condition. The mass transfer characteristics of the reacting flow are least influenced by velocity slip while the temperature jump at the wall alters the mass transfer characteristics the most. The hydrogen oxidation rate on the catalytic wall is strongly affected by the rarefaction effects, especially in the entrance region. The results show that the thermal diffusion effect in calculating the diffusive mass flux of species has a considerable effect on the species distribution inside the channel and cannot be neglected. The effects of slip/jump boundary conditions can change significantly at different inlet fluxes and channel heights.

► Effects due to slip/jump on a catalytic wall are examined in detail.
► Temperature-jump at the wall has the highest impact on the flow and temperature fields.
► Thermal diffusion effects cannot be neglected in the entrance region of the channel.
► Temperature-jump increases the catalytic H2 conversion rate substantially.
► Concentration-jump decreases the catalytic H2 conversion rate.