Stagnation point flow of a chemically reactive fluid in a catalytic porous bed

Abstract

The heat transfer and reaction characteristics of a chemically reactive flow near the stagnation point of a catalytic porous bed with finite thickness are investigated theoretically. Due to the catalytic effect, the activation energy is reduced such that chemical reaction in the bed becomes possible even at relatively low flow temperatures. The steady state and initial transient period in the gas phase upstream and in the catalytic porous bed are studied using both the singular perturbation method and the finite difference method. For the perturbation analysis, a single layer model is sufficient when the bed is relatively thin, of the order of the characteristic thermal diffusion length scale. For a thick bed, a multiple layer analysis is necessary. Results from the steady-state analysis show that for a higher chemical reactivity, lower flow velocity gradient, lower activation energy, and lower mass diffusion rate, the conversion rate from reactants to products is higher so that a thinner bed can be used to reach complete reaction. Moreover, due to the high thermal conductivity of the solid porous material, temperature profiles are modified by the heat release through chemical reaction only slightly for a thin bed. The flow temperature is affected by the reaction more significantly for a thicker bed because more heat is released from the reaction, and the increased importance of convection effect. Numerical results for the transient case exhibit the same characteristics as in the steady state.