Effects of Finite Rate Heterogeneous Kinetics on Homogeneous Ignition in Catalytically Stabilized Channel Flow Combustion

The homogeneous (gas-phase) and heterogeneous (catalytic) ignition of fuel-lean premixtures is investigated analytically and numerically in two-dimensional laminar channel-flow configurations with uniform incoming properties and isothermal catalytic walls. First-order matched activation energy asymptotics are employed, along with a one-step catalytic reaction and a one-step large activation energy gaseous reaction. Parametric description of the chemically frozen state leads to a closed-form heterogeneous ignition criterion in terms of non-dimensional variables that are relevant to confined flows. Formulation of the weakly reactive state yields a closed-form homogeneous ignition criterion that includes explicitly the homogeneous and heterogeneous reactivities through the relevant gaseous and surface Damköhler numbers (Dag and Das, respectively). Both ignition criteria are valid over the range 0.002 x/(bRePr) 0.16, 1.5 TW/TIN 3, and 0.9 Le 2.0, where x is the streamwise distance, Re the incoming Reynolds number based on the channel half-height b, Pr the Prandtl number, TW/TIN the wall-to-inlet temperature ratio, and Le the Lewis number of the fuel. Numerical simulations have shown good agreement between the numerically and the analytically predicted homogeneous ignition distances. A reduction of the surface reactivity (Das) promotes homogeneous ignition due to the associated increase of the near-wall fuel levels, and this effect is manifested in the homogeneous ignition criterion via a corresponding increase of the characteristic transverse diffusion time scale with decreasing Das. It is shown that there exist infinite combinations of surface and gaseous reactivities yielding the same homogeneous ignition distance, suggesting caution in the interpretation of catalytically stabilized combustion (CST) experiments. Moreover, the homogeneous ignition distance is much more sensitive to the gaseous rather than to the surface reaction pathway, thus exemplifying the importance of validated homogeneous reaction schemes under CST-relevant conditions. © 2002 by The Combustion Institut