Transient ignition and combustion of diluted hydrogen/air mixtures by a thin catalytic wire

Abstract

This paper reports on a study of transient ignition and combustion of hydrogen/air mixtures by a heated, thin catalytic wire in a natural convection environment. Modeling of the process is accomplished via a reduced set of heterogeneous kinetic processes which include dissociative adsorption and desorption of both reactants, three fast surface reactions of the Langmiur-Hinshelwood type, and the desorption reaction of the adsorbed product, H2O(s). The overall surface reaction rate is found to be limited by the adsorption rate of molecular oxygen, which depends on the concentration of molecular oxygen close to the surface of the wire and the distribution of empty sites in the catalyst. The analysis allows the determination of the critical conditions for ignition and the ignition delay time as a function of the important physicochemical parameters. The resulting wire temperatures and the critical value of the external heating rate at ignition are computed. The analysis shows how the ignition temperature increases as the external heating rate increases. The configuration dependence of the ignition temperature is taken as an indication that the use of a critical Damköhler number provides a better ignition condition than the ignition temperature concept. A self-sustained combustion regime for strong diluted mixtures is described.