Gas Phase Ignition of a Premixed Combustible by Catalytic and Non-Catalytic Cylindrical Surfaces

Abstract

An analysis is carried out of the gas phase ignition of a premixed combustible gas flowing under mixed convective conditions over a hot, non-catalytic or catalytic, cylindrical surface. The surface temperature of the cylinder is assumed to be constant. In the case of the catalytic surface it is also assumed that the catalytic reaction is infinitely fast so that chemical equilibrium is reached along the surface of the cylinder. In the gas, a finite rate chemical reaction with a large activation energy is considered. The analysis makes use of the boundary layer approximation to describe the gas flow and of first order matched asymptotic expansions to define ignition. Explicit expressions are derived for the critical gas phase Damköhler number for ignition as a function of the angular location. The results show that the value of the angular coordinate at which ignition takes place increases as the convective velocity and activation energy increase and as the initial temperature and initial fuci concentration decrease. It is also shown that for the same surface temperature, ignition will occur further downstream from the forward stagnation point if the surface is catalytic than non-catalytic. This is the result of the reduction of fuel concentration near the wall due to the presence of the catalytic reaction. In a practical case, however, where the temperature of the catalytic surface is expected to be larger than that of the non-catalytic one, ignition will occur closer to the forward stagnation point when the surface is catalytic, thus suggesting the use of catalytic surfaces for flame holders.