Combustion of a carbon surface in a stagnation point flow field

Abstract

In the present paper the combustion behavior of a carbon surface exposed to the stagnation flow of a moist oxygen-nitrogen gas mixture is investigated theoretically. Calculations have been performed for finite homogeneous reaction rates as well as for the two limiting cases of a frozen and of an infinitely fast homogeneous reaction. For various velocity gradients, oxygen mass fractions, and humidities of the ambient flow, the theoretically determined dependence of the combustion rate upon the surface temperature is compared with the experimental combustion rate data of Matsui et al. It is shown by these comparisons that the Boudouard reaction C + CO 2 = 2CO must be relatively slow. In this case the rate of combustion is strongly influenced by the homogeneous reaction rate in the extended surface temperature range from about 1200 to 2000K. In this regime the decrease of the rate of combustion with increasing humidity, which has been observed experimentally by Matsui et al., is confirmed by the numerical results. This decrease is due to the acceleration of the homogeneous reaction by small amounts of water vapor, and it occurs only in the case of a slow Boudouard reaction. The theoretical results concerning the effects of the velocity gradient and the ambient oxygen mass fraction on the burning rate also show good agreement with the experimental data of Matsui et al. It is shown that the interactions between chemical and transport processes, which determine the combustion rate, cannot be reproduced by simply considering the limiting cases, as has often been done in the past.