Effects of multi-dimensionality on a diffusion flame

Abstract

A numerical study of an axisymmetric, coflowing, methane-air diffusion flame was made with detailed chemistry including the NO x formation reactions compiled by Miler and Bowman. The calculated flame structure was compared with that of a counterflow diffusion flame. It was found that there is a remarkable similarity in the flame structure, in the physical plane and in the mole fraction vs. conserved scalar plane, of the two flames with the same representative diffusion time of the outer diffusion layer. The agreement is more remarkable for the mole production rate profiles rather than the temperature and the concentration profiles. The mole consumption rates of the reactants per unit flame surface area of the two flames are controlled by their supply rates to the reaction zone by molecular diffusion through the outer diffusion layers, rather than by the reaction rates inside the reaction zone, and are inversely proportional to the square root of the representative diffusion time. Furthermore, the production rate of NO, which is produced and consumed by the complicated reactions inside the reaction zone, is also mostly governed by the supply rates of the original reactants from the outer diffusion layers by molecular diffusion. The production rate was found to be rather insensitive to the representative diffusion time. The results obtained suggest that the representative diffusion time is the most appropriate parameter to link diffusion flames in different flow fields, and that we can make use of counterflow diffusion flame calculations to predict the essential characteristics of diffusion flames in other flow fields that do not lead to local extinction.