The asymptotic structure of weakly strained moderately rich methane—air flames

Abstract

The asymptotic structure of weakly strained moderately rich premixed methane-air flames stabilized in stagnation-point flows is analysed. In the asymptotic analysis the influence of strain on the mass flux, on the temperature and on the concentration of chemical species in the flame is treated as a perturbation from their values in the corresponding unstrained flame. Perturbation analysis is carried out to the first order. The deviation of the mass burning rate in the strained flame from the mass burning rate in the corresponding unstrained flame is obtained in terms of the Markstein number. Chemical reactions taking place in the flame are described by a reduced chemical-kinetic mechanism made up of four global steps. The analysis gives a sufficient number of equations from which the Markstein numbers can be calculated. Numerical calculations are performed using a chemical-kinetic mechanism made up of elementary reactions. Markstein numbers are obtained and they are found to agree reasonably well with those calculated using the results of an asymptotic analysis. Markstein numbers are also found to agree reasonably well with those deduced from experiments. An interesting finding of the present study is that the Markstein number for moderately rich flames depends on the Lewis number of both fuel and oxygen.