Contributions of self-absorption and soot on radiation heat transfer in a laminar methane—air diffusion flame

Abstract

A numerical study is performed to understand the contributions of self-absorption and soot radiation in a laminar methane—air diffusion flame. Simplified models have been adopted for the combustion and soot chemistry. A gray radiation model was developed using the first-order moment method to evaluate the emission and absorption of radiation. The absorption coefficient is determined considering (i) the influence of the gaseous (CH4, CO2, H2O, and CO) species and soot (taken as the benchmark results for the current study), and (ii) only the influence of the gaseous species. A comparison of the two results identifies the contribution of soot on radiation. An optically thin radiation model is also used to estimate the contribution of self-absorption in the flame. Finally, a radiatively adiabatic treatment in the model is employed to account the total effect of radiation. The effects of each contribution have been ascertained by looking at the variation in the radiative heat loss and the resulting temperature distribution of the flame. Differences in the soot volume fraction and thermal NO formation with the different radiative treatments have also been studied. Radiation is found to have very important role on the overall flame structure as well as on the soot formation and NO emission in a methane—air diffusion flame. Neglecting soot in the calculation of radiation underpredicts the radiative heat loss from the flame, particularly in the soot-laden region. Though the effect on the macroscopic flame shape is marginal, the peak temperature difference at the flame tip is found to be substantial. On the contrary, the predictions using the optically thin approximation are very close to the benchmark results.