Universal Relationships in Sooting Methane-Air Diffusion Flames

Abstract

The laminar flamelet concept is based on the premise that scalar properties in laminar diffusion flames are nearly universal functions of mixture fraction. It has been well-tested and proven for temperature and the major species, however, few studies have addressed its applicability for minor species especially the radical species. In this study, we present a direct numerical simulation of an axisymmetric, laminar, methane-air diffusion flame to examine these universal relationships, including the major and minor chemical species, and the radical species. The numerical model solves the axisymmetric, time-dependent, reactive-flow Navier-Stokes equations coupled with sub-models for soot formation and radiation transport, and includes a detailed reaction mechanism for methane-air combustion. Quantitative comparisons with existing experimental data show a slightly wider computed flame compared to the experimental flame, however, the peak values and radial locations of temperature and soot volume fraction line up well with the experimental measurements. To study the universal relationships, scatter plots are made for temperature and the major and minor species throughout the entire flame and compared with existing experimental measurements. Excellent agreement is obtained between the computations and experiments for temperature and mole fractions of CH4, O2, OH, H2O, O, H, CO2 and N2, as a function of mixture fraction in the fuel lean, stoichiometric and fuel rich regions of the flame. The computations underpredict the concentration of H2 and CO in the fuel rich region, however, excellent agreement is obtained in the fuel lean and stoichiometric regions. The computations also overpredict the concentration of CH3 in the stoichiometric and fuel rich regions, however, the peak concentration occurs at the same mixture fraction for both the experiments and computations. The scatter plots indicate that many of the species studied, including the minor species and radical species, can be considered universal functions of mixture fraction, however some of the species show more scatter than others. The species which showed the least scatter were those whose production rates were fastest in the stoichiometric and fuel lean regions of the flame. Those species whose production rates were highest in the very fuel rich region showed less universality with mixture fraction.