UNIVERSAL RELATIONSHIPS IN SOOTING METHANE-AIR DIFFUSION FLAMES

The laminar flamelet concept is based on the premise that scalar prope rties in laminar diffusion flames are nearly universal functions of mi xture fraction. It has been well-tested and proven for temperature and the major species, however, few studies have addressed its applicabil ity for minor species especially the radical species. In this study, w e present a direct numerical simulation of an axisymmetric, laminar, m ethane-air diffusion flame to examine these universal relationships, i ncluding the major and minor chemical species, and the radical species . The numerical model solves the axisymmetric, time-dependent, reactiv e-flow Navier-Stokes equations coupled with sub-models for soot format ion and radiation transport, and includes a detailed reaction mechanis m for methane-air combustion. Quantitative comparisons with existing e xperimental data show a slightly wider computed flame compared to the experimental flame, however, the peak values and radial locations of t emperature 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 e ntire flame and compared with existing experimental measurements. Exce llent agreement is obtained between the computations and experiments f or temperature and mole fractions of CH4, O-2, OH, H2O, O, H, CO2 and N-2, as a function of mixture fraction in the fuel lean, stoichiometri c and fuel rich regions of the flame. The computations underpredicted the concentration of H-2 and CO in the fuel rich region, however, exce llent agreement is obtained in the fuel lean and stoichiometric region s. The computations also overpredict the concentration of CH3 in the s toichiometric and fuel rich regions, however, the peak concentration o ccurs at the same mixture fraction for both the experiments and comput ations. The scatter plots indicate that many of the species studied, i ncluding the minor species and radical species, can be considered univ ersal functions of mixture fraction, however some of the species show more scatter than others. The species which showed the least scatter w ere those whose production rates are 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 wit h mixture fraction.