Modeling soot formation and burnout in a high temperature laminar diffusion flame burning under oxygen-enriched conditions

A simplified model of soot formation, based on a laminar flamelet approach and developed, in earlier studies, with a view to subsequent turbulent flame prediction, is extended to include oxidation. The model is evaluated against detailed measurements in a two-dimensional laminar diffusion flame on a Wolfhard-Parker burner. The freestream compositions are modified to raise the stoichiometric condition (ζst = 0.475), thereby adjusting the flame shape to make the burn-out regime readily accessible to measurement and to substantially raise the temperature levels throughout the flame—peak measured temperature was ≈ 2550 K. These temperatures are more relevant to many practical systems than those typically realized in small-scale laminar flame experiments and therefore provide some insight into the extrapolation necessary for practical application. The principal oxidizing species is shown to be the hydroxyl radical, introduced into the prediction from a flamelet calculation as a function of the mixture fraction, the characteristic scalar variable for the complete composition field. The simplified representation of sooting processes in terms of volume fraction and number density is then readily adapted to incorporate soot oxidation and is shown to satisfactorily embrace the enhanced temperature range.