Detailed numerical study of soot surface growth and oxidation in laminar diffusion flames

he focus of the present study is to obtain detailed knowledge of the soot formation and oxidation processes in laminar diffusion flames. The present work studies the effects of various flame properties on soot growth and oxidation, and how they affect a flame’s sooting behaviour. Numerically modelling of soot formation in laminar coflow diffusion flames of vaporized gasoline/ethanol blends at atmospheric pressure is performed. The numerical results are compared with experimental data to gain improved understanding of ethanol addition to gasoline on soot formation. Four gasoline/ethanol blends are investigated to quantify how soot loading varies with the amount of ethanol blending in the fuel. The results of experimental and numerical modelling agree relatively well in terms of the levels of soot volume fraction. Both results show a decrease in soot loading as more ethanol is added in the fuel stream. The work continues by numerically studying the oxidation of soot in laminar ethylene/air coflow diffusion flames. A new model for soot oxidation, a complex process in numerical soot modelling, is developed based on the observation that soot ageing reduces surface reactivity. Using this new model, it is possible to capture the correct behaviour of both smoking and non- smoking flames in various flame configurations. Along with a detailed soot sectional model, the new model predicts the correct soot volume fractions, smoke emission characteristics, and primary particle diameters for different flames without any variation in model parameters. The work extends to study soot surface reactivity in the growth and oxidation regions. Laminar ethylene/air and methane/air coflow diffusion flames are numerically studied to develop a unique soot surface reactivity model. A newly developed surface character model simultaneously accounts for soot surface reactivity in surface growth and oxidation by considering soot ageing and its effects on the particle surface. The new model, which eliminates tuning of one modelling parameter, reconciles the quantification of the evolving soot surface character for both growth and oxidation. The model is shown to be uniquely capable of predicting soot concentrations and smoke emissions within experimental uncertainty in a wide range of laminar diffusion sooting flames.