A soot model for transient, spherically symmetric n -heptane droplet combustion

A soot model including soot nucleation, growth, and coagulation and O 2 and OH oxidation is presented for transient combustion of spherically symmetric n -heptane droplets. Influences of gas and soot radiation were included in the model. The various parameter values in the model were determined by using data from available results of experiments conducted in microgravity. Agreement between the measured and predicted transient soot volume fraction profiles was reasonably good. The model predicted maximum soot mass produced during the lifetime of a droplet very accurately. No adjustments were made in any of the model constants as runs were carried out for droplets of various sizes. Model predictions are also included for times extending beyond that measured in the experiments, i.e., beyond 0.8 s and for droplet diameters not covered by the experiments. Situations when the ambient oxygen concentration is changed or when the ambient temperature is increased were also considered. The soot field structure was elaborated by plotting the various rate terms. The soot nucleation term is broad in extent but much smaller in magnitude compared to the soot growth term, which is restricted to a comparatively narrow zone, situated closer to the droplet. The simulation also indicated that the total soot mass began decreasing as the flame contracted toward the droplet in the later stages of the droplet burning process, indicating the influence of soot oxidation and the importance of reduction in residence time for soot formation. To the authors' knowledge, this is the first article to discuss the development and validation of a comprehensive (i.e., inclusive of soot nucleation, growth, and oxidation) soot model for transient droplet combustion with elaborate comparison with experimental data.