Soot formation and oxidation in normal and inverse diffusion flames

A numerical study has been carried out to address the sooting characteristics of normal and inverse diffusion flames. The simulation was performed on the basis of single-step kinetics with a laminar flow assumption and non-unity Lewis number. Five different coaxial normal and inverse flame combinations with different momentum ratios have been investigated and compared. The results were experimentally validated using an identical burner with concentric cylindrical ports, where flame temperature measurements were employed for flame length comparison while a paper filter was utilized for soot concentration measurement. It was revealed that the momentum diffusion between the parallel streams governs the flame length and size. The results also showed that the single-step kinetics is able to predict the maximum soot formation in the normal flame annular region, provided that the Lewis number is less than unity. In inverse diffusion flames, the region of the soot surface growth is too small and the flow residence time is too short to allow significant soot production. For all combined inverse/normal diffusion flames, the soot emitted by the inner inverse flame had almost no influence on the total soot production. The thermal radiation by soot increased with the increase in both inner and outer flame lengths. The inner flame contributed to a 13 per cent increase in the total thermal radiation with keeping the same exhaust soot concentration level.