Analysis of Graphitic Crystallites Evolution of Soot Particles in a Diffusion Flame

Soot particles are one of the primary pollutants produced by the incomplete combustion of hydrocarbon fuels. To understand the fundamental mechanism of soot nanostructure evolution in a flame, the relationship between the soot behavior, i.e., inception, growth and oxidation, and variation of the soot nanostructure, was investigated experimentally. Soot particles in a laminar ethylene co-flow diffusion flame were analyzed non-intrusively by using laser spectroscopic techniques. Laser-induced fluorescence (LIF) and laser-induced incandescence (LII) were used to measure the spatial distribution of polycyclic aromatic hydrocarbons (PAHs), OH radical and soot. Time-resolved LII (Tire-LII) was applied to evaluate the soot primary particle size. Furthermore, soot was sampled along the flame axis and collected on a quartz–fiber filter. Internal nanostructure of the soot particle was characterized by laser Raman spectroscopy. The Raman spectra of the soot showed that the graphitic crystallite size contained in the soot particle was different at the sampling position. The graphitic crystallite size increases during the soot growth process. Then, the soot particle starts to shrink in the oxidation region, whereas the graphitic crystallite keeps increasing. Decreases of the graphitic crystallite size starts from the high OH concentration region.