Investigation of soot particle morphology and nanostructure in n-heptane laminar diffusion flames at elevated pressures

The morphology and nanostructure evolution of soot particles in flames are crucial for understanding soot formation and evolution mechanisms. However, experimental data on soot properties in laminar diffusion flames of liquid fuel at elevated pressures remain scarce. This study investigates soot particle evolution in n-heptane flames at pressures ranging from 1.0 to 7.0 bar using thermophoretic sampling, focusing on soot morphology (primary particle diameter, aggregate size and fractal dimensions) and nanostructure parameters (fringe length, fringe tortuosity, fringe spacing). Additionally, a fringe extraction algorithm for high-resolution transmission electron microscopy (TEM) image is proposed, incorporating particle center pre-annotation and a chord search strategy for identifying fringes from the complex branched structure and low-contrast images. The results indicate that elevated pressure promotes soot growth, yielding a nanostructure with more graphitic order. This is manifested in larger primary particle diameters, increased aggregate sizes, longer fringe lengths, reduced fringe tortuosity and narrower fringe spacing. On the other hand, elevated pressure also shifts the transition between the soot growth zone and the oxidation zone further upstream (towards the burner tip). These findings provide fundamental data for elucidating soot evolution mechanisms in high-pressure liquid fuel combustion.