The evolution of soot morphology and nanostructure in laminar diffusion flame of surrogate fuels for diesel

- Morphology and nanostructure of soot in diffusion flame were investigated.
- The fuels include n-heptane, n-butanol and a n-heptane/n-butanol (1:1) mixture.
- The size of primary particles and aggregates and fractal dimension were measured.
- Fringe length, tortuosity and inter-fringe spacing of soot were calculated.
- Soot in n-butanol flame is least and has the greatest graphitic degree in flame tip.

The morphology and nanostructure of soot particles in laminar diffusion flame has been investigated in this work. The fuels considered were diesel surrogate fuels, including n-heptane, n-butanol and the volumetric mixture 50% n-heptane/50% n-butanol (H50B50). Thermophoretic sampling and TEM (transmission electron microscopy) analysis were utilized to study the morphology and nanostructure of soot particles at different points on the centerline inside the flames. The soot morphological characteristics include primary particle diameter, projected area of aggregate, and fractal dimension. The nanostructure is quantitatively described by fringe length, tortuosity and inter-fringe spacing.Overall, n-butanol flame produced smaller primary particles and aggregates, followed by H50B50, and n-heptane flames. The primary particle diameter shows a trend of increasing first and then decreasing due to soot growth and oxidation respectively. The fractal dimension of aggregates almost keeps increasing along the centerline, indicating more and more compact soot structure. As the particles travel along the flame, a graphitic 'onion-like' structure develops where the core remains amorphous and the outer shell graphitises. In the process of soot growth, the fringes become shorter and less curved, meanwhile inter-fringe spacing increases due to electron repulsion between the adjacent carbon layers. Subsequently, the degree of graphitization would increase, concretely, fringe length increases, while tortuosity and inter-fringe spacing decrease. At the flame tip, most microcrystalline carbon layers in the outer shell are oxidized, which causes a sharp increase in the tortuosity, but does not have obvious effect on the other two parameters. In addition, for mature soot near the flame tips, particles in n-butanol flame has the maximum fringe length and tortuosity while the minimum inter-fringe spacing.