Effect of Fuels and Oxygen Indices on the Morphology of Soot Generated in Laminar Coflow Diffusion Flames

The morphological characteristics of soot are of primary importance to quantify its effect on climate forcing and human health and also to interpret the signals acquired in optically based soot diagnostics. In the present study, the morphology of soot particles produced in laminar coflow diffusion flames was investigated under different fuel and oxidizer conditions. Particles were sampled thermophoretically at the centerline at different heights above the burner in laminar diffusion flames of three common fuels, namely, ethylene, propane, and butane, at two sooting statuses, i.e., under the smoke point and at the smoke point. The oxygen content in the oxidizer stream was systematically varied. Different morphological parameters of mature soot, including primary particle number, aggregate radius of gyration, fractal dimension and prefactor, and overlapping, were obtained based on analysis of transmission electron microscopy images. The diameter and number of monomers were calculated by using automated methods based on the Euclidean distance mapping and the relative optical density methods, respectively. The other parameters were derived from these two parameters and some empirical correlations. The fractal dimension under different fuel and oxidizer conditions falls in a relatively narrow range from 1.68 to 2.05, while the derived fractal prefactor varies significantly from 0.24 to 2.66. An exhaustive comparison of the morphological parameters obtained in this study with the literature data suggests that our results fall within a range similar to those reported in previous studies. The present results suggest that the soot morphology does not display a clear trend of variation with the change in oxygen index for all three hydrocarbon fuels studied, at least for the fractal dimension and primary particle diameters obtained in this study.