Fractal Morphology Analysis of Combustion-Generated Aggregates Using Angular Light Scattering and Electron Microscope Images

Abstract

Experimental studies of the fractal morphology of flame-generated aggregates are described here, considering not only the fractal dimension, D f , but also the fractal prefactor (lacunarity), k g , both of which are shown to be needed to fully characterize aggregates. Measurements were made using angular light scattering (ALS) and thermophoretic sampling followed by transmission electron microscopy (TEM) for soot aerosols found in laminar and turbulent flame environments. D f and the prefactor k g were simultaneously inferred from ALS measurements using the optical properties of aggregates composed of small primary particles. TEM-based inferences of these fractal properties involve analysis of aggregate-projected images from which the actual morphologies are obtained by correlating the radius of gyration to the outer radius (half of the maximum length) of an aggregate. Both of our procedures for determining the detailed morphology of aggregates yield D f = 1.7 ± 0.15 and k g = 2.4 ± 0.4 for carbonaceous soot, in good agreement with earlier TEM measurements involving multiple angle images. Furthermore, we show that these values also seem to be valid for other materials such as alumina aggregates, suggesting that not only the fractal dimension but also the fractal prefactor are universal properties of aggregates found in combustion environments due to similar mechanisms of aggregation. This universality for D f and k g is observed at various positions in four different flame types with nine various gaseous and liquid fuels for aggregates' with mean primary particle radii and number of primary particles in aggregates in the range 12-26 nm and 2-10 4 , respectively.