Determination of the volume fraction of soot accounting for its composition and morphology

Abstract

The soot volume fraction, fv, is essential for combustion engineering, air quality and climate modeling. It is commonly obtained from mobility or optical data assuming soot spheres of constant composition that tend to overestimate fv by up to an order of magnitude. Here, a method is presented for estimation of the fv from soot mobility or optical diagnostics data accounting for the realistic soot morphology and composition. With mobility data, the fv is determined by a relation for soot morphology (fractal-like structure) and the average primary particle diameter from microscopy, gas adsorption or power laws. Such fv and the corresponding soot volume/mass size distributions are in excellent agreement with accurate but tedious soot mass-mobility measurements in premixed flames along various heights above the burner (HAB) as well as from the exhaust of diffusion flames and diesel engines. With light extinction data, the fv is obtained by explicitly accounting for soot composition (through its optical band gap and refractive index, RI) and the light absorption enhancement (due to multiple light scattering within soot agglomerates) in the absorption function, E(RI). So as nascent soot matures by surface growth and agglomeration, its optical band gap decreases while its mobility diameter, carbon to hydrogen (C/H) ratio and fv increase, increasing the soot E(RI) from 0.22 to 0.40, in excellent agreement with laser induced incandescence data from flat premixed ethylene flames. Using this variable E(RI), the soot fv from light extinction data in these flames is up to 50% lower than that using standard constant E(RI) for soot spheres.