Detailed soot field in a turbulent non-premixed ethylene/air flame from laser scattering and extinction experiments

Abstract

A soot-containing turbulent non-premixed flame burning ethylene in atmospheric-pressure air was investigated by conducting nonintrusive laser scattering and extinction experiments at various axial and radial flame locations. Mean soot properties of principal interest—soot volume fraction, spherule (primary particle) diameter, and aggregate size and fractal dimension—were characterized based on the Rayleigh–Debye–Gans scattering theory, which can properly account for the actual particulate size and morphology. In situ evidence for the formation of precursor particles and their carbonization to mature soot, as well as the onset of the aggregation process, was observed low in the flame. Maximum mean soot volume fraction and spherule diameter were about 1 ppm and 28 nm, respectively, both peaking at similar axial locations. The mean number of spherules per aggregate continuously increased along the flame centerline with the cluster–cluster aggregation mechanism leading to a fractal dimension of 1.8. Radial variations of mean soot field were also observed with the possible exception of the aggregate radius of gyration. Decoupling of spherule and aggregate sizes during the present analysis of optical measurements allowed the separation of soot surface growth, oxidation, and aggregation processes. Such accurate descriptions of soot dynamics in a lightly sooting turbulent flame are valuable in assessing computational soot models and other particulate diagnostics.