Progress in characterization of soot formation by optical methods

Abstract

A two dimensional in situ optical technique is used to measure absolute soot volume fractions, particle number densities, and mean particle sizes in moderately sooting laminar and turbulent diffusion flames with high spatial and temporal resolution. These data are of special interest for the development and validation of models for the formation and oxidation of soot. The technique (RAYLIX) is based on the simultaneous two dimensional detection of Rayleigh scattering and the laser induced incandescence (LII) in combination with the detection of the integral extinction from one single laser pulse. All signals are induced by a single pulse of a frequency doubled Nd–YAG laser. Besides mean particle sizes it is of special interest to derive information about the particle size distribution by the detection of the temporal decay of the LII signal. Information about the particle size distribution can then be obtained by simulating this decay using a LII model in combination with multidimensional non-linear regression. With this strategy the parameters describing the particle size distribution as well as the temperature of the surrounding gas phase are varied so that the calculated decay of the LII signal is in accordance with the measured one. In addition to these quantities also probability–density functions (PDF), correlation functions and length scales are derived from the soot volume fraction in the turbulent flames. These quantities are of special interest for the modelling of turbulent reacting flows.