Determination of soot particle size using time-gated laser-induced incandescence images

A laser-induced incandescence (LII) image-processing method has been developed to determine two-dimensional polydisperse size distribution of soot particles as well as their monodisperse equivalent mean diameters. In this method, two appropriate intervals confined by time-gated LII images are chosen to calculate the corresponding signal decay times coupled with exponential fits. The local soot particle sizes are determined based on the best-fit comparison of experimental LII signal decay times to the simulated database. According to the method, the experimental results on a laminar diffusion ethylene/air flame show that the monodisperse equivalent mean particle sizes obtained are identical to those from point measurements, and in agreement with the calculations from experimental lognormal distributions of polydisperse soot particles. Nevertheless, remarkable discrepancies occur at some locations for both results compared to transmission electron microscopy (TEM) measurements of thermophoretically sampled soot. Thus, an attempt is made to explore the shielding effect of aggregates which may be responsible for the discrepancies, and narrower distributions are obtained with considering the effect. Besides, the findings of three LII images for simultaneously evaluating particle size distributions at various locations show the potential application of the method in practical turbulent flames.