Determination of small soot particles in the presence of large ones from time-resolved laser-induced incandescence

Information about the polydispersity of soot can in principle be gained from time-resolved laser-induced incandescence (LII) using pre-assumed particle-size distributions. This paper introduces an alternative method, called two-exponential reverse fitting (TERF) that is based on combined mono-exponential fits to the LII signal decay at various delay times. The method approximates the particle-size distribution as a combination of one large and one small monodisperse equivalent mean particle size and does not require a distribution assumption. It also provides a ratio of the contribution of both size classes. The systematic error caused by describing LII signals by mono-exponential decays is calculated as less than 2% for LII signals simulated for monodisperse aggregated soot with heat-up temperatures for which evaporation is negligible. The effects of particle size, heat-up temperature, aggregate size, and pressure on this error are evaluated. The method is tested on simulated LII signals for lognormal and bimodal size distributions and applied to LII data acquired in a laminar non-premixed ethylene/air flame at various heights above burner. The results are compared to transmission electron microscopy (TEM) measurements of thermophoretically-sampled soot. The particle size of the large particle-size class evaluated with the method showed good consistency with TEM results, however the size of the small particle-size class and the relative contribution could not be compared due to missing information in the TEM results for small particles. These limitations of TEM measurements are discussed and the effect of the exposure time of the sampling grid is evaluated.