Investigating the apparent anomalous cooling of soot during laser-induced incandescence experiments

Abstract While time-resolved laser-induced incandescence (TiRe-LII) has become a standard laser-based diagnostic for soot, there remain unexplained observations in some datasets. One such effect is the so-called “anomalous cooling”, in which the pyrometric temperature decays faster than can be explained by conventional heat transfer models immediately following the peak temperature. This work investigates this phenomenon through experiments on soot entrained in different bath gases and irradiated in the low-fluence regime, where particle sublimation is minimal. The anomalous cooling phenomenon is caused by the contribution of particles in the probe volume that have been heated beyond the sublimation threshold to the overall incandescence signal, due to nonuniform laser fluence. Particles in these “hot spot” regions feature a faster cooling rate due to sublimation, contributing to the effect of apparent anomalous cooling. Particle-size polydispersity also plays a notable but minor role. The effect depends on the bath-gas composition, which is attributed to differences in species-specific heat transfer.