Experimental investigation of soot evolution in a turbulent non-premixed prevaporized toluene flame

The formation, growth, and oxidation of soot in turbulent prevaporized toluene diffusion flames stabilized on a jet-in-hot-coflow (JHC) burner are investigated in this study. Flame structure, local gas temperature as well as local soot volume fraction and primary soot particle diameter, are simultaneously detected by means of OH planar laser-induced fluorescence (PLIF), non-linear excitation regime two-line atomic fluorescence (nTLAF) of indium, and time-resolved (TiRe) laser-induced incandescence (LII), respectively. The collected data sets were used to generate joint statistics of soot properties and flame characteristics and provided new insights into the interaction of the OH layer and soot in turbulent flames. The interaction of OH and soot as a driving mechanism for soot oxidation is of particular interest as it has been proven to be challenging to model. Statistics of soot volume fraction and primary particle size in the OH layer are employed to gain deeper insights into the soot oxidation process. Mean soot volume fraction and primary soot particle size conditioned on temperature and OH signal intensity indicate that, due to differential diffusion of soot with respect to the chemical species, high soot volume fraction and primary soot particle diameter of up to 50 nm are present at low temperatures and low OH concentration. In the soot oxidation region, statistical analysis of the soot parameters disclose that clusters of high soot volume fraction mostly consist of large primary particles. Observations from instantaneous images and the presence of large primary particles inside the OH layer suggest that the oxidation is not sufficiently fast to burn the soot completely.