Séminaire exceptionnel IUSTI – 3 oct. 2024 – 14h salle 357

Mechanism and modeling of laser-induced annealing of young soot

Fengshan Liu – NRC Ottawa, Canada

Soot maturity has drawn considerable research attention in the last decade as a result of improved understanding of soot evolution from inception to mature aggregates. During this process, the H/C ration decreases and the degree of carbonization and graphitization increases. The physical and optical properties of soot also evolve continuously. The physical and chemical transformations of soot imply that it is questionable to assume a constant refractive index and a density of soot throughout a flame, either a premixed or a diffusion one. Due to the low absorption efficiency of young soot, it is challenging to measure its concentration or size using laser-based techniques. Experimental evidence has confirmed that both furnace heating and pulsed lasers can cause graphitization of soot and carbon black, suggesting that the absorption efficiency of young soot will increase after laser heating. This phenomenon has been termed annealing in the literature of pulsed laser irradiation of soot. By far, the laser-induced annealing has been modeled exclusively as a thermal process. In this presentation, the thermal induced annealing models proposed in the literature were used in an laser-induced incandescence (LII) model to simulate two recent experimental studies: the double-pulse experiments and cw-LII (continuous wave LII) experiments of young soot conducted at Lund University. The results suggest that thermal annealing models failed to reproduce the main features of experiments. A novel annealing model taking into account both photon-induced and thermal-induced annealing was proposed. The results from this proposed annealing model are in qualitative agreement with the experiments. It was also shown in this talk that the current soot aggregate heat conduction model strongly overpredicted the soot aggregate heat conduction cooling in modeling the cw-LII experiments. There is currently a lack of plausible explanation to the failure of the soot aggregate heat conduction model established in modeling pulsed LII. Ongoing research is purposed to provide solid evidence to support the assumption that photon-induced annealing plays an important role in laser-induced graphitization of young soot and to understand why the soot aggregate heat conduction fails in modeling cw-LII of young soot.