Combining information on particle size and optical properties for in situ detection of largely oxidized soot particles: A demonstration of feasibility in laminar coflow diffusion flames

The urgency of transitioning toward low-emission energy systems is driving the development of combustion technologies with minimal pollutant emissions. This includes soot, which consists of solid carbon nanoparticles produced under locally fuel-rich conditions as a result of formation and oxidation processes. Therefore, gaining a clear understanding of soot oxidation and controlling it may play an essential role in reducing the total amount of soot released by newly designed combustors. In this work, we investigate the possibility of detecting, along the flame, the presence of largely oxidized soot particles and of inferring the zones where oxidation is the predominant process. To this end, we combine information on particle size and optical properties from Laser-Induced Incandescence (LII) measurements. The feasibility of the approach is demonstrated for laminar coflow diffusion flames, using both experimental measurements and numerical simulations. First, a recently developed kinetic discrete sectional model accounting for the morphology and nanostructure of carbon nanoparticles is validated, for the first time, on two-dimensional laminar coflow flames. This is done by comparison with experimental data in terms of volume fraction fv, primary particle diameter dp, and gas temperature T0. Second, a proof-of-concept analysis is performed using the numerical source terms to identify and explain the relationship between the oxidation process and the particle characteristics, specifically particle size and composition. Finally, we show that the experimental absorption function E(m) qualitatively tracks the numerical evolution of particle composition and demonstrate that the combined use of dp and E(m) enables the identification of particles largely affected by oxidation and the localization of oxidation-dominated regions.