Effect of Toluene Content on Soot Particle Morphology and Evolution in Coflow Diffusion Flames of Diesel Surrogate Fuels

Toluene is a representative of aromatics in diesel surrogate fuels, and its influence mechanism on the formation and evolution of soot particles has attracted much attention. In this work, the evolution of the microscopic morphology of soot particles in n-heptane laminar diffusion flame doped with toluene of 0% to 30% was investigated experimentally, and the effects of toluene ratio on particle dynamics such as inception, surface growth and oxidation were analyzed numerically. The experimental results showed that as the toluene ratio increases, the primary particle size increases, and the aggregates first grows from short chains to long chains, then develops to multi-branched chains, which results in D-f first decreasing and then increasing, reaching the minimum at 10% toluene. The numerical results showed that the peak inception rate increases linearly with the increase of toluene ratio. The peak rate of HACA surface growth increases slightly due to the increase of particles that could provide active sites and reaches the maximum at ratio of 10% toluene, then decreases slightly due to the limited production of H and C2H2. On the other hand, the PAHs condensation rate increases monotonously, which makes the aggregates more likely to form a loose multi-branched chain structure, and in the subsequent oxidation process, the weak points of the connections between the particles are more likely to be oxidized, resulting in the fracture and fragmentation of the aggregates. For particles oxidation, toluene blending has an opposite effect on soot oxidation by O-2 and OH. As the toluene ratio increases, the O-2 oxidation rate increases gradually due to the increase of soot volume fraction, while the OH oxidation rate decreases due to the inhibited production of OH. However, the total oxidation rate of particles increases with the increase of toluene ratio.