Characteristics of soot formation in laminar co-flow diffusion flames of n-heptane/anisole blends under preheating reactant temperature

In the present work, the characteristics of soot formation in laminar co-flow diffusion flames of n-heptane doped with anisole under different preheating reactant temperatures were experimentally investigated. The combined diagnostics method of the planar laser-induced incandescence (LII) and the line-of-sight attenuation (LOSA) was employed to measure the soot volume fraction and soot yield in flames under varying preheating co-flow air temperatures. Results show that preheating co-flow air leads to the enhancement of the intensity of flame luminosity, but the flame luminous intensity captured by the 410-nm filter first increases and slightly decreases with anisole content at a relatively high temperature (e.g. 573 K). The enhancement effect of preheating temperature on the peak axial soot volume fraction in n-heptane/anisole flames is weaker than that in n-heptane flames and it weakens with the anisole content, but radially-maximum soot volume fraction along the wing pathline has a different evolution with the effect of preheating temperature and anisole content. Furthermore, the evolution of the maximum soot volume fraction, the volumetrically integrated soot volume fraction and the maximum soot yield is assessed to manifest that the sooting propensity of n-heptane/anisole flames is promoted by preheating temperature due to the promotion of fuel decomposition and PAH formation. The temperature sensitivity of the sooting propensity of n-heptane/anisole flames is higher than those of n-propylbenzene and 1,2,4-trimethylbenzene flames, and it presents a non-monotonic evolution trend with anisole contents which may be attributed to the combined effect of n-heptane, anisole and nitrogen (carrier gas) on the fuel pyrolysis and the reactions of PAH formation.