Effect of Fuel Molecular Structure and Premixing on Soot Emissions from n‐Heptane and 1‐Heptene Flames

Most liquid fuels contain compounds with one or more unsaturated CC bonds. Previous studies have observed that the fuel reactivity and ignition behavior are strongly influenced by the presence and number of double bonds in the fuel molecular structure. Here, we report a numerical investigation on the effect of fuel unsaturation on PAHs and soot emissions in partially premixed flames (PPFs) burning n-heptane and 1-heptene fuels. A detailed soot and fuel oxidation model is validated against gaseous species measurements in n-heptane PPF and soot measurements in ethylene diffusion flames. Simulations are performed to examine the effects of double bonds on PAHs and soot emissions at different strain rates and levels of premixing. For both fuels, the global flame structure is characterized by a rich premixed reaction zone (RPZ) on the fuel side and a nonpremixed reaction zone (NPZ) on the oxidizer side. PAHs and soot are mainly formed in the region between the RPZ and stagnation plane. The presence of double bonds results in higher amounts of C2H2 and PAHs, and thereby significantly higher soot emission in 1-heptene flames than that in n-heptane flames. As the level of premixing is reduced, spatial separation between the two reaction zones decreases, while C2H2 and PAH concentrations, and therefore soot emission, increase. The effect of fuel unsaturation on PAH and soot emissions becomes more pronounced as the level of premixing and/or the strain rate is reduced. A reaction path analysis was performed to identify the dominant routes for the formation of acetylene, benzene, and pyrene. Acetylene and benzene are known to be important precursors for larger PAH species, while acetylene also plays an important role in soot surface growth through the HACA mechanism. The analysis indicated that the major route for benzene formation in the RPZ is through the recombination reaction of propargyl radicals, which are mostly formed from allyl radicals. The other route is through the reaction of vinyl with butadiene. The presence of a double bond leads to higher concentrations of propargyl and butadiene and thus increased benzene formation in 1-heptene flames relative to that in n-heptane flames. The presence of a double bond also increases the amount of C2H2 formed in 1-heptene flames due to the higher C4H5 concentration. Thus, the presence of a double bond promotes β scission reactions, leading to the increased production of C2H2, C6H6, and C16H10 and thus higher soot emissions in 1-heptene flames.