Analysis of the nanostructure evolution of soot in n-heptane/iso-octane with 2,5-dimethylfuran addition: A combined experimental study and ReaxFF MD simulations

The soot formation of n-heptane/iso-octane doped 2,5-dimethylfuran (DMF) with different ratios was experimentally investigated through laminar diffusion flame and numerically simulated by using the pyrolysis simulations method of reactive force field molecular dynamics (ReaxFF MD). The results showed that the laminar diffusion flame height of n-heptane/iso-octane increased with increasing DMF doping ratio. At a consistent sampling height, the primary soot particle sampling numbers collected were first decreasing and then increasing, and the size of the primary soot particle was first increasing and then decreasing accompanied by the DMF doping ratio increasing. Furthermore, the transmission electron microscope (TEM) analysis provided that DMF initially inhibited and subsequently promoted the primary soot particle growth in the n-heptane/iso-octane flame. The core-shell ratio increased and then decreased with the increase of the DMF doping ratio, which indicated that the maturity of soot decreased and then increased. In n-heptane/iso-octane doped with DMF ReaxFF MD pyrolysis simulation, it was divided into three stages 0–0.5 ns (the first stage), 0.5–3 ns (the second stage), and 3–4.5 ns (the third stage). Fuel decomposed in the first stage and reacted violently in the second stage. The soot precursors continued to react, molecule size kept to increase, and the polycyclic aromatic hydrocarbons (PAHs) continued to grow. In the third stage, the carbon number of the largest molecules were slowly increasing, and the development of soot entered mature stage, where the H/C ratio was slowly decreasing. Through ReaxFF MD simulations, it was found that the maturity of soot exhibited decreasing and then increasing trend with the increase of DMF doping ratio. The graphene-like structures were mainly concentrated around the H/C ratio of 0.297 region.