Molecular Dynamics Simulation Of Soot Particles Generated From Octane Isomers Based On Reaxff Force Field

The carbon emission standards of countries all over the world are increasingly strict,but fossil fuels are still the current energy base.Octane isomers are the main substitutes of various fuels,and a deep understanding of soot evolution during their combustion is helpful for combustion efficiently and cleanly of fuels.In this paper,the molecular dynamics simulation of reactivity force filed（ReaxFF MD）was used to study the formation process of soot nanoparticles of three lightly branched octane isomers（2-methylheptane,2,5-dimethylhexane,3-ethylhexane）and n-octane at high temperature,and the influence of hydrogen on the formation of soot nanoparticles was analyzed under different hydrogen doping conditions.ReaxFF MD was used to study the reaction process evolution path,intermediate product change trend and oxygen action mechanism of the four octane isomers at high temperature.ReaxFF MD and CHEMKIN simulated that the molar fractions of C₂H₄,C₃H₆ and C₄H₈ were in the same order of magnitude and the change trends were consistent under the same conditions.It was found that the primary reaction process of the fuel molecule was dominated by the C-C bond cleavage within the molecule,and the bismethyl and monoethyloctane isomers exhibited similar chemical reactivity.It was observed that most of the oxygen molecules mainly reacted with C₃ and lower carbon molecular fragments below C₃.Based on the detailed reaction paths of four octane isomers,the formation time and mechanism of the largest soot nanoparticles in each stage were analyzed,and the evolution process from soot nanoparticles to primary soot particles was studied.It was found that the formation time of initial polycyclic species(C₁₆)and large PAH(C₃₁)decreased with the increase of methyl branch chain,but the time required for monoethyl was longer than that for monomethyl.C₃ could promote the formation of short carbon chain and initial polycyclic species more effectively than C₂ olefin species.The time for light branched isomers to generate large nano soot particles(C₁₀₀)accounts for about 43%of the whole reaction process,while the proportion of n-octane is as high as 51%.Finally,the formation trend of soot particles is 2,5-dimethylhexane≈3-ethylhexane>2-methylheptane>n-octane.In the process of primary soot particle formation,A parallel aggregation pattern similar to the AMLH mechanism is present throughout the stage,prompting the formation of stacked nanoparticles,while the low energy barrier reaction of the important chemical mechanism CHRCR allows rapid bonding between PAHs.Through the simulation under different hydrogen doping conditions（20%,40%,60%,80%and 100%of octane number）,it is found that only a very small amount of hydrogen indirectly affects the fuel molecular consumption.The increase of hydrogen slows down the subsequent dehydrogenation of olefins to alkynes,and slightly prolongs the time of C₁₆formation.The initial number of hydrogen doping is inversely proportional to the carbon content of the final soot nanoparticles,but directly proportional to the hydrogen-carbon ratio.The chemical effect of hydrogen per unit can reduce the final carbon number of soot nanoparticles produced by n-octane,2-methylheptane,2,5-dimethylhexane and3-ethylhexane by 0.46,0.45,0.53 and 0.48 respectively.