ReaxFF MD Investigation of the High-Temperature Combustion of Six Octane Isomers

Abstract

This work attempts to investigate the combustion mechanism, reactivity, and coking performance of six octane isomers using reactive molecular dynamics (ReaxFF MD) simulations and the VARxMD code. The kinetic analysis results were firstcompared with the experimental data to verify the validity of the simulation results. It was found that the combustion of six isomers mainly involves C−C bond cleavage, isomerization reactions, dehydrogenation, and oxidation. The cleavage of C−C bonds is the dominant route of initial reactions in combustion. While the isomerization reaction is a major distinction between normal and branched alkane isomers. The formation mechanism of coke precursors was further discussed by pathway analysis. The results showed that the existence of branch chains increases the possibility of the formation of coking precursors, and monoethyl isomers show the highest generation trend. The methyl substitution location also has an obvious effect on the formation mechanism of coke precursors. For meta-methyl isomers, coke precursors are formed by dehydrogenation of macromolecules at high temperature, while those are caused by collisions between carbon atoms for ortho-methyl isomers. The product distribution showed that the number of ethylene groups in n-octane combustion is larger than that of other isomer systems, while there is more CO formed in 2,2,3,3-tetramethylbutane combustion. For reactivity, the n-alkane is more reactive than other isomer systems, and increasing the number of methyl substitutions also enhances the reactivity of branched isomers for multimethyl isomers. Hopefully, the mechanism information obtained in this work could improve the understanding of differences in combustion of octane isomers and provide insights into the structural effect during the fuel combustion process at the molecular level.

Efficient Reaction Space Exploration with ChemTraYzer-TAD

“…We ran NVT simulations at temperatures of 1000, 2000, 3000, 4000, and 5000 K with the ReaxFF force field and parameters developed by Ashraf and van Duin, which have been used in numerous studies, for example, − with a time step of 0.1 fs. The simulations were thermostated with the Langevin thermostat with a damping constant of 10 fs.…”

Recent ReaxFF MD studies on pyrolysis and combustion mechanisms of aviation/aerospace fuels and energetic additives

“…And it can be observed from the number of isooctane pyrolysis products and intermediates that C2 species play an important role in the pyrolysis of isooctane. Liu et al 37 investigated and compared the combustion mechanism, reaction performance and coking status of six octane isomers. The results…”

“…And it can be observed from the number of isooctane pyrolysis products and intermediates that C2 species play an important role in the pyrolysis of isooctane. Liu et al 37 investigated and compared the combustion mechanism, reaction performance and coking status of six octane isomers. The results show that both n -octane and isooctane undergo the stage of isomerization during combustion, but the activity of isooctane decreases due to the existence of a branched chain compared with n -octane, and the trend of coking is obvious with the increase of the branched chain.…”

Oxidation and hydrogenation of monolayer MoS2 with compositing agent under environmental exposure: The ReaxFF Mo/Ti/Au/O/S/H force field development and applications

“…From the ReaxFF literature, a bond order cutoff of 0.3-0.5 is commonly used for the recognition of molecule fragmentation (Wang et al, 2020;Wang et al, 2022;Hahn et al, 2018;Liu et al, 2021). We determine the cutoff bond lengths through the bond scan technique with an increment of 0.1 Å, where a cutoff bond length is computed as the bond order between the two target atoms is reduced to 0.5.…”