Revealing the Initial Chemistry of Soot Nanoparticle Formation by ReaxFF Molecular Dynamics Simulations

This work presents long time ReaxFF MD simulations of fuel-rich combustion for up to 10 ns to explore the initial mechanism of soot nanoparticle formation. A 24-component rocket propellant 1 (RP-1) model based on the major components of RP-1 fuel was employed. Simulations were performed by GPU-accelerated code GMD-Reax, and reactions therein were revealed with the aid of VARxMD. Simulated evolution of physical and chemical properties of the largest molecule exhibits the overall structural transitions of three stages for incipient ring formation, nucleation, and graphitization from fuel molecules to the formation of a single soot nanoparticle. The incipient ring formation takes place in stage 1 by large ring generation from activated aliphatic polyyne-like chains, ring number increase from internal bridging between carbon atoms of large rings, and consequent formation of PAH-like molecules with aliphatic side chains. Nucleation of a nanoparticle in stage 2 is the result of coalescence of PAH-like molecules, accompanied by ring closure reactions that occurred at side chains of the PAH-like molecules, and following formation of internal bridged bonds. Graphitization of the nanoparticle in stage 3 is dominated by the transformation from C5/C7 rings to C6 rings with C3 rings as intermediates. This work demonstrates that ReaxFF MD simulation might be a promising approach for qualitatively characterizing morphological evolution and the underlying chemical complexity of soot formation using multicomponent fuel models.