Rate Coefficients Of Polycyclic Aromatics Hydrocarbons Oxidation Reactions At High Temperatures:Methods And Applications

Polycyclic Aromatic Hydrocarbons(PAHs)referring to a class of organic compounds with multiple aromatic rings,and being formed during the incomplete combustion of hydrocarbons,are recognized as the precursors of soot particle formation as well as the major components of soot chemical structure.The reaction kinetics studies of PAHs under combustion conditions are essential for constructing the detailed growth and oxidation reaction models of PAHs and soot.In the meanwhile,due to the large molecular weights of PAHs,their unique reaction kinetics are also very important for understanding other large molecular systems.In this dissertation,the oxidation reaction kinetics of PAHs under high temperatures(1500-2500 K)have been systematically investigated at both microscopic and macroscopic scales,through the use of quantum chemistry calculations,master equation modeling and molecular dynamics simulations.Firstly,the thermal decomposition reactions of PAH oxyradicals were investigated.The potential energy surfaces were found to have the characteristics of multi-wells and multi-channels,containing a class of shallow wells existing.Moreover,due to the large molecular weights of PAHs,most of the thermalized PAHs occupy very high energy levels well above the reaction thresholds at high temperatures.Consequently the microcanonical reaction rates are close to or even far above the rates of collisional energy transfer,leading to the overlapping of time scales of chemical reactions and collisional energy transfer processes.These characteristics have brought several difficulties for calculating the phenomenological reaction rate coefficients.Based on the assumption that the shallow wells are in microcanonical equilibrium with the neighboring wells,a shallow-well merging method has been developed to eliminate the interference of shallow wells on calculating the phenomenological reaction rate coefficients.Then for the single-well dissociation reactions,the effective rate coefficients have been determined by an optimal fitting method developed in this work.Further investigations indicate that these PAH kinetics problems stem from the high temperatures,the very high densities of states,the initial Boltzmann distributions,as well as the collisional energy transfer rates.In view of the significant influence of collision energy transfer on PAHs reaction kinetics at high temperatures,the Lennard-Jones(L-J)parameters of PAHs were theoretically calculated for computing collision frequencies,and the trajectory simulations of collisional energy transfer of PAHs were performed for obtaining accurate energy transfer parameters.The spherical average method for calculating L-J parameters,and the canonical sampling method for selecting initial vibrational energy in the trajectory simulations of collisional energy transfer were found be suitable for PAHs.The dependences of collisional energy transfer on PAHs sizes,structures as well as the types of bath gases were investigated.The results indicate that the dependence of L-J parameters on PAHs structures could be revealed more significantly by theoretical methods than the empirical predictions.Finally,the oxidation reaction kinetics of PAHs containing a five-membered ring under high temperatures were investigated through the use of the phenomenological rate coefficients calculation method and the L-J parameters and energy transfer parameters obtained in this dissertation.The potential energy surfaces were calculated by density functional methods,and the master equation modeling was performed to obtain the temperature and pressure dependent phenomenological rate coefficients via the optimal fitting method.In addition,the influence of collisional energy transfer parameters on the reaction kinetics was discussed.