| Combustion of fossil fuels provides around 85% of total energy supply for modern society, and meanwhile causes many environmental problems and social problems such as air pollution and energy crisis. Aromatic hydrocarbons are major components of fossil fuels, and are commonly used in jet surrogate fuels as well. Therefore investigations on the chemical kinetics of aromatic hydrocarbon combustion will help us understand the combustion behaviors of practical fuels and jet surrogates and predict key parameters of practical combustion processes. On the other hand, aromatic hydrocarbons have greater sooting tendencies than other hydrocarbons, making their combustion an ideal system to study soot formation mechanism. However, most previous stuides of aromatic hydrocarbon combustion focused on benzene and toluene which have the simplest molecular structures among aromatic fuels. The lack of studies on aromatic fuels with complex side-chain structures limits the development of kinetic models of jet surrogate fuels and the understanding of soot formation mechanism.In this dissertation, low-pressure laminar premixed aromatic hydrocarbon flames at broad ranges of equivalence ratio (lean, stoichiometric, and rich flames) were experimentally studied using synchrotron vacuum ultraviolet photoionization mass spectrometry. The fuels studied includes four C0-C2 mono substituted aromatic hydrocarbons (benzene, toluene, styrene, and ethylbenzene) and three C1 bisubstituted aromatic hydrocarbons (o-xylene, m-xylene, and p-xylene). Kinetic models of these fuels were developed to reproduce the experimental results. Rates of production (ROP) analysis were performed for the deep insight of the chemical kinetics of aromatic hydrocarbon combustion.Experimentally, photoionization efficiency spectra of all observed mass peaks were measured to identify the intermediate pools of 20 flames studied, with special interest on isomers and radicals. Quantum chemical calculations on molecules with unknown ionization energies (IEs) were performed to get the calculated IEs for intermediate identification. More than 80 intermediates with molecular weights between 15 and 168, including many oxygenated aromatic hydrocarbons, were detected in the lean aromatic hydrocarbon flames, while more than 100 intermediates with molecular weights between 15 and 240, including dozens of polycyclic aromatic hydrocarbons (PAHs), were identified in the rich aromatic hydrocarbon flames.Mole fraction profiles of major flame species and intermediates were evaluated from the spatial distributions of signals measured by scanning burner position at several photon energies, from which the variation trends of mole fractions with varying equivalence ratio were concluded. Comparing mole fractions of intermediates indicated the similarities and differences among the flames of different fuels, particularly showing the varying mole fractions of styrene, benzyl radical, benzene, and some typical PAHs with different fuels. Furthermore, temperature profiles of all flames studied were measured using a Pt-6%Rh/Pt-30%Rh thermocouple with 0.100 mm in diameter.In the modeling work, kinetic models of seven aromatic hydrocarbons were developed in the order of structural complexity of fuels from a classical kinetic model for hydrocarbon combustion (USC Mechâ…¡model), which were updated with many recently studied reaction pathways and newly reported rate coefficients of some key reactions. In particular, new fuel submechanisms of benzene, toluene, and ethylbenzene were constructed in this work; and it is the first time to report the flame models of styrene, o-xylene, m-xylene, and p-xylene among the combustion community. Validation of all models was performed by comparing experimental and modeling results in order to provide the accuracy of these models and the performance of model predictions.ROP analysis was the main approach to analyze the major reaction pathways in fuel decomposition and aromatic hydrocarbon growth processes. In fuel decomposition processes, some intermediates with lighter molecular weights than fuels, such as styrene, phenylacetylene, benzyl radical, benzene, phenyl radical, cyclopentadienyl radical, vinylacetylene, propargyl radical, and acetylene afforded most carbon flux and therefore became the major decomposition products. It was also validated that oxidation reactions and pyrolysis reactions controlled the fuel decomposition processes in the lean and rich aromatic hydrocarbon flames, respectively. In the aromatic hydrocarbon growth processes, phenyl radical, benzyl radical, phenylacetylene, styrene, and indenyl radical were found to be precursors of typical bicyclic and tricyclic PAHs such as indene, naphthalene, acenaphthylene, and phenanthrene, with the ring enlargement driven by addition of small intermediates like ethynyl radical, acetylene, propargyl radical, vinylacetylene, and cyclopentadienyl radical. According to the ROP analysis, major reaction pathways converting fuels to CO and forming PAHs were drawn in schemes of carbon fluxes.Similarities and differences among the chemical kinetics of different aromatic hydrocarbon flames were discussed. It was concluded that after the primary decomposition processes of fuels, most decomposition products and decomposition pathways played similar roles in different aromatic hydrocarbon flames. In the rich flames of most aromatic hydrocarbons, Hydrogen-Abstraction-Carbon-Addition (HACA) mechanism, resonantly stabilized radical addition mechanism, and dehydrogenation mechanism collaborated in the aromatic hydrocarbon growth processes. The predominant differences in chemical kinetics related to fuel structural features were mainly caused by the mole fraction variation of some key intermediates including phenyl radical, benzene, benzyl radical, phenylacetylene, styrene, and so on. Particularly in aromatic hydrocarbon growth processes, more complex side-chain structure the fuel molecule contains, more major PAH precursors and higher concentrations of PAHs its flame has, which interprets the relation of sooting tendencies of all aromatic hydrocarbons studied with the complexities of their molecules (Benzene |
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