A reduced and robust reaction mechanism for toluene and decalin oxidation with polycyclic aromatic hydrocarbon predictions

In this work, a reduced toluene-decalin reaction mechanism containing 108 species and 566 reactions was proposed for computational fluid dynamics (CFD) simulations and polycyclic aromatic hydrocarbon (PAH) formation predictions in combustion engines. The present mechanism was validated with the available literature data including ignition delay time (IDT) determined in shock tubes and rapid compression machines (RCM), species mole fractions measured in premixed flames and jet stirred reactors (JSR), and laminar flame speeds. Moreover, a sensitivity analysis was performed on toluene and decalin flames to further investigate the main formation pathways of four representative PAHs: benzene (A₁), naphthalene (A₂), phenanthrene (A₃) and pyrene (A₄). In general, the simulation results exhibited a reasonable agreement with the experimental data. The negative temperature coefficient (NTC) behaviors of decalin IDTs were accurately reproduced by the proposed mechanism. The PAH species concentrations were also well captured for ethylene and benzene flames. The sensitivity analysis results indicated that the main formation reactions for PAHs have a strong link with ring structures. The decompositions of toluene and decalin primarily contributed to the formation of A₁. For toluene, A₂ was formed by the reactions A₁- + C₄H₃ = A₂ and IC₄H₅ + A₁ = A₂ + H₂ + H, while for decalin, the self-combination reaction of C₅H₅ became the main pathway. In addition, the reactions of the aromatic molecules and radicals significantly promoted the formation of A₃ and A₄ for both toluene and decalin.