Two-stage oxidation phenomenon of n-propylbenzene at elevated pressure

n-Propylbenzene (A1C₃H₇) is a representative aromatic component of surrogate fuels for jet fuels. In order to reveal low-temperature oxidation characteristics of A1C₃H₇ under high pressure, the experimental and modeling studies on the oxidation of A1C₃H₇ in a jet-stirred reactor were conducted under equivalence ratios (Φ) of 0.4, 1.0, and 2.0, temperatures of 535–1015 K, and a pressure of 12.0 atm. The oxidation species were detected and quantified using GC and GC–MS. Key products, including 1-propenylbenzene, styrene, and benzaldehyde, were identified in this study. Two-stage oxidation behavior of A1C₃H₇ was observed at Φ = 0.4, which has not been previously reported. This behavior is characterized by a notable, nearly constant slope of fuel consumption at lower temperature as the temperature increases, followed by a pronounced acceleration of fuel consumption at higher temperature. A comprehensive kinetic model, comprising 323 species and 2017 reactions, was developed and validated using experimental data related to oxidation, pyrolysis, ignition delay times, and laminar burning velocities. The model reasonably reproduces these experimental results. Rate-of-production analysis reveals that the main consumption pathways of A1C₃H₇ involve H-abstraction by OH radicals at the propyl group side. Under fuel-lean condition, low-temperature chain-branching pathways lead to the generation of OH radicals at lower temperatures, driving Stage I oxidation of A1C₃H₇. As temperature rises, the main channel for OH production shifts to the reaction H₂O₂(+M) = 2OH(+M), leading to a rapid increase in OH radicals and an accelerated consumption of A1C₃H₇ during Stage II. Sensitivity analysis indicates that the most significant reaction promoting A1C₃H₇ consumption during Stage I is H-abstraction by OH radicals forming 1-phenylpropyl radicals. In Stage II, this trend shifts to H₂O₂(+M) = 2OH(+M). A1CHOOCH₂CH₃ = A1CHCHCH₃ + HO₂ and CH₃O₂ + HO₂ = CH₃O₂H + O₂ are identified as the most inhibiting reactions for A1C₃H₇ consumption during Stage I and Stage II, respectively. The experimentally observed two-stage oxidation of A1C₃H₇ reveals that the alkylbenzene with single C₃ substituent can undergo pronounced low-temperature chain-branching channels.