Role of O₂ on nitrous oxide fuel blend ethylene auto-ignition sensitivity

An experimental and kinetic modeling study of the influence of O₂ addition on the ignition behaviors of Ar diluted nitrous oxide/ethylene blend was performed. Ignition delay time (IDT) measurements were taken by detecting pressure evolution profiles in a heated rapid compression machine (RCM) at temperatures and pressures ranging from 910–1500 K to 35–45 bar. Results show that the addition of O₂ significantly promotes the ignition of N₂O/C₂H₄/Ar blend. With the O₂ blending ratio increase from 10 % to 100 %, the IDT first decreases and then increases. Subsequently, the IDT data were employed to validate several kinetic models in literature. A reaction model developed in our previous work [Combustion and Flam, 2020, 221: 64-73] can reproduce the measured data and predict the promotion effect of O₂ on N₂O/C₂H₄/Ar blend well. Reaction pathway and rate of production (ROP) analyses show that high decomposition temperature of N₂O inhibits the ignition of N₂O/C₂H₄/Ar blend, whose ignition is mainly driven by reactions of N₂O (+M) = N₂ + O (+M) and N₂O + H = N₂ + OH. The dominant pathway is chain branching process: C₂H₄ + H/OH→C₂H₃ (+M)→C₂H₂ + O→HCCO + H₂→CH₂CO + H→CH₃CO (+M)→CO + CH₃. However, the presence of O₂ in mixture induces reaction flux changes and decreases the global activation energy for chemical reaction. C₂H₃ radical is the main intermediate product during the ignition initial stage of C₂H₄ blends, whose ROP can reflect ignition process of C₂H₄ blends. For N₂O/C₂H₄/O₂/Ar and C₂H₄/O₂/Ar ignition, C₂H₃ radical pool can be rapidly established via H-abstraction reaction of C₂H₄. However, the reaction of N₂O + H = N₂ + OH enhances the ROP of OH radical in N₂O/C₂H₄/O₂/Ar blends, resulting in C₂H₃ radical pool rapid establishment and a faster ignition.