Towards a kinetic understanding of the NO _x sensitization effect on unsaturation hydrocarbons: A case study of ethylene/nitrogen dioxide mixtures

Ignition delay time (IDT) measurements for ethylene (C ₂ H ₄ )/nitrogen dioxide (NO ₂ )/oxygen/argon mixtures with various blending ratios of NO ₂ /C ₂ H ₄ ?=?0/100, 15/85 and 50/50 were performed in a shock tube. The sensitization effect of NO ₂ on C ₂ H ₄ ignition was investigated at pressures ranging from 1.2 to 10 atm, in the temperature range 920-1780?K, and at equivalence ratios from 0.5 to 2.0. Similar to our recent studies of NO ₂ effects on CH ₄ and C ₂ H ₆ ignition (Deng etal., 2016; Deng etal., 2017), the effect of NO ₂ addition on measured IDTs of the unsaturated hydrocarbon C ₂ H ₄ also exhibits both temperature- and pressure- dependences. NO ₂ addition generally promotes the reactivity of ethylene and the promotion effect is more pronounced for the mixtures with high NO ₂ content at higher pressures (p?>?4.0 atm), at lower temperatures (T?<?1200?K) and for fuel-rich mixtures. However, NO ₂ shows only a limited effect at lower pressures (p?<?4.0 atm) and at higher temperatures (T?>?1250?K). A detailed kinetic mechanism of C ₂ H ₄ /NO ₂ was proposed by refining critical reactions through incorporating ab-initio calculations from this study and recently published rate constant data. This mechanism is capable of reproducing the new data reported in this work and literature ones as well and has thus been used to perform sensitivity and flux analyses to kinetically interpret the interaction of NO ₂ with C ₂ H ₄ .