An experimental and kinetic study the effect of nitrogen dioxide addition on autoignition of n-heptane

Vitiation effect is vital in the combustion-heated wind-tunnel experiments. NO2 is an active vitiated species which can affect the combustion properties of hydrocarbons. Ignition delay time experiments were performed to investigate the NO2 effect on the ignition behavior of n-heptane behind reflected shock waves. The experiments were carried out at pressures of 2 and 10 atm, and temperatures of 700- 1400 K with stoichiometric n-heptane/air mixtures. 0.5% and 1.0% NO2 were added to study the vitiation effect on n-heptane ignition. The NO2 effect depends significantly on temperature and NO2 concentration. NO2 addition can promote n-heptane oxidation and dramatically reduce the ignition delay time of n-heptane at high temperatures, but has negligible effect on the autoignition of n-heptane around 700 K. The promoting effect of NO2 is stronger with higher NO2 concentration at high temperatures of T > 860 K. However, at low temperatures of T < 860 K, 1.0% NO2 has less promoting effect compared to 0.5% NO2. A chemical kinetic mechanism for n-heptane/NO2 oxidation was proposed and validated against current experimental data. Good agreements were achieved under all experimental conditions. Sensitivity and reaction pathway analyses were performed to investigate the NO2 kinetic effect. NO2 + CH3 = NO + CH3O and H-abstraction from n-heptane by NO2 are dominant reactions for the NO2 promotion at both high and low temperatures, leading to a promoting effect of NO2 on n-heptane ignition. At low temperatures, C7H15 + NO2 = C7H15O + NO and N(O)2 + OH (+ M ) = HNO3 (+ M ) have strong inhibiting effect on n-heptane ignition, which will weaken the promoting effect of NO2 at low temperatures. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

Vitiation effect is crucial in combustion experiments, with NO2 playing a significant role in affecting the ignition behavior of hydrocarbons like n-heptane. The addition of NO2 can promote n-heptane oxidation and reduce ignition delay times at high temperatures, while having a minimal impact at lower temperatures. The effect of NO2 is dependent on temperature and NO2 concentration levels.