Effects of hydrocarbon impurities, vibrational relaxation, and boundary-layer-induced pressure rise on the ignition of H2–O2‒Ar mixtures behind reflected shock waves

Abstract

The results of an experimental and theoretical study of the ignition of H2–O2‒Ar mixtures behind reflected shock waves are reported. The experiments are performed with mixtures containing from 0.15 to 8.0% H2 and from 0.75 to 2.0% O2 at temperatures of 980–1800 K and a total gas concentration of (1.0 ± 0.1) × 10‒5 mol/cm3. The progress of the process is monitored by recording the time evolution of the pressure behind the reflected shock wave and the intensity of the chemiluminescence of electronically excited OH∗ radicals (λ = 308 ± 2.5 nm). A numerical model capable of predicting the effects of additional factors, such as hydrocarbon impurities, the vibrational relaxation of the test mixture, and boundary-layer-induced pressure rise, is developed and used to simulate our own and published experimental data on the ignition of H2–O2‒Ar mixtures. It is demonstrated that the best agreement between experimental and theoretical results is achieved when all the additional factors are taken into account. A sensitivity analysis shows that the effects of the vibrational relaxation of the test mixture and the presence of hydrocarbon impurities are significant only for lean mixtures, whereas the influence of the boundary-layer-induced pressure rise is important across a wide range of stoichiometries at long ignition delay times. Additionally, an analytical model is developed, which takes into account the finite time of the chain‒propagation reactions O + H2 and OH + H2. The predictions of the numerical and analytical models are demonstrated to be in close agreement for a wide range of mixture compositions and experimental conditions.