Ignition of hydrogen and oxygen in counterflow at high pressures

Ignition of hydrogen and oxygen in the "third limit" was studied in counterflow for both premixed and nonpremixed systems with activation energy asymptotics. The results of the analysis were then compared to numerical calculations for further verification of the findings. In the analysis, simplifications to the chemical mechanism were formally derived, and criteria determining the accuracy of these simplifications were identified. It was demonstrated that the chemical steady-state approximations of O and OH are accurate for a wide range of conditions, while the steady-state approximations of H and HO₂ were both found to be inaccurate in certain regimes. Using these approximations, a reduced mechanism was derived and used to explain the ignition behavior of H₂ and O₂. It is shown that, for both the preximed and nonpremixed configurations, ignition occurs through the effect of heat release on the Arrhenius term of reaction 15, H₂O₂+M→2OH+M, and expressions were derived that are able to qualitatively predict the relationship between the pressure, temperature, and strain rate at ignition. The difference in ignition temperature between the premixed and nonpremixed configurations is associated with the effect of reactions between H and HO₂. For the premixed case, the rate of destruction of H through H+O₂+M→HO₂+M is much slower due to the lower concentration of O₂ in the reaction zone, thus reactions between H and HO₂ play a greater role in the ignition process.