On the role of excited species in hydrogen combustion

Recently updated hydrogen combustion mechanism was combined with ozone decomposition reactions and extended by reactions of excited species: O(1D), OH(2Σ+), and O2(a¹Δg). The reliability and the accuracy of the rate constants pertinent to these excited species were evaluated. Many reactions proposed in the literature and implemented in other kinetic schemes were found irrelevant or insignificant. The new mechanism for hydrogen combustion was then validated against commonly accepted sets of laboratory experiments. It was expected that new reactions incorporated into the model should not affect its predicting ability for “thermal” combustion of H2, i.e. in the absence of excited species in the initial mixtures. The model validation showed that predictions of ignition, oxidation, flame burning velocities and flame structure of hydrogen-oxygen-inert mixtures are indistinguishable or very close to those of the basic mechanism at all condition, except for hydrogen oxidation in a flow reactor close to explosion limit. It was further demonstrated that singlet oxygen formed in reaction H2 + O2(1Δ) = H + HO2 at ppm levels may notably accelerate the process. Kinetic role of O(1D) and OH(2Σ+) in the “thermal” combustion of H2 was found negligible. In addition, hydrogen + air flame enhancement by singlet oxygen was modeled. It was demonstrated that the burning velocity increase with 1% of O2(a¹Δg) seeded into the air is rather modest. Moreover, purely thermal effect due to additional enthalpy brought to the mixture exceeds chemical flame enhancement by the singlet oxygen.