Dynamics of excited hydroxyl radicals in hydrogen-based mixtures behind reflected shock waves

The chemiluminescence originating from OH*, the excited hydroxyl radical, is one of the most extensively used diagnostics to characterize auto-ignition delay time of gaseous mixtures behind reflected shock waves. We have carried out new experiments and modeling of this diagnostic as well as analyzed previous results for hydrogen-based mixtures, including H ₂-O₂, H₂O₂-H₂O, H ₂-N₂O and H₂-O₂-N₂O. The experiments were analyzed with a detailed chemical reaction model which included mechanisms for OH* creation, quenching and emission. Simulations of the reaction behind reflected shock waves were used to predict OH emission profiles and compare this with measured results as well as profiles of temperature and the ground state concentrations of OH. Analysis of OH* rates of progress demonstrates that a quasi-steady state approximation is applicable and an algebraic model for OH* concentrations can be derived that relates emission to the product of concentrations of O and H for H₂-O ₂ and H₂O₂ mixtures and an additional contribution by the product of H and N₂O when N₂O is an oxidizer.