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

The chemiluminescence originating from OH¹, 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 H₂–O₂, H₂O₂–H₂O, H₂–N₂O and H₂–O₂–N₂O. The experiments were analyzed with a detailed chemical reaction model which included mechanisms for OH¹ creation, quenching and emission. Simulations of the reaction behind reflected shock waves were used to predict OH¹ emission profiles and compare this with measured results as well as profiles of temperature and the ground state concentrations of OH. Analysis of OH¹ rates of progress demonstrates that a quasi-steady state approximation is applicable and an algebraic model for OH¹ concentrations can be derived that relates emission to the product of concentrations of O and H for H₂–O₂ and H₂O₂ mixtures and an additional contribution by the product of H and N₂O when N₂O is an oxidizer.