Kinetic Studies of Hydroxyl Radicals in Shock Waves. III. The OH Concentration Maximum in the Hydrogen-Oxygen Reaction

Abstract

The chemical reaction zone in shock waves in H2–O2–Ar mixtures has been studied in detail using an ultraviolet line absorption technique to measure the OH radical concentration. The observed reaction course consists of an induction period followed by a sharp rise of [OH] to a maximum and then slow disappearance of OH. The [OH] maxima measured in rich, lean, and stoichiometric mixtures at 1000∘≦T≦2600∘K and over a sevenfold range of density show that the reaction occurs in two distinct steps. The intermediate condition, in which [OH], [H], and [O] reach values greatly in excess of their equilibrium ones, is approximately a state of equilibrium among the three independent bimolecular reactions which is reached before any significant recombination has occurred. The relationship of these results to experiments near the explosion limits and in flames is emphasized. The occurrence of this overshoot in atom and free radical concentrations, which is very pronounced in hydrogen-oxygen flames but is not observed in many hydrocarbon flames, is related to the decrease in number of moles which occurs in the oxidation of hydrogen but not of most other fuels.