The temperature dependence of H + OH recombination in phosphorus oxide containing post-combustion gases

Abstract

The temperature dependence of the rate of H + OH recombination is experimentally investigated using laser photolysis of high-temperature (1500–2500 K) water vapor. Parametric adjustment of reaction rate constants to fit a simulated OH signal with the experimental OH decay data gives a three-body reaction rate constant for H + OH + M → H 2O + M that is larger than expected at low temperature and lower at high temperatures. This result could have negative consequences for hypersonic propulsion systems since much of the thrust producing radical recombination occurs at high temperature. When PH 3 combustion products are present in the photolyzed gas mixture at low concentration, the OH decay is much faster and dominated by catalytic reactions that include phosphorus oxides and acids. Parametric reaction rate analysis indicates that two reactions, H + PO 2 → HOPO and H + HOPO → H 2 + PO 2 are responsible for the increase in the overall rate of H + OH recombination. The temperature dependence of the phosphine catalyzed recombination reaction is adequately described by the rate constant estimates previously reported.