The overestimation of O radicals in shock tunnel test flows and its effect on supersonic combustion

Reflected shock tunnels are often used for testing supersonic combustion in test flows representing hypersonic flight. The chemical composition of these flows may contain pre-existing radicals produced upstream in the nozzle supply region where the test gas is stagnated at extremely high temperatures and pressures. These radicals are seen as a key departure from truly matching flight conditions, however there is evidence that the concentration of one radical, atomic oxygen , is significantly overestimated. As has an incredibly strong impact on the ignition delay of fuels such as hydrogen, it is imperative that its concentration, in particular, is known with confidence. This work argues that the choice of reaction scheme is crucial when simulating reflected shock tunnel nozzle flows, with a common 5-species scheme artificially inflating concentration. By using a 6-species reaction scheme including NO 2 , the concentration of O is decreased by many orders of magnitude to trace amounts. This paper presents comparisons between 5- and 6-species air schemes for simulations of nozzles and subsequent combustion experiments, that indicate the importance of including additional oxygen-containing species, such as , in reflected shock tunnel nozzle simulations. • Atomic oxygen in the freestream of reflected shock tunnel test flows can significantly affect supersonic combustion experiments. • There is evidence that the concentration of atomic oxygen is overestimated by existing 5-species reaction schemes in tunnel nozzle calculations. • By using reaction schemes that include other oxygen-containing species, in this case NO 2 , the estimated atomic oxygen decreases to trace amounts. • The test flows of reflected shock tunnels are therefore more representative of flight conditions than previously thought, that is there are less radicals in the reflected shock tunnel freestream.