Effects of oxygen dissociation on hypervelocity combustion experiments

Results of a comparative experimental study are reported which was conducted to measure the effects of the test gas oxygen dissociation produced in reflected shock tunnels on hypervelocity combustion data. An identical combustor model was tested in a reflected shock tunnel with test gas containing approximately 50% by mass of oxygen in dissociated form, as either atomic oxygen or nitric oxide, and in an expansion tube having test gas containing negligible dissociated oxygen. Comparisons were made at two test conditions which were energy equivalent to flight conditions at Mach 13.5 and Mach 17. Because combustion induced pressure rise in supersonic ducted flow has a complex dependence on many flow parameters, it is important to carefully define the test conditions for the two facilities. A method was developed and applied for selection of comparison test conditions in the shock tunnel such that differences in measured combustor duct pressures should be sensitive to dissociation enhanced heat release alone. Comparison of measured combustor pressure distributions shows a very similar wave structure is produced with a small but measurable increase in pressure rise due to dissociation occurring in the shock tunnel. The maximum absolute combustion pressure difference detected over a range of hydrogen-air equivalence ratios was 11%. This range of difference is also predicted by equilibrium combustion simulations and its small magnitude is a result of the relatively high combustor inlet temperatures (>2000 K°) at which the tests were run.