Combined schlieren and OH PLIF imaging study of ram accelerator flowfields

OH PLIF and schlieren imaging were applied to investigate shock-induced combustion phenomena on a 40° wedge in an expansion tube. OH PLIF was utilized to determine the regions of combustion in the flow field, while schlieren imaging provided complementary shock wave visualization. Stoichiometric H2/O2 gas mixtures, at three different levels of nitrogen dilution (75 %, 80%, and 85 %) were tested at two different test flow conditions in these experiments. A pressure transducer was mounted in the wedge to obtain a record of the surface pressure history on the model. Three test cases yielded shockinduced combustion behind an attached shock at the tip of the wedge. Depending on the sensitivity of the mixture employed, the flame front either rapidly converged with the shock, or slowly diverged away from it. The measured wave angles and surface pressures in these tests were, in general, well-modeled by shock-polar theory using frozen thermochemistry. Two other test cases, using the most sensitive gas mixtures, produced a closely-coupled flame front behind a detached shock wave near the wedge tip. The measured surface pressure in this latter case was better modeled by a shock polar using equilibrium chemistry. Simple finite-rate chemistry modeling of the ignition zone agrees well with the experimental results in all cases.