Improving high enthalpy expansion tube condition characterisation using high speed imagery

Traditionally, Pitot rake test models have been used to take time and spatially resolved pressure measurements in impulse facilities for flow characterisation. These measurements allow the test section gas state to be inferred using analytical and more complex computational models. They also provide an indication of experimental test time and core flow diameter. However, being based on only one state variable, they are not definitive, and this gives motivation for developing methods based on other state variables. When expansion tubes are used for the study of hypervelocity planetary entry phenomena, generally the post-shock state in the test section strongly radiates, a phenomenon which is very sensitive to temperature. This paper provides a simple method which uses a high speed camera in addition to a conventional Pitot rake to improve estimates of experimental test time and core flow size by imaging post-shock radiative emission over the probes in the Pitot rake. This method can also be used with specific narrow bandpass optical filters to examine when the emission from key species remain constant in the test flow. The selection of and suitability of various optical filters for an air test gas is examined in this paper. Experimentally, it was found that the radiative emission generally rises quite abruptly at the end of the test time, and sometimes in situations where the pressure remains constant. It was also seen that different optical filters can show their abrupt rise at different times, giving differing values for the end of the test time. For the test condition studied, experiments performed at the entrance and exit of the test facility’s nozzle found that the test time in terms of flow radiative emission was similar in both cases