Using optically filtered high-speed imaging to characterise expansion tube operating conditions

Abstract

Traditionally, Pitot rake test models have been used to take time- and spatially resolved impact pressure measurements in impulse facilities for flow characterisation. When expansion tubes are used for the study of hypervelocity planetary entry phenomena, generally the post-shock flow in the test section strongly radiates. This paper outlines 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 remains constant in the test flow in both time and space. The selection and suitability of various optical filters for a high-enthalpy Earth entry scenario are examined in this paper, as well as the effect of radiation from flow contamination. 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 focusing on different spectral features can show their abrupt rise at different times, giving differing values for the end of the test time. Unfiltered measurements were found to be compromised by contaminant radiative emission which was seen to be randomly distributed in time and space towards the end of the test time.