Spectroscopy of high speed expanding argon flows

Abstract

Experiments were conducted to study excited state populations of a gas undergoing a steady expansion. The X2 expansion tube located at the University of Queensland, Australia produced hypersonic test flows for the study. A simple two-dimensional wedge geometry was selected to simulate the flow structure found in atmospheric reentry. Argon was used as a test gas to model the evolution of a monatomic species as it is rapidly heated through a shock wave followed by an expansion fan. An initial campaign demonstrated the measurement technique using two spectrometers and a high speed camera. The spectra were analysed to extract excitation temperatures. These varied from 10000K to 15000K from the shock to the expansion fan, \nfollowed by the excitation temperature freezing. Numerical simulations of the two-dimensional wedge were also conducted and showed a much lower starting excitation temperature, with a steady rise until the expansion fan and a freezing effect after the expansion fan. Interesting precursor radiation phenomenon was seen in the free-stream flow before the shock. A new higher density argon condition has been developed for probing the expansion fan further to measure the gas as it expands around the rear of the vehicle. Increased density allows measurements of a more exhaustive spectral region, extending from 300nm to 4000nm, greatly increasing the number of energy levels mapped, and information gained on the state of the gas. Continuum radiation has been detected and analysed in an attempt to infer the temperature of the electrons. Precursor radiation has been detected more strongly in the higher density cases.