A time-resolved temperature measurement system for free-piston shock tunnels

This work presents the experimental preliminary results for a water-vapor based tunable diode laser absorption spectroscopy sensor to be used in a free-piston shock tunnel. The sensor uses free-space propagation of light from a vertical-cavity surface emitting laser (VCSEL) scanned at a rate of 50,000 measurements per second to determine the temperature in a hypersonic flow. The experiments were carried out in the T-ADFA free-piston shock tunnel at the University of New South Wales in Canberra, Australia. Three different flow enthalpies were examined, 13.6 MJ/kg, 10.56 MJ/kg, and 3.77 MJ/kg. Temperatures were extracted from the spectra using two methods: two line integrated absorption and nonlinear least squares fit. Both methods showed significant variations from tunnel run to the next, with standard deviations ranging from ± 36 to ± 183 K, depending on condition. Comparison of the two methods showed a discrepancy that is consistent with the nozzle flow transitioning from under expanded to over expanded flow during the test. Lastly, the nonlinear least squares fits showed an inconsistency in the HITRAN 2012 calculated strength of one of the lines.