Spectroscopic Characterization of a High-Enthalpy Plasma Arcjet Tunnel

The characterization and quantification of thermochemical properties in high-enthalpy wind tunnels are a current challenge in the field of hypersonics. Various non-linear and coupled phenomena lead to highly complex flows that are difficult to characterize due to the limited availability of non-intrusive techniques able to measure thermochemical properties. Due to the lack of understanding of the underlying physics, spatially and temporally resolved measurements of the plasma characteristics in hypersonic flows are critical to ensuring accurate modeling of flow properties. In the current study, emission and absorption spectroscopy are applied to the plasma flow of the 500-kW arc-heated High-Enthalpy Tunnel at the University of Tennessee (Tenn-HET) using both air and nitrogen as working gas. The spectral range from deep UV to visible has been investigated to collect data on flow composition and temperature in both plenum chamber and hypersonic freestream. Comparison between the experimental spectra collected in the plenum chamber and the theoretical predictions provided by SPARK 3.0 line-by-line code (SPARKLbL) allowed to infer vibrational and rotational temperature of ionized diatomic nitrogen (N2+) using a spectral fitting method. The Mach 6 freestream was primarily investigated using UV absorption from nitrogen oxide (NO) in air flow. Despite a larger uncertainty due to the low density of the flow, an estimation of the freestream temperature was also obtained. Qualitative evidence of thermochemical non-equilibrium in the freestream was provided by the detection of emission from the N2 first positive system.