Characterization of the T5 Reflected Shock Tunnel Freestream Temperature, Velocity, and Composition using Laser Absorption Spectroscopy

Abstract

We report on measurements of rotational and vibrational temperature, velocity, and concentrations of nitric oxide (NO), carbon monoxide (CO), water (H₂O), and potassium (K) in freestream flows generated by the T5 reflected shock tunnel facility at Caltech. Quantities were measured at a rate of 50 kHz by Tunable Diode Laser Absorption Spectroscopy (TDLAS) using four quantum cascade lasers (QCL) and one distributed feedback diode laser (DFB) simultaneously to sample a large collection of resolved rovibrational transitions of NO, CO and H₂O, and an electronic transition of K (D1). To isolate core flow measurements from the shear flow surrounding it, flow cutters (optical arms) were designed. Two sets of the optical arms with differing optical path lengths (13 cm and 23 cm) were used to assess the uniformity of quantities across the freestream core. Additionally, using angled beams (i.e. with nonzero flow-wise propagation), the flow velocity in the core was measured using laser Doppler velocimetry. A total of 9 experiments ("shots") were operated in this study, spanning 4 different nominal conditions with total reservoir enthalpies ranging from 8 MJ/kg to 21 MJ/kg, freestream densities ranging from 0.012 kg/m3 to 0.040 kg/m³, and flow velocities ranging from 3.5 km/s to 5.5 km/s. Measurements of the freestream generally suggest thermal equilibrium of the rotational and vibrational modal temperatures for both NO and CO. We compare temperature and NO concentration measurements with predictions from two-temperature, 5 species air simulations and find reasonable agreement at conditions with total enthalpy ~ 8 MJ/kg, but over-prediction in both temperature and NO concentration at conditions with total enthalpy ~ 18 MJ/kg. Potassium (K) was also detected in the freestream in many shots, enabling measurement of translational temperature and total static pressure from lineshape, but measurable concentrations were found to be transient and oscillatory.