High-speed laser-absorption measurements of non-equilibrium nitric oxide in the Sandia Hypersonic Shock Tunnel

This manuscript presents a quantum-cascade-laser-absorption-spectroscopy (QCLAS) diagnostic for the partial pressure and internal temperatures (rotational and vibrational) of nitric oxide (NO) in hypersonic flows. Two quantum-cascade lasers (QCLs) were used to measure four transitions of NO near 1887 cm\(^{-1}\) and 1930 cm\(^{-1}\) at 25 or 100 kHz using scanned-wavelength direct absorption. Tests were performed in the Purdue High-Pressure Shock Tube (HPST) using an NO–Ar mixture to confirm the accuracy of the diagnostic. The diagnostic was then applied to characterize the Hypersonic Shock Tunnel (HST) at Sandia National Laboratories. In the HST, two flow cutters were used to direct the measurement line-of-sight through the quasi-uniform core flow exiting the nozzle, thereby avoiding measurement complications associated with the thick boundary layers at the nozzle exit. In the HST, tests were performed with air velocities of 3, 4, and 5 km/s where the rotational and vibrational temperature of NO varied from 150 to 850 K and the partial pressure of NO was near 20 Pa. Additionally, dry bottled air and humid room air were used as test gases to quantify the impact of water contamination on the vibrational non-equilibrium of NO. Comparisons with two CFD predictions using unique rate constants for vibrational relaxation are also presented. The vibrational non-equilibrium of NO was more pronounced for 3 km/s tests, and water had a negligible impact on the thermal non-equilibrium of NO. Lastly, the measured rotational temperature of NO agreed well with CFD predictions, the measured partial pressure of NO was consistently above CFD predictions, and the vibrational temperature had moderate agreement with CFD predictions for 4 and 5 km/s tests, and poor agreement for 3 km/s tests.