Single-beam and -detector laser-absorption velocimetry, non-equilibrium thermometry, and nitric oxide measurements at near-MHz rates in hypersonic air

This manuscript presents a quantum-cascade-laser-absorption-spectroscopy (QCLAS) diagnostic for measuring the rotational and vibrational temperatures, partial pressure, and velocity of nitric oxide (NO) in hypersonic flows at rates up to 500 kHz. Two fiber-coupled distributed-feedback QCLs and a 3D-printed optical probe were used to measure the aforementioned properties via a single retroreflected beam and a single detector. This approach was taken to minimize spatial averaging and sensor complexity while still providing a self-referenced calibration-free velocity measurement. Two 3D-printed optical probe designs are presented, and design guidelines for minimally invasive probes in hypersonic test facilities are discussed. The diagnostic was applied in the freestream of a reflected-shock tunnel for flow enthalpies of 3.8, 10.3, and 12 MJ/kg, which correspond to velocities near 3, 4, and 5 km/s. The quasi-steady flow conditions were characterized and compared to CFD predictions. In general, the measured quantities agreed relatively well with CFD predictions except for the vibrational temperature of NO and the observation of some high-frequency oscillations in freestream conditions. The high measurement rate of the diagnostic was critical to quantifying the latter.