Quantum-cascade-laser-absorption-spectroscopy diagnostic for temperature, pressure, and NO X₂?_1/2 at 500 kHz in shock-heated air at elevated pressures

The design, validation, and application of a quantum-cascade-laser-absorption-spectroscopy diagnostic for measuring gas temperature, pressure, and nitric oxide (NO) in high-temperature air are presented. A distributedfeedback quantum-cascade laser (QCL) centered near 1976 cm 1 was used to scan across two transitions of NO in its ground electronic state (X2 5 1=2). A measurement rate of 500 kHz was achieved using a single QCL by: (1) performing current modulation through a bias-tee, and (2) targeting closely spaced transitions with a large difference in lower-state energy. The diagnostic was validated in a mixture of 95% argon and 5% NO, which was shock-heated to approximate to 2000 to 3700 K. The average mean percent differences between laser-absorption-spectroscopy (LAS) measurements and predictions from shock-jump relations for temperature, pressure, and NO mole fraction were 3.1%, 4.1%, and 6.5%, respectively. The diagnostic was then applied to characterize shock-heated air at high temperatures (up to -5500 K) and high pressures (up to 12 atm) behind either incident or reflected shocks. The LAS measurementswere compared to theoretical predictions fromshock-jump relations, pressure sensors mounted in the wall of the shock tube, and equilibrium values of theNOmole fraction. The average mean percent differences between LAS measurements and their aforementioned reference values were 3.2%, 10.8%, and 10.4% for temperature, pressure, and NO mole fraction, respectively. Last, a comparison between a measured NO mole fraction time history and a time-stepped homogeneous reactor simulation performed using two different chemical kinetics mechanisms is presented. (c) 2022 Optica Publishing Group