High-Temperature Infrared-Based Diagnostic for Nitric Oxide Using Tunable Diode Laser Absorption Spectroscopy

The formation of nitric oxide (NO) in high-enthalpy air behind shock waves provides an infrared-active target species with a well-defined rovibrational absorption spectrum that may be used to monitor non-equilibrium chemistry. Although prior studies using NO sensing have been conducted, there remains an unmet need for well-characterized experimental data at the extreme temperatures seen during hypersonic flight to further refine and validate theoretical models. This work seeks to develop and demonstrate an infrared-based, tunable diode laser absorption spectroscopy (TDLAS) diagnostic for in-situ temperature and NO number density sensing in high-temperature air that (1) extends the temperature range for NO diagnostics to that of interest for atmospheric reentry, (2) acquires rotational and vibrational temperature time-histories for high-temperature conditions, and (3) obtains NO species time-histories for high-temperature conditions. Measurements of NO number density and temperature were acquired using TDLAS behind reflected shocks in 2% NO diluted in argon (Ar) mixtures for temperatures ranging from 2000-5000 K at pressures of 0.68 and 1.53 atm. A single distributed feedback quantum cascade laser (DFB-QCL) was used to scan multiple lines in the fundamental band of NO near 5.2 μm. Specific transitions were selected based on prior use in high-enthalpy facilities and will be further refined for the particular test conditions considered in future work. For the low temperature experiments performed in this work, extracted rotational and vibrational temperatures and NO number density follow trends consistent with simulations. During vibrational relaxation there is an increase in vibrational temperature post-shock towards the rotational temperature at equilibrium, and rotational temperature and number density remain relatively constant due to heavy argon dilution. For the high-temperature experiments, inferred temperatures and number densities are consistent with simulations at early times, however, the rapid loss of absorbance signal obviates further quantitative inferences. Future work will consider lines optimized for high-temperature conditions to ensure sufficient absorbance levels across a wide range of test conditions. Further refinement of the diagnostic is expected to yield improved accuracy and increased time-resolution.