High-temperature vibrational relaxation and decomposition of shock-heated nitric oxide: II. Nitrogen dilution from 1900 to 8200 K

Abstract

This work investigates the high-temperature vibrational relaxation and decomposition of nitric oxide (NO) diluted in nitrogen (N2) to target the NO–N2 rates relevant to high-temperature air, thereby building off the argon (Ar) experiments investigated in Part I. [J. W. Streicher et al., “High-temperature vibrational relaxation and decomposition of shock-heated nitric oxide. I. Argon dilution from 2200 to 8700 K,” Phys. Fluids 34, 116122 (2022)] Again, two continuous-wave ultraviolet laser diagnostics were used to obtain quantum-state-specific time histories of NO in high-temperature shock-tube experiments, including absorbance (α) in the ground vibrational state of NO, translational/rotational temperature (Ttr), and number density of NO (nNO). The experiments probed mixtures of 2% and 0.4% NO diluted in either pure N2 (NO/N2) or an equal parts N2/Ar mixture (NO/N2/Ar). The NO/N2 experiments spanned initial post-reflected-shock conditions from 1900–7000 K and 0.05–1.14 atm, while the NO/N2/Ar experiments spanned from 1900–8200 K and 0.11–1.52 atm. This work leveraged two vibrational relaxation times from Part I (τVTNO−Ar and τVTNO−NO) and extended measurements to include the vibrational–translational and vibrational–vibrational relaxation times with N2 (τVTNO−N2 and τVVNO−N2). Similarly, this work leveraged the four rate coefficients from Part I (kdNO−Ar, kdNO−NO, kfN2O, and kzNO−O) and extended measurements to include NO dissociation with N2 (kdNO−N2). A few studies have directly inferred these rates from experiments, and the current data differ from common model values. In particular, τVTNO−N2 differs slightly from the Millikan and White correlation, τVVNO−N2 is four times slower than Taylor et al.'s inference, and kdNO−N2 is four times slower than the Park two-temperature model. The unique experimental measurements and dilution in N2 in this study significantly improve the understanding of the vibrational relaxation and decomposition of NO in high-temperature air.