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

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 (T_tr), and number density of NO (n_NO). 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.