Nitric Oxide Vibrational Relaxation and Decomposition Rate Measurements in Shock-Heated NO-Ar and NO-N2Mixtures

Nitric oxide (NO) formation and decomposition reactions are important intermediate steps for breaking oxygen (O2) and nitrogen (N2) bonds in shock-heated, hypersonic air flows. Collisions with NO additionally influence the vibrational energy distribution of O2 and N2 through vibrational-translational (VT) and vibrational-vibrational (VV) energy exchange. This study improves the understanding of high-temperature NO vibrational relaxation and decomposition by: 1) accessing high-temperature, low-pressure experimental conditions in a shock tube for NO diluted in N2 and argon (Ar), 2) acquiring temperature and NO number density time-histories using a sensitive, quantum-state-specific ultraviolet (UV) laser absorption diagnostic, and 3) inferring relevant vibrational relaxation rates from absorbance time-histories. The experimental conditions include 0.4% and 2% NO diluted in either Ar, N2, or equal parts Ar and N2, an initial temperature of approximately 3600 K, and sub-atmospheric pressures of 0.39 - 0.86 atm. This work measures NO absorbance (α) using two continuous-wave (CW) laser diagnostics at approximately 224.8150 nm (± 0.0006 nm) and 226.1025 nm (± 0.0001 nm), probing rotational states in the ground vibrational and ground electronic state (Fig. 2). Quantum-state-specific time-histories for translational/rotational temperature (Ttr) are inferred from the absorbance ratio at the two wavelengths, and number-density of NO (nNO) time-histories are subsequently inferred from Ttr and α at a single wavelength (Fig. 3, 4). The reported rate parameter results include three VT relaxation times (τVTNO-NO, τVTNO-Ar, τVTNO-N2), and one VV relaxation time (τVVNO-N2), inferred at 3600 K from comparison across the six test mixtures (Tables 1, 2). While slight NO decomposition was observed in the 2% NO/Ar experiment, the influence of NO decomposition was largely negligible at 3600 K. To exhibit the effect of NO decomposition, example absorbance, temperature, and number density time-histories from a 4490 K experiment are provided, which demonstrate stronger, multi-stage decomposition at the higher temperature condition (Fig. 5).