Spectrally-resolved ultraviolet absorption measurements of shock-heated NO from 2000 K to 6000 K for the development of a two-color rotational temperature diagnostic

Abstract

In high-enthalpy air flows, nitric oxide (NO) formation and removal involves multiple, complex energy transfer processes that occur under nonequilibrium conditions, thus motivating the need for diagnostic methods to probe such flows in laboratory settings. In this work, a rotational temperature diagnostic for NO was developed using a spectroscopic model (Stanford NO model) of the γ-bands of NO to inform optimal wavelength candidates for absorption using two continuous-wave (CW), ultraviolet (UV) lasers. Absorption cross-sections of shock-heated NO were measured with two CW UV lasers over a range of temperatures and wavelengths, and results were compared to the Stanford NO model. All measurements were completed behind reflected shocks in 0.4% and 2% mixtures of NO in argon (Ar). Fixed-wavelength cross-section measurements from 2000 K to 6000 K between 1 atm to 0.085 atm were made at nominal wavelengths of 226.1026 nm and 224.8155 nm. Additionally, fixed-temperature measurements were made over the wavelength range 226.1013 nm to 226.1035 nm at 0.8 atm. The combination of experiments provided temperature and wavelength sweep cross-sections to compare against the Stanford NO model. The formulation for the collisional broadening parameter, 2γ, was divided into two temperature regimes, transitioning at 2500 K. Additionally, line positions of the R11(26.5), R12(34.5), Q21(26.5), and Q22(34.5) transitions around 224.8155 nm and the Q11(12.5), R12(19.5), P21(12.5), and Q22(19.5) transitions around 226.1026 nm were adjusted to best fit the cross-section data. A vibrational equilibrium assumption was used to determine the translational and vibrational temperatures at the point of cross-section determination, while a coupled vibration-dissociation (CVDV) model characterized the extent of dissociation at the point of cross-section determination. Data from both temperature-sweep and wavelength-sweep measurements agree to within ± 10% of the Stanford NO model, with most cross-sections falling within ± 5% of model predictions. Oxygen (O2) cross-sections measurements were also taken behind reflected shocks in dilute 2% O2 in Ar mixtures at 2000 K and 6000 K to quantify the interfering O2 absorbance at the chosen diagnostic wavelengths.