Probing the Vibrational Relaxation of N2 and O2 by Use of CARS Spectroscopy to Model NTE-Turbulence

摘要

The thermochemical dynamics associated with hypersonic flight and turbulent flow is vital to understanding the effects that hypersonic turbulence has on objects or vehicles traveling at speeds above Mach 5 (~ 1708 m/s). Non-thermochemical equilibrium (NTE) exists downstream of strong shock fronts and encountered in the shear layers from hypersonic flight, and coupled with turbulence, it has significant effects on flow dynamics. NTE, characterized by high vibrational temperatures of N2 and O2, was observed, and the relaxation processes were measured to obtain time-resolved results. By inducing cold-flow NTE via RF-plasma, species in the flow were probed to determine specific vibrational temperatures at particular distances and times following initial NTE-preparation. The detection technique used in the experiment was coherent anti-Stokes Raman spectroscopy (CARS) and variations of this laser-based technique were optimized to maximize accuracy and signal-to-noise in the vibrational relaxation measurements. It was found that the boxCARS variant was most successful in probing the v=1 --> v=0 vibrational transition associated with the measurement. Also a dual pump-beam CARS setup enabled the probing of both N2 and O2 simultaneously; however, in this report only N2 vibrational relaxation was modeled because the lifetime of vibrationally-excited (v=1) O2 was too short and the O2 vibrational temperature was too low to model. The CARS spectra were obtained in a subsonic wind tunnel with a flow velocity of approximately 30 m/s and probing distances from 4.4-39.4 cm downstream the plasma. Five averaged vibrational temperature values were determined and they yielded a decay from 1882 ? 46 K (4.4 cm from plasma) to 1010 ? 16 K (39.4 cm from plasma) showing the relative rate of vibrational relaxation of N2. The vibrational relaxation was also modeled as a function of time after passing through the plasma, and a kinetic simulation was fit to the results. The spectral decay of the v=1 peak relative to v=0 (Iv=1/Iv=0) was found and compared to the decay of the vibrational temperature. Data analysis revealed that the results were in agreement with theory and the observed vibrational relaxation of N2 fit the simulated kinetic model accurately.