Measurement of mixing-induced thermal non-equilibrium in a supersonic shear layer using spontaneous Raman scattering

Abstract

Mixing-induced vibrational non-equilibrium of N2 is studied in the turbulent shear layer between a supersonic cold jet and a surrounding hot air coflow. The jet fluid is either air, N2, or Ar, and the heated coflow is air at a maximum temperature of 850 K. The rotational and vibrational temperatures of N2 are determined by fitting the measured time-averaged spontaneous Raman spectra to an analytical model that allows for different equilibrium distributions for the vibrational and rotational states. The mixing of the jet fluid with the coflow gases occurs over time scales of the order of 5 μs, which is found to be sufficiently fast to induce vibrational non-equilibrium in the mixture of hot and cold gases. Results show that the non-equilibrium can be measured, but not on the cold side of the shear layer where the vibrational population in the first hot band is negligible. The effect of fluctuating temperatures within the time-averaged Raman measurement was quantified with the use of Rayleigh thermometry and found to not significantly alter the Raman scattering results. It was also found that the non-equilibrium increases in the shear layer when N2 is removed from the jet fluid, indicating that the observed non-equilibrium is an averaged result of two competing processes that occur simultaneously at a molecular scale, i.e., vibrationally hot N2 is being cooled by a fast jet fluid and vibrationally cold jet fluid is being heated by a hot coflow fluid. An interesting inference of this view is that the averaging effect is always present, regardless of the measurement resolution.