Control of vibrational distribution functions in nonequilibrium molecular plasmas and high-speed flows

摘要

The control of the vibrational distribution of nitrogen by energy transfer to CO_2 is studied in two closely related experiments. In the first experiment, the time-resolved N_2(v = 0-3) vibrational level populations and temperature in the afterglow of a diffuse filament nanosecond pulse discharge are measured using broadband coherent anti-Stokes Raman spectroscopy. The rotational-translational temperature in the afterglow is inferred from the partially rotationally resolved structure of the N_2(v = 0) band. The measurements are performed in nitrogen, dry air, and their mixtures with CO_2. N_2 vibrational excitation in the discharge occurs by electron impact, with subsequent vibration-vibration (V-V) energy transfer within the N_2 vibrational manifold, vibration-translation (V-T) relaxation, and near-resonance V-V′ energy transfer from the N_2 to CO_2 asymmetric stretch vibrational mode. The results show that rapid V-V′ energy transfer to CO_2, followed by collisional intramolecular energy redistribution to the symmetric stretch and bending modes of CO_2 and their V-T relaxation, accelerate the net rate of energy thermalization and temperature increase in the afterglow. In the second experiment, injection of CO_2 into a supersonic flow of vibrationally excited nitrogen demonstrates the effect of accelerated vibrational relaxation on a supersonic shear layer. The nitrogen flow is vibrationally excited in a repetitive nanosecond pulse/DC sustainer electric discharge in the plenum of a nonequilibrium flow supersonic wind tunnel. A transient pressure increase as well as an upward displacement of the shear layer between the supersonic N_2 flow and the subsonic CO_2 injection flow are detected when the source of N_2 vibrational excitation is turned on. CO_2 injection leads to the reduction of the N_2 vibrational temperature in the shear layer, demonstrating that its displacement is caused by accelerated N_2 vibrational relaxation by CO_2, which produces a static temperat