Picosecond CARS measurements of nitrogen rotational/translational and vibrational temperature in a nonequilibrium Mach 5 flow

摘要

PicosecondUnstable-resonator Spatially Enhanced Detection Coherent Anti-Stokes Raman Scattering (USEDCARS) is used for the measurement of nitrogen Q-branch (ΔJ = 0) spectra in the subsonic plenum and supersonic flow of a highly nonequilibrium Mach 5 wind tunnel. Spectra are processed to infer rotational/translational (T_(rot)) and first-level vibrational (T _(vib)) temperatures in the 200-370 torr plenum simultaneously. Operation of the nominally high reduced electric field (E_(peak) ~ 500 Td), nsec pulsed discharge alone results in fairly significant vibrational loading, T_(vib) ~ 720 K/T_(rot) ~ 380 K; addition of an orthogonal low E (~ 10 Td) DC sustainer discharge produces substantial vibrational loading, T_(vib) ~ 2,000 K/T_(rot) ~ 450 K. Effects of injection of CO_2, NO, and H_2 downstream of the pulser-sustainer discharge are examined, which result in vibrational relaxation accompanied by simultaneous gas heating, T_(vib) ~ 800-1,000 K/T _(rot) ~ 600 K. CARSk measurements within very low-density flows in the Mach 5 expansion nozzle are also performed, with T_(vib) measured in both the supersonic free-stream and downstream of a bow shock created by a 5-mm-diameter cylindrical test object in the Mach 5 flow. Measurements within 300 μm of the cylinder leading edge show that for pure N_2, or N_2 with 0.25 torr CO_2 injection, no vibrational relaxation is observed behind the bow shock.