Condensation of SF₆ in seeded supersonic jets

Abstract

Fractional condensation in seeded supersonic free jets of SF₆ has been determined at different locations along the jet axis and at different stagnation pressures using a diode laser absorption technique. Rotational temperature and relative number densities of SF₆ have been determined from Q-branch absorption of the v₃ band. The fractional condensation increases with increased stagnation pressure as well as with increased dilution. The onset pressure for condensation of SF₆ in pure SF₆ jets is above 2000 torr, whereas in seeded jets, it is much lower (900-1200 torr). Calculated dimer mole fractions do not account for the observed degree of condensation. Experimentally observed local onset number density can be reasonably correlated with the trimer critical model of Fisher.