Onset of SF6 clustering in steady supersonic Laval nozzle flow

Abstract.

SF₆ clustering (3–6% in molar fraction) by homogeneous nucleation in argon was experimentally studied in a steady-state-flow supersonic Laval nozzle. The onset of clustering was detected by FT-IR spectroscopy. Measurements of the static pressure in the nozzle, together with the equations of isentropic flow, yielded the relation between the pressure of SF₆, P_c, and the temperature, T_c, at the observed onset of clustering in the range 60 K<T_c<110 K and 0.1 Torr<P_c<2.0 Torr. We found that there was no influence of the argon carrier gas pressure, which ranged from 1.9 to 21 Torr at the onset points, on the onset of condensation. In addition, the logarithmic value of the supersaturation ratio was found to linearly depend on T_c ^-3/2 at the onset as deduced by Wegener and Wu from the classical theory of homogeneous nucleation [Adv. Colloid Interface Sci. 7, 325 (1977)]. Our results indicate that supersonic expansion in the nozzle did not form heterogeneous clusters consisting of SF₆ and Ar but single-component clusters of SF₆ in a mixture of SF₆ and Ar under the experimental conditions studied herein. We obtained an onset curve (pressure versus temperature) for the SF₆ homogeneous clustering, which indicated the boundary for diminishing of the the SF₆ monomers in the supersonic Laval nozzle.