Onset of SF 6 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, Pc, and the temperature, Tc, at the observed onset of clustering in the range 60 K<Tc<110 K and 0.1 Torr<Pc<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 Tc-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.