Study on Droplet Nucleation Using Beryllium Injectors and Small Angle X- Ray Scattering (SAXS) Technique

摘要

The structures of condensed supercritical ethylene jets injected from an axisymmetric beryllium injector were explored using the small-angle X-ray scattering (SAXS) technique. The experiment was conducted at the 12-ID-B beamline at the Argonne National Laboratory. Two beryllium injectors with differences in exit orifice diameter and internal converging angle were utilized for ethylene injection. Scattering intensity was measured both inside and outside the injector for three injection conditions. For all three conditions, the presence of scatterers on the order of 20 A (2 nm) inside the injector was calculated from the SAXS measurements. With an assumed liquid ethylene density for small scatterers, unusually high liquid volume fractions were obtained. While the uncertainties related to the identity and source of the small scatterers were ignored for the present, this study shows that homogeneous nucleation takes place within a narrow spatial region around the nozzle exit for all three injection conditions. After the initial appearance of 700 A (70 nm) droplets, droplet size reaches the maximum value within the region between nozzle exit and Mach disk. Downstream of the Mach disc, an abrupt decrease in the line-of-sight average droplet size across was observed, and the line-of-sight average droplet size was found to remain fairly constant. The line-of-sight liquid volume fraction decays exponentially with the axial distance from the nozzle exit, with little or no effect from the Mach disk. For the injection condition with a large exit orifice diameter, large droplets are mainly distributed within the region bounded by barrel shock and Mach disk, while slightly smaller droplets are distributed in the region outside the barrel shock. Dependence of droplet size on exit orifice diameter or converging angle inside the injector was observed in the selected injection conditions in the present study.