HYDRODYNAMIC CHARACTERISTICS OF DUAL RUSHTON IMPELLER STIRRED VESSELS

The flows generated in vessel stirred by two Rushton impellers were in vestigated in two vessels of diameter (T) 100 and 294 mm with impeller s of diameter D = T/3 using flow visualization, power consumption, mix ing time, and 360 degrees ensemble-averaged and 1 degrees angle-resolv ed LDA measurement techniques. The flows depended strongly on the clea rance of the lower impeller above the base of the vessel (C1), the sep aration between the impellers (C2), and the submergence (C3) of the up per impeller below the top of the liquid column height (H). When these distances were varied three stable and four unstable flow patterns we re observed. Comparisons between the two LDA techniques showed that wh ile the 360 degrees ensemble-averaged measurements are useful for char acterizing the overall flow structure and turbulence level in the vess el care must be exercised when interpreting such data, since in the im peller region they include periodic variations in the mean velocity in addition to the turbulent fluctuations. The trailing vortex structure and flow periodicity produced by the Rushton impellers is shown to de cay significantly within a cylindrical region of height 1.2D and radiu s 1.0D centered around the middle of the vessel, when C1 = C2 = T/3. T he turbulence structure within this region is anisotropic, while outsi de this region it might be considered mostly isotropic. The main flow features scaled well between the vessels.