Free vortex shape in Uncovered Unbaffled Stirred Tanks (UUST) agitated by Rushton turbines

Abstract

The main feature of unbaffled stirred tanks is the highly swirling liquid motion, which leads to the formation of a central vortex on the liquid free surface, when the vessel is operated without top-cover (Uncovered Unbaffled Stirred Tanks, UUST). In this work, original data on fluid dynamics characteristics of (UUST) agitated by a Rushton turbine are presented. In particular, digital image analysis is employed to accurately assess vortex shape and validate an original model for its description. The model describes vortex shape in the case of sub-critical systems (when vortex bottom is placed above the impeller plane) as well as in the case of super-critical systems (when vortex bottom falls below the impeller plane). In the latter case, a sudden change of fluid-dynamic features is observed, since the vessel starts behaving as a self-ingesting system, with the generation of a gas-liquid dispersion. Data on the overall mass transfer both in the case of sub-critical and super-critical regimes are presented and compared with relevant data obtained in baffled vessel of similar size. Finally, original data about periodic oscillations of the liquid free surface, a phenomenon not investigated so far, are presented and preliminarily discussed