Oscillation dynamics of free vortex surface in uncovered unbaffled stirred vessels

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). One of the main drawbacks of such vessels, that limits their industrial applicability, is the possible onset of low-frequency sloshing of the free surface. In this work, original data on oscillation dynamics in UUST are presented. In particular, data focus on the oscillation amplitude as well as on their frequency. Data were obtained by means of a novel experimental technique based on digital image analysis. The effect of impeller geometry (Rushton turbine, pitched blade turbine, Lightnin A310 propeller) and that of scale-up were finally investigated. It was found that the natural oscillation frequencies only depend on vessel scale and geometry, while impeller type plays a role in the way the free surface oscillation frequencies change with impeller speed.