In an unbaffled stirred tank, rotation of the fluid induced by the impeller can lead to the formation of a stable vortex, where the fluid in the centre of the tank is depressed due to centrifugal forces. Depending on process objectives, vortex formation can be desirable or undesirable. If the vortex is large enough to touch the impeller, there can be a loss of mixing power, unwanted gas entrainment, foaming, vibrations, and mechanical stresses on agitator, shaft, and bearings. However under other circumstances it is desirable to form a deep stable vortex – such as to incorporate floating solids or a light secondary phase. Prediction of the vortex depth is, therefore, of practical value in designing an agitation system.
In this work the effect of fluid viscosity on the formation of a stable surface vortex was studied experimentally. Fluid viscosity, impeller speed, impeller diameter, and impeller submergence were varied for a 100L vessel with a Rushton Disk Turbine impeller.
The existing theory for vortex formation (Nagata (1975)) suggests that impeller Froude number adequately characterizes the vortex depth when viscous effects are minor. In the present work, we first show that at the 100L scale, the scaling with Froude number fails when viscous effects are significant. A correlation is proposed to describe the experimental data. This correlation incorporates the effects of Reynolds number and impeller submergence, in addition to the Froude number. This correlation is then validated against data taken at different tank scales.