A PIV study to quantify the relationships between viscoelasticity and flow behavior in stirred tanks

Quantitative data on fluid dynamics in stirred tanks with viscoelastic fluids are scarcely available. Therefore, a particle image velocimetry study on flow phenomena occurring when agitating viscoelastic liquids with radial and axial impellers is presented. Different flow regimes exist, which result from interactions between elastic normal stresses and inertial forces: While for all agitators an axial pumping effect is found in the elastic regime, (semi-)radial flow patterns result in inertia-dominated flows. In the transition regime, locally limited zones exist dominated by either elasticity or inertia. This leads to a flow compartmentalization, a related pseudo-stagnation flow and reduced circulation rates. Which of the flow regimes is present can be predicted by the use of the elasticity number. With it, circulation flow numbers and flow distributions are standardized: In elastic flow conditions, these are constant. Thus, a scaling of the log-normally distributed velocities to the tip speed in this regime is possible.