Using thrust to control the mixing process in stirred tanks with side-entering agitators and viscoelastic fluids

Stirring experiments were conducted in a pilot scale tank with side-entering axial impellers to study the mixing behavior of biogas fermenters. Propeller flows were analyzed using the PIV technique in dependence of propeller geometry in a viscoelastic fluid, modeling the rheology of organic substrates. Moreover, mixing times via decolorization and thrusts using strain gauges were measured, varying propeller geometry, rotational frequency and viscoelastic rheology. Three general propeller flow types caused by the interplay of elastic and inertial forces could be found and categorized with a newly defined axial elasticity number. In addition, a new correlation between axial flow rate and thrust could be derived, using a single proportionality constant for all analyzed propellers. With this relationship, mixing times could also be summarized and correlated. Thus, by the use of a singular proportionality constant and thrust, mixing times could be predicted with good accuracy independently of propeller geometry and mostly rheology below a critical elasticity number. With this correlation, mixing cycles in biogas fermenters can be optimized regardless of the propeller geometries and substrates in use with the potential to save energy from unnecessary stirring.