On the efficiency of turbulent mixing in rotating stirrers

Turbulent mixing is crucial in sterilizing engineering, where it is mandatory to produce the finest and the most homogeneous turbulence all over the fluid domain. This study shows how the turbulence imparted by Rushton impellers, which are typically used in several industrial productions, may be hardly improved by varying the impeller's geometry.An impeller of new design, a perforated paddle that completely removes both limitations of Rushton impellers, is illustrated. The mathematical modeling is based on the RANS equations and the k–ɛ model for turbulence closure; great care was lavished to understand the accuracy of the numerical solution. The results are presented in terms of a global indicator of mixing performance that combines both the Kolmogorov scale and the Gini coefficient.The numerical solution disclosed the formation of a peripheral turbulent spot and a seemingly slow motion zone next to the shaft of the rotating perforated paddle. The imparted turbulence may be usefully fine, but many revolutions are needed to grant the safe target level of spatial homogeneity of the turbulence. This was confirmed by both a dedicated statistics treatment and the conventional statistical analysis.

► The geometry of Rushton impellers is not important in terms of energy savings.
► Turbulence generated by a rotating perforated paddle is numerically simulated.
► Results are presented in terms of Kolmogorov scale and a new statistical treatment.
► The imparted turbulence may be usefully fine but without a safe spatial homogeneity.