Influence of viscoelasticity on mixing performance of primary and secondary circulation flows in stirred vessels

Abstract

The aim of this study was to experimentally verify the mixing performance of primary and secondary circulation flows appearing in turbulence in stirred vessels of Newtonian and viscoelastic fluids. Impeller torque measurements, flow visualization, and particle image velocimetry and planar laser-induced fluorescence measurements were performed. In the case of the Newtonian fluid, a tornado-like flow that was a combination of primary and secondary circulation flows was observed with small-scale turbulent eddies. This flow required a moderate torque power and shortened the mixing time. Conversely, a large-scale primary circulation flow of a slow rigid vortex with no small-scale turbulent eddies was observed in the viscoelastic fluid. Although the discharge flow was enhanced or diminished dependently on the Reynolds number and surfactant concentration, it induced slow large-scale secondary circulation flows in the stirred vessel. As a result, the tornado-like flow disappeared, and these flows resulted in a long time constant of the mixing. Even with such flow characteristics, while the low-concentration case indicates that a low torque corresponding to the driving power is needed to drive the flow, the high-concentration case suggests that the high torque is due to the occurrence of additional viscoelastic stress.