Effects of the Blade Shape on the Trailing Vortices in Liquid Flow Generated by Disc Turbines

Particle image velocimetry technique was used to analyze the trailing vortices and elucidate their relationship with turbulence properties in a stirred tank of 0.48 m diameter, agitated by four different disc turbines, including Rushton turbine, concaved blade disk turbine, half elliptical blade disk turbine, and parabolic blade disk turbine. Phase-averaged and phase-resolved flow fields near the impeller blades were measured and the structure of trailing vortices was studied in detail. The location, size and strength of vortices were determined by the simplified λ2-criterion and the results showed that the blade shape had great effect on the trailing vortex characteristics. The larger curvature resulted in longer residence time of the vortex at the impeller tip, bigger distance between the upper and lower vortices and longer vortex life, also leads to smaller and stronger vortices. In addition, the turbulent kinetic energy and turbulent energy dissipation in the discharge flow were determined and discussed. High turbulent kinetic energy and turbulent energy dissipation regions were located between the upper and lower vortices and moved along with them. Although restricted to single phase flow, the presented results are essential for reliable design and scale-up of stirred tank with disc turbines.