Effect of blade modifications on the torque and flow field of radial impellers in stirred tanks

We perform both high- and low-speed particle image velocimetry and torque measurements to characterize eight radial impeller types in an unbaffled stirred tank. The blade types consist of a set of regular flat blades, used as a baseline, regular blades of increased thickness, perforated blades, and fractal blades. We find a qualitative correlation between the blades' torque coefficient and both vortex coherence and turbulent kinetic energy, possibly explaining the torque differences of the tested impellers. Furthermore, we find that the proposed modifications increase the bulk turbulence levels and mass flow rates while at the same time reducing the shaft torque, showing promise for applications. Finally, we attempt a comparison between fractal and perforated geometries using data from this study and the literature.