Flow field characteristics and energy injection in a tank stirred by regular and fractal blade impellers

Abstract

Experiments with regular flat-blades and two fractal modifications have been performed in an unbaffled octagonal stirred tank (tank diameter T = 0.45 m), producing a turbulent regime. Shaft torque was monitored to assess differences in input of power of the different impellers. Fractal blade impellers showed a decrease of power number Np of over 12% in comparison to normal blades. Phase-locked PIV measurements in the impeller discharge region and spectral analysis of the radial velocity component revealed that the tip vortices produced by the regular blades are more coherent and have higher kinetic energy, being advected by a stronger radial jet compared with their fractal counterparts. This implies that the reduction in torque can be attributed to differences in the vortex formation mechanism. PIV measurements in the bulk of the flow show an increase in turbulence intensity of over 20%for the fractal blades relative to normal blades. These results suggest that fractal blade turbines are candidates for higher mixing efficiency.