Reduced energy consumption in stirred vessels by means of fractal impellers

TL;DR

This study explains how fractal impellers reduce energy consumption in stirred vessels by altering vortex dynamics and turbulence, leading to smaller separation regions and lower torque.

Contribution

It provides a physical explanation for energy savings with fractal blades using direct numerical simulations at Re=1600.

Findings

Fractal blades create jets that penetrate the recirculation zone.

The separation region volume is 7% smaller with fractal blades.

Reduced turbulent transport is linked to smaller trailing vortices.

Abstract

Earlier studies have shown that the power consumption of an unbaffled stirred vessel decreases significantly when the regular blades are replaced by fractal ones. In this paper, the physical explanation for this reduction is investigated using Direct Numerical Simulations at Re=1600. The gaps around the fractal blade perimeter create jets that penetrate inside the recirculation zone in the wake and break up the trailing vortices into smaller ones. This affects the time-average recirculation pattern on the suction side. The volume of the separation region is 7% smaller in the wake of the fractal blades. The lower torque of the fractal impeller is equivalent to a decreased transport of angular momentum; this difference stems from the reduced turbulent transport induced by the smaller trailing vortices. The major difference in the turbulent dissipation is seen in the vicinity of trailing vortices, due to fluctuations of velocity gradients at relatively low frequencies.