Effect of impeller design and rotation protocol on the power consumption of turbulent stirred tanks

Abstract

This thesis deals with topics concerning both passive and active control of stirred tanks. Regarding passive flow control, the effect of certain turbine blade modifications is investigated, most notably that of the blade perimeter increase in a fractal manner, applied on a conventional radial turbine stirring an unbaffled tank. It is found that the tested modifications show potential for applications, as by applying them, a drop in power consumption, an increase of the bulk turbulence intensity and the mass flow rate, and a suppression of the shed blade vortices' intensity and coherence is achieved. The latter, in particular, is argued to be a potential cause of the above-mentioned drop in torque/power consumption. Additional material from this section are the detailed comparison of fractal and perforated bluff bodies and a characterisation of the form drag distribution of radial turbines stirring unbaffled tanks. The latter was achieved by employing a novel pressure measuring technique.

Regarding the active flow control, this thesis focuses on the prediction of stirred tank power consumption in situations where the shaft speed is not constant, but rather time dependent. The motivation for this is that such speed control has been shown to promote mixing in the tank. Employing first principles, qualitative scaling laws and empirical correlations, analytical models for the prediction of the torque response, when the shaft speed undergoes smooth, or step changes are developed. The predictions are then experimentally validated using torque measurements. The above models could find application in the design process of variable speed systems.