The Effect of Stirrers at low Re Numbers

Abstract

During glass melt homogenisation, the stirrers are often required to show not only the homogenisation, but also a pumping effect. This effect can be achieved by means of two mechanisms. The first one is paddle uplift, applied for plane wings, airscrews, ventilation fans and turbines. This devices operates at high values of the Re number, usually 10⁵ - 10⁶ . Fig.1. The second phenomenon that can be used for pumping is a shearing force, appearing as a consequence of a drawing force which appears due to the movement of planes in a viscous liquid. Worms, extruders and feeders of various media, e.g. plastics, work on this principle. In liquids of high viscosity and with very slow movements resulting from this, inertia becomes negligible. The liquid moves only according to resultants of forces affecting at the moment. This flow is called creeping flow, or flow in Stokes' area. Worm pumps operate in this range. The mechanism of screw effect can be seen in Fig. 2. This devices operates at very low Re number e.g. for the transport of plastics polymers Re=0.006.