A non-Newtonian model of flow in forward and backward pumping screw regions of a modular tangential counter-rotating twin-screw extruder

Abstract

The flow of a non-Newtonian fluid in forward and backward pumping elements of a tangential counter-rotating twin-screw extruder has been modeled. A simulation has been carried out for a power law fluid model using hydrodynamic lubrication theory and a modified flow analysis network procedure. The model predicts pressure fields from which mean flux fields and screw characteristic curves may be computed. Large backward leakages between the screws are predicted for the forward pumping elements. This leakage induces distributive mixing. The backward flow is much greater for staggered as opposed to matched screw flights. The pumping characteristics are much less than for two equivalent single-screw extruders. The non-Newtonian behavior reduces the relative amount of backward leakage between the screws but, as in a single-screw extruder, non-Newtonian viscosity behavior reduces pumping ability. For backward pumping screws, there is a forward surging flow between the screws. The screw characteristic curves indicate negative pressure gradients for positive fluxes. These screw characteristic curves are much less non-linear.