CFD analysis of the frame invariance of the melt temperature rise in a single-screw extruder

In analytical analyses for single-screw extruders the simplified approach of rotating the barrel and keeping the screw fixed is often used instead of rotating the screw and fixing the barrel. Although the flow field is independent of the reference frame, as has already been shown to a satisfactory degree (Rauwendaal et al., 1997), the question of the dependence of the melt temperature rise on the reference frame is still being challenged, e. g. Campbell et al. (2001, 2008). In this work we develop a finite-volume CFD code allowing for the three-dimensional simulation of the flow and temperature rise in both reference frames. The question of frame invariance is addressed by simulating the flow of a Newtonian-like polycarbonate both in a two-dimensional cross-section of a single-screw extruder and in a three-dimensional model with two full turns of the screw. Our results show that the kinematics and the melt temperature rise are equal for screwand barrel-rotation and thus independent of the reference frame. Furthermore, the presence of a clearance flow has a negligible influence on the temperature rise.