Entry flow vortices in polymer melt extrusion: A review

Abstract

Although, circular or planar abrupt entry flows are geometrically very simple hydrodynamic problem highly viscoelastic polymer melts makes it very complex with extreme differences in velocities and stresses across the geometry. Despite these flows are very common in polymer melt extrusion industry their strongly non-viscometric and transient nature represents exceedingly challenging task for experimental as well as theoretical investigation and consequently complicates their fully understanding. Polymer melts flowing through abrupt entry contractions exhibit several unique features of which the vortices are one of them. Occurrence of infinitesimal stress singularity in the salient corner leads to presence of weak concave Newtonian viscous vortex. Moreover, polymer melts with increasing extensional to shear viscosity (Trouton) ratio as a function of flow rate exhibit strong convex elastic vortex caused by complete reorientation of stress field near the re-entrant corner (infinite stress singularity point) as a result of momentum balance in the flow direction. This leads to separation of the flow into the primary "funnel-shaped" flow around the centre line/plane on which the secondary recirculation flow(s) in the corner(s) (vortices) are superimposed. Polymer melt captured in the vortex very slowly rotates in the direction opposite to the main flow direction (2D simplification) or takes a helical path moving also in the third direction (real 3D flow). Since the first visual experimental observation performed by Tordella as well as preliminary theoretical prediction made by Langlois and Rivlin at the end of the 1950s this phenomenon represents one of the most fundamental rheological problem ever. In this review paper, the most important experimental as well as theoretical papers focused on entry flow vortices are reviewed and discussed. © 2017 Author(s).