The effect of viscoelasticity on stress fields within polyethylene melt flow for a cross-slot and contraction-expansion slit geometry

Abstract

The sensitivity of the principal stress difference (PSD) profiles to material viscoelasticity is demonstrated for two flow geometries using three different polyethylenes. Studies were performed using both experimental optical techniques and computational simulations, in the latter case to evaluate the ability to model these complex flows. The materials were characterised using linear and extensional rheology which was fitted to a multimode POM-POM model implemented in the Lagrangian–Eulerian code flowSolve. A contraction–expansion (CE) slit geometry was used to create a mixed, but primarily simple shear flow, whilst a cross-slot geometry provided a region of high extensional shear and high strain. In both flows, the PSD developed from an initial Newtonian profile to increasing levels of asymmetry between the inlet and the outlet flow. More specific phenomena, such as downstream stress fangs in the CE slit and the formation of centreline cusps and “W”-shaped cusps in the cross-slot, were also observed. The simulations of PSD development within the CE slit geometry quantitatively captured the experimental results. In the case of the cross-slot geometry, the qualitative features of the PSD development were well captured, although the results were quantitatively less accurate.