The effect of boundary curvature on the stress response of linear and branched polyethylenes in a contraction–expansion flow

The effect of flow-boundary curvature on the principal stress difference (PSD) profiles observed through a contraction–expansion (CE) slit flow is evaluated for three different polyethylenes exhibiting increasing levels of branching. Studies were performed using both experimental optical techniques and computational simulations, in the latter case to evaluate the ability of constitutive models to predict these complex flows. The materials were characterised using linear and extensional rheology, which were fitted to the multi-mode ROLIE-POLY and POM-POM models depending upon material branching. Three CE-slit geometries were used; with sharp corners, and with rounding equal to one quarter and one half of the slit length. These created a mixed, but primarily simple shear flow, with different levels of extension and shear depending upon the level of curvature. The PSD developed from an initial Newtonian profile to increasing levels of asymmetry between the inlet and the outlet flow as the level of material branching increased. The rounding was found to lead to the delocalisation of PSD within the flow and removal of the stress singularity from the corner of the CE-slit. It also led to a decrease in the pressure drop across the geometry and an “opening out” of features such as downstream stress fangs to create downstream “crab-claws”. Matching between experiments and simulations for the time evolution of flow from start up for each material in the various geometries illustrated good agreement for both models.