FLOW VISUALIZATION AND BIREFRINGENCE STUDIES ON PLANAR ENTRY FLOW BEHAVIOR OF POLYMER MELTS

Both flow visualization and flow birefringence results are presented for polystyrene and low density polyethylene (LDPE) in planar entry flow for contraction ratios of 4:1 and 8:1. It was found that vortex growth was a different function of Weissenberg number (We) for each polymer and hence fluid elasticity could not be solely used to account for the onset and growth of vortices. Based on extensional stress measurements along the centerline, normalized by means of the down-stream wall shear stress, it was proposed that vortex growth and intensity are a function of the extensional viscosity and to some degree the shear viscosity. In particular, LDPE, which exhibits a strain hardening extensional viscosity and an early onset of shear-thinning, exhibits large and intense vortices while polystyrene, which does not exhibit strain hardening, exhibits only small low intensity vortices. The purpose of the study was to emphasize the importance of a fluid's extensional properties. relative to its shear properties, in determining entry flow behavior.