Uniaxial extensional characterization of a shear thinning fluid using axisymmetric flow birefringence

Computations are reported for a shear thinning, elastic fluid in axisymmetr ic stagnation flows, in an attempt to predict flow birefringence data on a 5 wt % high molecular weight polystyrene solution. For multimode Phan-Thien -Tanner (PTT) and Giesekus models, several parameter sets are chosen in whi ch the degree of elongational hardening is systematically adjusted. The pri mary objective is to see whether the extensional properties of the fluid ca n be inferred from comparisons between these computations with normal stres s measurements in flows dominated by uniaxial extension. Using the PTT mode l it is possible td vary extensional predictions while maintaining a good f it of shear viscosity data. Up to moderate Weissenberg number, these compar isons suggest a modest degree of extension hardening. Computations in rear stagnation flow using the PTT model fail at the intermediate Weissenberg nu mber. A multimode Giesekus model with parameters determined only by fitting shear data performs exceptionally well in forward stagnation flow at all W eissenberg numbers, but is deficient in rear stagnation flow at the two hig hest experimental Weissenberg numbers. Attempts to selectively enhance exte nsional hardening at high rates do improve normal stress predictions in the rear stagnation region, but only at the expense of degraded predictions of shear stress in rear stagnation flow, and all stresses in forward stagnati on flow. (C) 1999 The Society of Rheology. [S0148-6055(99)00501-5].