NTUA

Greece

Keywords: Rheology, Rheometry, Bagley Correction

The excess pressure losses due to end effects (mainly entrance) in the capillary flow of a branched polypropylene melt were studied both experimentally and theoretically. These losses were first determined experimentally as a function of the contraction angle ranging from 10o to 150o. It was found that the excess pressure loss function decreases for the same apparent shear rate with increasing contraction angle from 10o to about 45o, and consequently slightly increases from 45o up to contraction angles of 150o. Numerical simulations using a multimode K-BKZ viscoelastic and a purely viscous (Cross) model were used to predict the end pressures. It was found that the numerical predictions do agree well with the experimental results for small contraction angles up to 30o. However, the numerical simulations under-predict the end pressure for larger contraction angles. The effects of viscoelasticity, shear and elongation, on the numerical predictions are also assessed in detail. Shear is the dominant factor controlling the overall pressure drop in flows through small contraction angles. Elongation becomes important at higher contraction angles (greater than 45o). It is demonstrated in abrupt contractions (angle of 180o) that while the entrance pressure loss is slightly dependent on the extensional viscosity, the vortex size is strongly dependent on it. It is suggested that such an experiment (visualisation of entrance flow) can be useful in evaluating the validity of constitutive equations and it can also be used to fitting parameters of rheological models that control the elongational viscosity