Three-dimensional simulation of capillary rheometry for an estimation of extensional viscosity

Abstract It is difficult to directly measure the steady extensional viscosity of thermoplastic composite materials, especially at high extension rates. The famous Cogswell method was derived analytically from the pressure drop of entrance flow in commercial capillary rheometers for estimating the extensional viscosity. However, using Cogswell´s extensional viscosity has always resulted in over-predictions of pressure drop. Recently, the GNF-X (eXtended Generalized Newtonian Fluid) model with a weighted shear/extensional viscosity was proposed to show the typical extension-induced vortex growth in entrance flow simulations. Under given various values of Trouton’s ratio for extensional viscosity, the GNF-X model is introduced to perform three-dimensional flow simulations of capillary rheometry over a range of apparent shear rates. The difference between the predicted pressure drops and the relevant experimental data is minimized such that the estimation of extensional viscosity is optimized herein.