Determination of extensional viscosity from capillary rheometer entrance pressure drop measurements using a numerical simulation of contraction flow

A numerical model based on an augmented spectral method has been developed to simulate the flow in contraction regions of inelastic fluids which exhibit a high extensional viscosity to shear viscosity ratio. The model has been used to evaluate the use of an existing analytical model for determining extensional viscosity values. The evaluation demonstrates that the extent of agreement between the analytical and the numerical approach depended on the fluid behavior, however, the agreement was in general poor. The use of the spectral numerical contraction flow model, in place of analytical models, for determining extensional viscosity data from capillary entrance pressure drop data is shown to be an improvement on the analytical technique examined giving good agreement with experiment in the case of a DMC slurry. However, limitations in the technique have been identified for use with fiber-filled materials. Recommendations to extend the approach by using other constitutive equations in the contraction flow model and so enable application of the technique to a wider range of fluids have been made.