Modifications to non-linear rheological models of viscoelastic fluids

Abstract

Four constitutive equations have been used to describe the behaviour of viscoelastic fluids viz., (i) upper convected Maxwell, (ii) Oldroyd 4-constant, (iii) Bogue-White and (iv) Bird-Carreau. These were modified to improve their ability to predict the observed behaviour of viscoelastic fluids in viscometric and oscillatory shear flows. The fluids used during the experimental work are four different concentrations of polyacrylamide (Separan AP 30) solutions, i.e. 0.6, 0.8, 1.0, and 1.2%, in glycerine / water mixtures. The experimental data are obtained under simple shear and small-amplitude oscillatory shear flow conditions. The viscosity data, obtained from steady shear experiments, and the dynamic viscosity and storage modulus obtained from oscillatory shear experiments have been used to determine the model parameters. The performance of the original and modified models have been studied by comparing their predictions of the viscosities, in steady, and the dynamic viscosities, in oscillatory shear flows. The average root mean square has been used as a criterion for the comparison. Finally, the results of the predictions made by the modified models to the first normal stress coefficients are given to see how they can predict a material function other than those used to determine the model parameters. This may justify the application of these modified models in complex flows such as free coating.