Estimação de parâmetros de modelos constitutivos usando reômetro multipasse e simulação em OpenFOAM

Abstract

To select the most suitable constitutive model and to estimate its set of parameter for describing the rheological behavior of a given viscoelastic fluid, it remains a challenging task in polymer rheology. In this context that the present work is inserted, aiming at the use of computational fluid dynamics and experimental techniques in order to validate a methodology for parameter estimation of nonlinear terms of constitutive models, based on shear viscosity and the difference of the principal stresses. For this, the processing of a low density linear polyethylene (LLDPE) sample was performed in a multipass rheometer of double piston, coupled to an optical module, which allows the acquisition of flow induced birefringence images. The bright and dark fringe patterns observed in the images were related to the stress field. Polymer processing was performed at 140, 150 and 160°C and at three different velocities using a slit-die geometry. Experiments on rotational and capillary rheometers were also performed to obtain the dynamic modules and the shear viscosity. In relation to the computational step, from the dynamic modules, the material relaxation spectrum was determined and the routine for the parameter estimation of nonlinear terms of constitutive models was implemented coupled to the viscoelasticFluidFoam solver, available in OpenFOAM, open source software of computational fluid dynamics. The results showed that Giesekus and exponential Phan-Thien-Tanner constitutive models with the estimated parameters were able to represent the viscoelastic fluid behavior and to predict satisfactorily the experimental observations obtained in the multipass rheometer, validating the proposed methodology.