467195 Numerical Simulation of Viscoelastic Liquid Flow in Stirred Vessels

Monday, November 14, 2016: 8:57 AM
Union Square 1 & 2 (Hilton San Francisco Union Square)
Markus Kolano, Manuel Brehmer and Matthias Kraume, Chair of Chemical and Process Engineering, Technische Universit├Ąt Berlin, D-10587 Berlin, Germany

Most of the fluids encountered in chemical, food or cosmetic industries exhibit non-
Newtonian flow behaviour. Especially when dealing with polymeric and biological fluids, viscoelastic effects can occur, altering flow patterns and dynamics in comparison to Newtonian fluids.

In general, viscoelastic fluids can be viewed as a combination of a viscous and an elastic material. In contrast to Newtonian fluids, time-dependent stress relaxation is observed. When subjected to shearing, viscoelastic fluids develop normal stress differences, leading to phenomena like the Weissenberg effect. Furthermore, stretching of these fluids causes elastic forces. Additionally, the viscosity dependence on shear rate is commonly non-linear.

In stirred tanks, these effects can lead to cavern formation and inversion of flow fields around impellers i.e., influencing mixing times, power input, shear stress, etc.. For equipment design, the ability to quantify and predict these effects numerically is therefore of high interest in order to be able to choose optimal stirrer and tank geometries and process conditions.

Due to their complex flow behaviour, numerical simulation of viscoelastic fluids is a
challenging task, especially when rotating geometries are considered. In most CFD software, it has not been possible to do such simulations yet. Hence, a tool for calculating viscoelastic flows with rotating geometries based on the Finite-Volume-Method in OpenFOAM was developed. By combining the split-stress tensor approach and viscoelastic differential constitutive equations with the sliding-mesh technique, it is now possible to perform simulations for viscoelastic fluids strirred by complex, rotating geometries.

With this tool, simulations were run for multiple stirrer geometries at various stirring rates and for different constitutive equations, using aqueous solutions of Xanthan and Carboxymethyl cellulose as model fluids. Since an exact description of the rheological properties of viscoelastic fluids is essential for quantitatively accurate flow simulations, rheological measurements of these solutions were conducted and material parameters for different constitutive equations were determined. For validation, the simulations were compared to flow field data acquired through particle image velocimetry measurements.

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