Mixing of fluids with viscoelastic flow behaviour in stirred tanks

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Although the field of classical mixing has been the subject of considerable investigation in the open literature, far-reaching gaps of knowledge exist in the use of viscoelastic fluids. Due to the lack of a fundamental understanding of physical relationships, experimental data and because of mathematically complex models, the effects of elastic flow properties on stirring processes are still often neglected in science and industry. The proposed project aims to change this by creating an experimental database and by developing missing numerical tools (CFD).The experimental investigations are carried out in stirred tanks with six-bladed disk and pitched blade turbines with varying stirrer to tank diameter ratios. They represent standard geometries of radial and axial pumping stirrers, respectively. With these, dependencies between flow fields, power inputs, mixing times as well as geometric and rheological parameters are determined. This is achieved by the use of model fluids, whose (viscoelastic) rheology is specifically adjusted and measured with rotational and extensional viscometers.By using laser-optical measuring methods (particle image velocimetry and planar laser-induced fluorescence), spatially and temporally resolved flow field and mixing information will be determined for the first time for the above-mentioned stirring geometries in viscoelastic fluids. With these methods, characteristic flow phenomena such as flow field reversals, the formation of elastic flow compartments and their influence on power characteristics will be visualized and quantified.The experimental results also serve as validation basis for the numerical analysis. Based on finite volume solvers developed in preliminary works, the suitability of differential, viscoelastic constitutive models for the calculation of flow fields, mixing times and power inputs in stirred vessels will be investigated. Preliminary simulation results prove the ability of multimode models (Giesekus, Phan-Thien-Tanner) to reproduce the above-mentioned flow phenomena qualitatively for the first time. While very good quantitative agreement is achieved especially when flow field reversals occur, extensional viscosities still need to be considered for more accurate results concerning size and shape of elastic flow compartments. Finally, a comparison of the models enables the derivation of rules which model is best suited for the simulation of stirred tanks, depending on viscoelastic flow properties and operating conditions as well as on accuracy and calculation effort.By correlating mixing characteristics obtained from these investigations with geometric and operating parameters, scale-up criteria will be assessed or developed, which are selectively verified by experiments.