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978-3-8439-5336-8, Reihe Lebensmitteltechnologie
Flow of highly concentrated suspensions with a non-Newtonian matrix: Measurements and modelling for die extrusion applications
183 Seiten, Dissertation Universität Hohenheim Stuttgart-Hohenheim (2022), Softcover, A5
Flow of concentrated suspensions is challenging with respect to its characterization, prediction and process control. Local inhomogeneities, nonlinear rheology and sensitivity of the viscosity to minor changes in the formulation complicate the description of their flow in generalized rheological models. To assist formulation- and process engineers, a fundamental understanding of the relation between formulation and flow is required.
The aim of this thesis was to create a phenomenological model that relates product formulation to the pressure drop during channel flow and create a best practice to determine the rheological parameters of this model. Central to this analysis is the separation between wall slip and shear flow. Alternative slip analyses were developed, including a new analysis that uses a 3D-printed die with a rough internal surface that ensures a no-slip condition at the wall. Not only does it allow for the determination of the slip velocity, following a subtractive approach, it also allows for the direct quantification of the shear viscosity. Measurements provide evidence for an inhomogeneous bulk as a result of shear-induced migration. The original slip analysis cannot account for the changing local rheology of the suspension and physically unreasonable results are a direct consequence.
With the proposed method it was possible to model the shear viscosity as a function of solid volume fraction and liquid phase rheology. The relative consistency coefficient was described using a modified Krieger-Dougherty relation and the relative flow index using an empirical function with a single fit parameter.
The proposed model and best practice concerning parameter quantification form the basis for a comprehensive analysis of the influence of the suspension formulation on its flow behaviour. It allows for the anticipation of compositional variation and a priori prediction of changes in flow conditions for numerous flow situations.