This paper aims to investigate a method to perform non-isothermal flow simulations in a complex geometry for generalised Newtonian fluids. For this purpose, 3D numerical simulations of starch based products are performed. The geometry of a co-rotating twin-screw extruder is considered. Process conditions concern high rotational speed (up to 1800 rpm), different flow rates (30, 40 and 60 kg/h) and water contents (22% and 36%), for a total of 54 simulations. To cope with the geometry complexity a Mesh Superposition Technique (MST) was adopted. The pseudoplastic behaviour of the fluid is taken into account by considering viscosity as function of shear rate (Ostwaldde Waele relationship) and temperature (Arrhenius law). Simulated temperature variations are compared with measurements at same process conditions for validation. Qualitative behaviour of temperature T and shear stress along the screw are analysed and comparisons of different process conditions are presented. By these simulations a database is formed to develop a process control strategy for novel extruder operating points in food technology.
Since their massive development in the first half of 20th century, extruders have been experiencing a non-stop growth in all industrial fields, not only in plastic industry but also in food processing. The need to develop new products and increase the existent production encourages manufacturers to develop new geometrical screw configurations and to promote research to better understand the complex nature of extrusion processes. Especially in the area of breakfast cereals and convenience food many works have been carried out in order to explain the rheological properties of starch based extrudates. Senouci and Smith [
The present work is intended to provide a method to set non-isothermal flow simulations in a complex geometry for generalised Newtonian fluids. The new contribution is the analysis and discussion of the flow behaviour of the studied extrudate comparing a wide range of process conditions, which cover new and extremely high operating points, up to 1800 rpm. For this aim the MST implemented in the POLYFLOW® software is applied to a twin-screw co-rotating geometry and non-isothermal 3D numerical simulations of starch based products are preformed. The analysis of the extrudate flow behaviour is carried out from a thermal and kinematic point of view for screw speeds ranging from 400 rpm to 1800 rpm. Three different feed rates (30, 40 and 60 kg/h) and two water contents (22% and 36%) are also considered for a total of 54 simulations. The modelled zone is the melt-conveying zone of the extruder, before the die, where the mixture flows as molten monophasic dough. Counter pressure is taken into account. Material parameters were provided by the Department of Food Process Engineering and Department of Applied Mechanics of Karlsruhe Institute of Technology, that performed the experiments and measurements. Validation is made with experimental data for the temperature across the simulated area. Profiles of temperature and shear stress along the simulated area are qualitatively analysed. Quantitative comparisons of all process conditions are also discussed. The performed simulations investigate novel extrusion process conditions in food technology and offer a base of numerical data which can be used to develop a control strategy for optimisation and control of the extrusion process.
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