Abstract
ABSTRACTContinuous mixing and dispersing process flows produced by scraped surface heat exchangers (SSHE) in food technology influence the microstructure of multiphase food systems and hence desired quality aspects (e.g. specific texture properties and temperature resistance). Such process flows in general depict non‐Newtonian fluid behavior. To explain and optimize the structuring mechanism of food systems (due to mixing and dispersion) treated in such process apparatus the knowledge of the local flow behavior is necessary.In this paper a scraped surface apparatus with special narrow annular gaps including two wall scraper blades is chosen as model process (scraped surface heat exchanger (SSHE)). To get optimizing criteria in the SSHE for mixing and dispersion of shear‐thinning fluids which include structuring components, a numerical particle tracking method (NPT) was developed and used to investigate local flow behavior for various scraper blade geometries and rotational velocities. The flow fields considered are received from numerical flow simulations (finite volume method (FVM)), which have been validated with experimental velocity field measurements (digital‐particle image velocimetry method (D‐PIV)). Besides the flow field and pressure contours, values of elongational and shear rates (as components of the deformation rate tensor, causing flow structuring contributions) are compared along characteristic particle tracks in order to get a quantitative information on the mechanical history which is experienced by the structuring units. Related flow structuring contributions are defined in terms of elongational and shear energy dissipation, integrated over the particle residence times along the tracks. Hence the effects of the rotational velocity ω, the scraper blade angle β and the scraper blade gap rs on the flow structuring contributions are discussed and suggestions of SSHE geometries for food processing are given.
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