Effect of the rotating domain size in multiple reference frame simulations of two radial flow impellers in a baffled stirred tank
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computational fluid dynamics (CFD) study was conducted in order to simulate the flow induced by both a turbine of four flat blades, and a high shear impeller (HSI) Norstone® type. The investigation was carried out in a stirred baffled-tank operating in laminar and turbulent flow conditions. To account for the rotation of the impeller-shaft array, the multiple reference frame (MRF) approach is employed, which in its core demands a demarcation for a surface separating the computational domain into two very distinctive regions: the rotating reference frame (RRF) and the static reference frame (SRF). This research project focuses specifically on investigating the role of the location of the separating surface over the accuracy of numerical approximations. To this end, several cylindrical volumes are constructed surrounding each of the impellers. In order to conduct simulations of the stirred-tank working at Reynolds numbers below 115, the laminar model was considered, whereas for Reynolds numbers above 21000, the standard κ-ε turbulence model was employed. To validate the digital replicas employed in the numerical simulations, power number measurements corresponding to laminar and turbulent flow regimes were obtained from a lab-scale cylindrical stirred-tank equipped with either of the two impellers previously mentioned. The computer results reveal that in the case of laminar flow, the position of the surface separating both regions for the two evaluated impellers do not have a substantial effect on the corresponding numerical approximations. However, its position influences numerical accuracy as the Reynolds number increases, i.e., the higher this is, the larger the RRF must be. The optimal extension for the RRF-region for the four flat blades impeller found in this work in turbulent flow were employed for reproducing satisfactorily experimental results of a published system. An analysis based on grid density for the two digital replicas built for this study, revealed that numerical power number values and local velocity profiles are more sensitive to changes in the extension of the RRF domain rather tan to an increase in the number of grid points for the mesh representing the stirred-tank system equipped with the four flat-blade turbine. On the other hand, in the case of the stirred-tank system equipped with the Norstone® impeller, sensitive changes were only substantial in the case of the velocity profiles.
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