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Shear rate and direct numerical calculation of the Metzner-Otto constant for a pitched blade turbine

Márquez-Baños, Valaur E., De La Concha-Gómez, Aarón D., Valencia-López, José J., López-Yáñez, Adrián, Ramírez-Muñoz, Jorge
Journal of food engineering 2019 v.257 pp. 10-18
blades, fluid mechanics, impellers, mathematical models, shear strength
Metzner-Otto's concept regarding the existence of an average shear rate (γ˙av) around the impeller was numerically undertaken for shear-thinning fluids. This study focuses on investigating the region around the pitched blade turbine (PB4) which could be considered as the region where Metzner-Otto's constant can be extracted directly from non-Newtonian simulations as Ks=γ˙av/N (where N is the impeller speed). For this purpose, ten succeedingly increasing volumes enclosing the impeller were defined in the mesh with the objective of studing γ˙av in these zones. Results show that γ˙av in the volume swept by the blades for the PB4 is similar with that reported for the Rushton turbine. However, the region where Ks can be computed for the PB4 extends approximately up to 1.22 impeller radii and 2.5 impeller blade heights. The Metzner-Otto's constant in this region (Ks = 8.86) was found to be in good agreement with our experimental (Ks = 8.53) result and with reported data (Ks = 8.58).