CFD analysis of the flow pattern and local shear rate in a scraped surface heat exchanger

A numerical investigation of a scraped surface heat exchanger (SSHE) was undertaken using the commercial CFD code FLUENT to characterize the shear rates for Newtonian and Non-Newtonian fluids. Simulations are carried out in standard geometries of SSHE. The electrochemical method was first employed to achieve experimental measurements of the shear rates. A two-dimensional model was created to perform the simulation, keeping the dimensions and flow parameters of the experiment. Because of the symmetry of the geometry, a bidimensional resolution of the continuity and momentum equations was conducted. A hybrid mesh was retained with a grid refinement between the tip of the blades and stator where high shear rates occur. A single reference frame approach was then applied to obtain the laminar steady-state flow induced by the rotation of blades in the geometry. A grid refinement in the zone localized between the tip of the blades and the stator is used. A comparison of predictions with experimental measurements was carried out, showing relative agreement between shear rates (Smax) predicted and measured for three fluids (HV45, CMC and guar gum). The little differences observed were principally explained by the 2D simulation which neglects Taylor vortices, when Taylor number (Tag) exceeds a critical value. In the case of HV45, rotating velocity can influence the scraper angle position due to the floating blade. This can induce a change in the gap between the tip of the blade and stator assembly (from 90 to130 μm) that was investigated here.