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Non-Newtonian Flows

Non-Newtonian flow through an orifice

 Non-Newtonian fluid passing through an orifice

Credit: Bart Hallmark

Streamlines and residence time predictions for the steady state flow of guar gum at a concentration of 5 g/L through an asymmetric contraction. The guar gum has been modelled as a viscoelastic fluid, described by a single-mode Giesekus model, and the streamlines have been coloured by residence time. The simulation shows that a significant proportion of the flow passes through a helical vortex upstream of the contraction, (A), greatly increasing its residence time. This has implications for the design of process equipment since it shows that fluid rheology significantly affects residence time which, in turn, could affect final product properties.

Many industrial food processing operations involve the use of complex fluids – they may be viscoelastic, viscoplastic or visco-elasto-plastic. Processing of these fluids can involve their exposure to carefully controlled profiles of, for example, temperature or shear rate to encourage a change of phase, to enable pasteurisation or to effect homogenisation or bubble break-up. Additionally, hygiene is paramount: unexpected areas of stagnation in, say, filling nozzles, may lead to hotspots of bacterial growth that could cause contamination in the final product.

The simulation above demonstrates that the flow profiles of complex fluids in contraction geometries, such as those found in nozzles or homogenisers, may not resemble those expected from a Newtonian fluid. In particular, flow structures such as corner vortices can be present: these can be either attached or detached from the main bulk flow. Symmetric geometries tend to favour the development of vortices that are detached from the main bulk flow, hence leading to regions of stagnation. The simulation shown overleaf shows that asymmetric contractions may result in vortices that are helical, and attached to the bulk flow. Rather than resulting in stagnation regions, these flow structures can greatly increase the residence time of a proportion of the material and expose it to a significantly different shear-rate profile. These effects could result in an inhomogeneous product and poor quality control.