Shear and skin friction on particles in power-law fluids agitated by flat-blade and fluid foil impellers

Abstract

The shear rates that exert angular deformation on spherical particles have been measured. The particles are mimiced by a spherical probe. The probe has been immersed in various impeller-agitated power law fluids. The fluids are aqueous dispersions of polymers, e.g. CMC, xanthan gum and starch. The probe has been positioned in various points of a stirred vessel and at various angles. Angle-averaged shear rate distributions were produced. The distributions obtained are characteristic for the specific impeller flow patterns. The flow patterns have been identified by computational fluid dynamics (CFD). Two types of impellers representative for the flat and the fluid-foil blade design, i.e., a Rushton flat-blade turbine (RT) and a Narcissus impeller (NS) are studied. The effects of rheological properties and blade design on the ‘shear-rate-on-particles’ distribution are examined. The local shear field non-uniformity has been uncovered and compared in terms of the CFD-generated time-averaged velocity and deformation rate profiles. The ‘shear-rate-on-particles’ distribution apart from the impeller is found to follow qualitatively the time-averaged inner flow shear rate distribution. Referring to impeller speed 5–12.5Hz, the dimensionless wall shear rate varied between 200 and 1000. In power law fluids, the shear rate on particles decreased up to 50%. The fluid-foil NS-generated shear field was found comparable to the shear field induced by conventional flat-blade turbines and appeared in cases less sensitive to polymer presence. The shear rate produced by the fluid-foil impeller in the highly shear-thinning model solution (n∼0.4) exceeded the flat-blade RT-imposed shear rate. The analysis has been extended to skin friction drag on particles. It is shown that, while exerting an undoubtedly greater angular deformation in water-like fluids, in polymer presence the conventional flat-blade turbine introduces a flow geometry that imposes particle drag that is close or in some cases even less than the one generated by the fluid-foil impeller. The fact implies a weak shape effect of radial turbines on shear-sensitive particles or particle dispersions in power law liquids.