Agitation of a thixotropic shear-thinning fluid exhibiting a yield stress is investigatedudboth experimentally and via simulations. Steady-state experiments are conducted at threeudimpeller rotation rates (1, 2 and 8 s−1) for a tank stirred with an axial-impeller and flow-fieldudmeasurements are made using particle image velocimetry (PIV) measurements. Threedimensionaludnumerical simulations are also performed using the commercial CFD codeudANSYS CFX10.0. The viscosity of the suspension is determined experimentally and is modelledudusing two shear-dependant laws, one of which takes into account the flow instabilitiesudof such fluids at low shear rates. At the highest impeller speed, the flow exhibits the familiarudoutward pumping action associated with axial-flow impellers. However, as the impellerudspeed decreases, a cavern is formed around the impeller, the flow generated in the vicinityudof the agitator reorganizes and its pumping capacity vanishes. An unusual flow pattern,udwhere the radial velocity dominates, is observed experimentally at the lowest stirring speed.udIt is found to result from wall slip effects. Using blades with rough surfaces prevents thisudpeculiar behaviour and mainly resolves the discrepancies between the experimental andudcomputational results.
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