Determination of 3-D flow fields in agitated vessels by laser-Doppler velocimetry: Effect of impeller type and liquid viscosity on liquid flow patterns

Radial, axial and tangential velocities were measured in a dished-bottom stirred vessel with a Rushton turbine, and two axial agitators: the Mixel TT and the Lightnin A310, for turbulent (in plain water) and near-laminar (in 1% CMC solution) flow regime. The long-term flow patterns, comprising a single circulation loop for the two axial agitators and a dual one for the Rushton turbine, mask a low-frequency periodic disruption. For all three agitators, a region at the upper part of the vessel was found, where liquid rotates counter-clockwise to the impeller rotation. The Rushton turbine was the most effective in terms of overall liquid circulation, the Lightnin A310 exhibited a poor circular-ion, with velocities in some parts lower than 1% oi. the impeller tip speed, while the Mixel TT yielded the best pumped flow rate to power ratio. When the liquid viscosity was increased, the pumped flow rate decreased for all agitators, but the reduction in the amount of liquid circulated was more considerable. In the viscous liquid, the axial agitators developed a radiality, with 10-15% of the flow being ejected radially rather than axially.