IMPROVING WALL VELOCITY IN NON-NEWTONIAN MIXING TANKS
Graham, L.J.W., Nguyen, B., Wu, J. and Kilpatrick, T.
The aim of this project was to determine a retrofit impeller design for tanks for non-Newtonian slurries, such that an increased tank wall velocity (and hence reduced scale growth) could be achieved within the constraint of the existing motor power.
The study was carried out using a scaled-down model mixing tank at CSIRO Fluids Engineering laboratory. The tank was fully instrumented for torque and speed measurements. The non-Newtonian viscous slurry flow in the de-silication tanks was modelled with a clear Carbopol solution with similar non-Newtonian rheology to the industrial material. Velocity profiles near the tank wall were measured at several vertical heights in the tank using Laser Doppler Velocimetry via a fibre optic probe mounted on an industrial robot. Tanks studied included those with multiple Lightnin A310 impellers as the original configuration.
Several different modified agitator configurations were tried with the best results for the wall velocity being for the case where the agitators were adjusted to increase the operating speed without changing the power. It is thought that this is due to the shear thinning nature of the slurry allowing a reduction in the effective viscosity (and hence tank resistance) due to increased shear rate as the agitator speed is allowed to increase. Improved agitator designs were found which could provide median velocity increases of the order of 20-30% near the tank wall without extra agitator power as demonstrated by velocity measurements.