CAVERN SIZES IN AGITATED FLUIDS WITH A YIELD STRESS

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Abstract
Highly viscous, non-Newtonian Xanthan gum solutions and two transparent model fluids with similar Theological properties have been studied under aerated (up to 1 vvm) and unaerated conditions in a 0.29m diameter agitated vessel. Rushton disc turbines of size 1/3 and 1/2 of the tank diameter have been used alone and also in conjunction with 6-bladed, 45°-pitch axial flow turbines of the same size at speeds up lo 24 rev/s, enabling specific power inputs of up to 15 W/kg to be imparted. Flow patterns were studied by flow visualisation and hot film anemometry. When the fluids have a yield stress, the fluid divides into a turbulent well-mixed cavern which increases in size with increasing speed with the remainder stagnant. A model for the size of the cavern fits the experimental data well for both aerated and unaerated mixing. Large diameter combinations produce good mixing at about 1 to 2 W/kg which is about 1/3 to 1/4 of that required with small diameter combinations. Single disc turbine impellers are unsatisfactory.