CAVERN SIZES IN AGITATED FLUIDS WITH A YIELD STRESS

Highly viscous, non-Newtonian Xanthan gum solutions and two transparent model fluids with similar rheological 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 113 and 112 of the tank diameter have been used alone and also in conjunction with 6-bladed, 45 degrees-pitch axial Row turbines of the same size at speeds up to 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 I to 2 W/kg which is about 113 to 114 of that required with small diameter combinations. Single disc turbine impellers are unsatisfactory.