Cavern Formation in Stirred Vessels Containing Herschel-Bulkley Fluids: Experiments and Simulations*

The aim of the experimental aspect of this study was to achieve flow similarity over scales in stirred vessels containing viscoplastic fluids in which a ‘cavern’ is formed when they are agitated by a central impeller. Outside of this region, the material is stagnant because the stresses the material is experiencing are not large enough to overcome the material’s yield stress. Although the flow of these fluids in vessels has been widely researched, limited studies have looked at the flow of viscoplastic fluids over scales. Aqueous Carbopol solutions were used and through agitation with Rushton turbine impellers, the resulting caverns were highlighted using food and UV fluorescent dyes. Fluid flow was assumed to be matched if geometrically similar caverns were produced over three geometrically-similar scales. Numerical simulations elucidated the effects of impeller size and speed on cavern formation in Herschel-Bulkley fluids using a 3-D CFD solver. Numerous models have been developed to predict cavern size [Solomon et al(1981), Elson et al(1986), Amanullah et al(1998)]. Comparisons were made between the experimental and numerical results. It was found that the cavern sizes were in relatively good agreement at pre-defined impeller speeds and Reynolds numbers.