Macro-instabilities of a Suspension in an Axially Agitated Mixing Tank

Abstract

This paper deals with an experimental assessment of the occurrence of flow macro-instabilities in a mechanically stirred suspension and visual observation of the origination and extinction of macro-vortices. The mean frequency of the occurrence of macro-instabilities in operational conditions was also observed. The experiments were carried out in a cylindrical vessel with an inner diameter of 0.19 m, and an axial stirrer with six pitched 45° blades (PBT) was used. The diameter of the stirrer was equal to half of the vessel diameter. The mean frequencies of the occurrence of macro-instabilities were determined at the stirrer frequency for just suspended conditions in dependence on solids concentration and impeller clearance (height of the stirrer above the bottom of the vessel).Two regions of origin of macro-instabilities and one region of extinction were determined visually. It was found that the mean frequency of occurrence of macro-instabilities increases with increasing stirrer frequency at constant concentration of solids. At constant stirrer speed, the frequency of occurrence of macro-instabilities decreases with increasing concentration of solids. At higher concentration of solids, a sharp change toward lower mean frequencies of macro-instabilities was observed.