Characteristics of fluid flow and particle suspension in a solid-liquid draft-tube stirred tank

The three-dimension velocity distribution of single liquid phase, solid concentration distribution and particle off-bottom suspension characteristics in the liquid-solid phases were systemically investigated in a draft-tube stirred tank of 0.8 m diameter. The results showed that the axial velocity of liquid phase was far larger than radial and tangential velocities inside and outside the draft tube. And there was a second circulation zone close to the outside wall of the draft tube. The shallow conical bottom leads to a critical just off-bottom suspension impeller speed (NJS) at least 14% lower than that in the flat bottom. As expected, NJS increased as the solid concentration increased. NJS decreased slightly as the solid concentration exceeded 50%. The homogeneity of vertical solid concentration distribution was enhanced with the increase of solid concentration. The results are of importance to the optimization of industrial draft-tube stirred reactors.