Suspension of solid particles in agitated vessels—I. Archimedes numbers ≲ 40

Abstract

Complete suspension of fine-grained particles is achieved at mean circulation velocities of the fluid exceeding the settling velocities of the particles by several orders of magnitude. It is therefore argued that boundary layer effects are significant. In this paper, through an analysis of the boundary layer flow in an agitated vessel two theoretical criteria for the required minimum stirrer angular velocities are derived. The evaluation of own experiments proves the significance of one of them for Archimedes numbers Ar ≲ 40, i.e. for particles completely immersed in the viscous sublayer. For higher Archimedes numbers, i.e. for particles protruding into the buffer layer and into the turbulent near wall layer, criteria based on the boundary layer flow turn out not to be suitable.