Characteristics of flotation, sedimentation and entrainment of particle in agitated liquid

The characteristics of particle entrainment from free surface or bottom wall into mechanically agitated water have been studied to clarify the inclusion behavior in liquid metal. The critical agitation speed, n_c, at which a 50% of particle is entrained into bulk water is measured under steady state by using buoyant particles (glass balloon and silica balloon) and sedimental particles (glass bead). It is found that the dependence of n_c, on the particle diameter, d_p, is different between the buoyant and the sedimental particle. For the case of the buoyant particle, n_c is independent of d_p, whereas for the sedimental particle n_c increases with increasing d_p. To account for this difference, two transition times are investigated. One is the entraining time, t₁, which is the time needed to reach a steady state after a sudden imposition of agitation with various speeds. The other is the floating/sinking time, t₂, which is the time required to reach a steady state after a sudden decrease in agitation speed from a high speed achieving uniform particle dispersion to lower agitation speeds. It is thought that the balance between t₁ and t₂ should determine the amount of entrained particles in bulk water under steady-state experiments. For the case of the buoyant particle, the dependencies of t₁, and t₂ on d_p are almost same, so that the value of n_c does not depend on d_p. On the other hand, for the case of the sedimental particle, the dependence of t₁, on d_p is smaller than that of t₂, so that n_c increases with increasing d_p. By the VTR observation of the particle entrainment from the free surface or bottom wall, particles are found to be entrained mainly by vertical vortices formed in the agitation vessel.