Boundary-enhanced sedimentation due to settling convection

Sedimentation rates are significantly enhanced when the process occurs in containers of certain shapes or orientations. For example, sedimentation in a conical vessel or a tilted tube may be several times faster than sedimentation in a vertical tube of the same height. This enhancement results from a naturally occurring convection.

Monodisperse particles were observed during settling from a viscous, incompressible, and Newtonian fluid contained beneath an upward-pointing cone. At particle concentrations of about 0.05 per cent by volume, convective velocities reach ten times the particle settling velocity, and sedimentation is complete in 40 per cent of the time necessary in a vertical tube.

Continuum fluid mechanics successfully models this settling convection. The mechanism is particle momentum transfer to fluid. The model requires numerical solution but its functionality can be investigated in terms of dimensionless parameters.