Abstract
The Plate Settler Spacing team is currently investigating the Floc Roll-Up Phenomenon in the tube settler. By developing a velocity gradient model that supports our various experimental results, we hope to both analytically and experimentally determine the critical velocity floc particles experience when they begin to roll up the settler tube and into the effluent rather than settling back down the tube and into the floc blanket.
Overview of Methods
When an incompressible fluid flows through a cylindrical tube its velocity relative to the walls changes as a function of the tube radius. In general, this velocity distribution is parabolic: the greatest velocities are achieved at the center of the tube (where R=0) eventually tapering off to 0 at the walls. The parabolic nature of the distribution arises from cylindrical symmetry as well as the fact that the fluid does not move at the walls (the "no-slip" condition).
This gradient in the velocity profile contributes to the force that a floc experiencing roll-up feels. Flocs actually begin to roll up when the velocity at their edge exposed to the flow exceeds the settling velocity that the floc particle experiences. The settling velocity of a floc particle is dependent on floc diameter and floc density. Conversely, the critical velocity a floc experiences is dependent on floc diameter and the inner tube diameter.