Vortex Plate for Enhancing Particle Settling

Abstract

The feasibility of enhancing suspended solids settling by using the newly proposed vortex plates in clarifiers, instead of conventional smooth lamellae, was studied using computational fluid dynamics (CFD) modeling and laboratory experiments in which suspended particles were mimicked by crushed walnut shells and glass beads. The vortex plate was formed by attaching perpendicular ribs to the plate, forming slots of 25×25 mm(depth×width) and placing the plate parallel to the longitudinal clarifier axis at an angle of 60° from the horizontal. Rib walls were placed either in vertical planes, perpendicular to the clarifier longitudinal axis, or were slightly sloping in the main flow direction (20° about the vertical). Three hydraulic concepts were explored with respect to enhancing suspended particle settling: (1) the use of flow energy to generate steady vortices inside the slots and thereby entrain particles into the slots, where they would be sheltered from the fast horizontal flow and could settle without much hindrance; (2) enhancing the particle settling by increasing the contact surface area and thereby reducing the length of travel of settling particles; the same principle is used in conventional lamellar settlers but the surface area of a vortex plate is three times that of a smooth lamella; and (3) increasing the particle collision frequency within the swirling flow inside slots to prompt particle flocculation. The CFD modeling and experimental observations confirmed the formation of strong vortices in the parallel slots of the vortex plate. Such vortices entrained the passing by particles and retained some of them in slots, which provided a quiescent settling zone. Both the simulation and measured results indicated that the vortex plate contributed to a slightly improved removal of suspended particles. A CFD particle tracking model was applied to clarifiers with two vortex plates or two smooth plates and indicated that the vortex plate removed about 8% more particles than the smooth plate. In laboratory tests with plate arrays, the vortex plate array also contributed to better particle removals, especially for slower settling particles and larger inflow rates (by up to 26%).