The efficient design of ultraviolet light (UV) systems for water and wastewater treatment requires detailed knowledge of the patterns of fluid motion that occur in the disinfection channel. This knowledge is increasingly being obtained using Computational Fluid Dynamics (CFD) software packages that solve the equations governing turbulent fluid-flow motion. In this work, we present predictions of the patterns of flow and the extent of disinfection in a conventional reactor consisting of an open channel with an array of UV lamps placed with their axes perpendicular to the direction of flow. It is shown that the resulting flow is inherently unsteady due to the regular shedding of vortices from the submerged lamps. It is also shown that the accurate prediction of the hydraulic residence time and, consequently, the extent of disinfection is strongly dependent on the ability of the CFD method to capture the occurrence and strength of the vortex shedding, and its effects on the turbulent mixing processes.

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