Surface Tension Effects on Discharge Capacity of Coanda-Effect Screens

Abstract

Coanda-effect screens exclude coarse and fine debris from a variety of water intakes. Water overflows an inclined wedge-wire screen with tilted wires that shear high velocity flow from the bottom of the water column. The screens hydraulically self-clean, making them ideal for remote, nonpowered sites. Flow conditions vary widely over the length of most screens. Previous testing related flow capacity to gravitational, surface tension, and viscous forces, but the range of flow conditions was limited versus potential applications. This study tested small sections of prototype-scale screens at varying slopes, and discharge coefficients were related to Froude and Weber numbers. Tests with variable water temperatures proved that screen performance is independent of Reynolds number and viscosity, but depends strongly on surface tension. Several screen geometries (i.e., wire size, shape, and slot size) were tested, and the performance of all screens could be modeled with power curve functions of the Weber number modified by the Froude number. Individual screens exhibited some unique performance characteristics, but 10 of the 13 screens could be modeled effectively as a group.