Hydraulic Performance of Coanda-Effect Screens

Abstract

A theoretically based computational model is presented for predicting the hydraulic performance of Coanda-effect screens. These screens use a tilted-wire, wedge-wire screen panel to remove thin layers of high-velocity flow from the bottom edge of a supercritical flow. Typical slot openings are 1 mm or less, and the screens are self-cleaning with no moving parts. The discharge characteristics of several screen materials were evaluated in laboratory tests, and a relation was developed for computing the discharge through a tilted-wire screening surface as a function of the Froude number, the specific energy, and the Reynolds and Weber numbers. A model for the performance of complete Coanda-effect screen structures predicts the wetted length of screen required to accept a given flow, or the flow rate through the screen and the bypass flow over a screen that does not accept all of the flow. Predictions from the model compare favorably to results from clean-water laboratory tests of several different prototype-size screen structures. The model will allow designers to accurately size screens and evaluate design alternatives.