Developing Low-Head Coanda-Effect Screens
* Can Coanda-effect screens be applied in situations with less available head than that required for free-flow screen operation?
Need and Benefit
Coanda-effect screens are becoming widely used for debris, fish, and sediment exclusion at water diversions and at turnouts in irrigation delivery systems. They provide effective removal of fine material and have low maintenance requirements because the screens are hydraulically self-cleaning.
The most advanced design tool available to Reclamation for predicting the hydraulic performance of Coanda-effect screens is the Coanda computer program developed by the principal investigator and documented in a 2003 design guide (report R-03-03). The program is based on the numerical model described in (Wahl 2001), which was developed from laboratory tests conducted on Coanda-effects screens operating in a free-flow condition. The free-flow condition is presently the most common implementation. It implies supercritical flow above the screen face and discharge into a non-pressurized chamber (i.e., at atmospheric pressure) beneath the screen. These two factors lead to significant head drop. This head drop requirement cannot always be satisfied.
To reduce the required head drop, end users are now attempting to install Coanda-effect screens in submerged flow configurations, with some successes and some failures. To develop reliable designs, there is a need for a more thorough investigation of submerged flow performance, which occurs when there is a high tail water (i.e., subcritical flow) above the screen face and/or a pressurized receiving chamber. These conditions allow operation with less total head loss. The project will also explore issues related to installation in a so-called bottom-screen configuration, where sediment exclusion and plugging by sediment are primary concerns.
Wahl, Tony L., 2001, "Hydraulic Performance of Coanda-Effect Screens". Journal of Hydraulic Engineering, Vol. 127, No. 6, June 2001, pp. 480-488.
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