Effects of Floating Rafts as Anti-Vortex Devices at Horizontal Intakes

Abstract

Air- entraining vortices created by swirling flows on intakes cause serious problems such as; increasing loss of hydraulic load and discharge at water intake structures, loss of efficiency, operational problems, cavitation and vibration problems in hydraulic machines. Hence the position of the intake should be justified for the most critical scenario as the reservoir is at dead or at minimum storage level to avoid the occurrence of air-entraining vortices. Although intakes are designed by considering the formation of air-entraining vortices, they cannot be prevented due to approach flow conditions and submergence. Therefore, some structural changes should be considered in order to avoid the occurrence of air-entraining vortices. One of these methods is using anti-vortex devices to prevent the formation of air-entraining vortices. In this experimental study, floating rafts at different sizes were tested as anti-vortex devices to prevent the formation of air-entraining vortices at single and multiplehorizontal intake structures under symmetrical and asymmetrical approach flow conditions. Three identical pipes of diameter Di=0.265 m were tested at a wide range of discharge with varying sidewall clearances. Experiments were conducted for three different combinations of the intake structures: single, double and triple water intakes were operated, respectively. Different side wall distances in the approach channels of the intake structures were specified previously to create symmetrical and asymmetrical flow conditions. The side walls were located according to these distances before each experiment. For single and double water intake structures, Wraft=10 cm and triple water intake structures, Wraft=20 cm raft width were found to be successful for vortex prevention. Moreover, Wrafts/Di values that gave successful results are 0.38 for single and double intake structures and 0.75 for triple intake structures.