Experimental studies on swirling flows at vertically upward intakes

Abstract

The experimental investigations presented in this thesis are concerned with the aspects of sump geometry and sump parameters, which govern the flow conditions at vertically upward intakes. Detailed studies are made to understand the effects of back?wall clearance, channel aspect ratio and approach?flow angle on critical submergence; effect of aspect ratio on circulation and sump?inlet geometry on swirl?flow conditions near the bell mouth. In addition, the effects of longitudinal and vertical positions of control gate or breast wall on the swirl angle are investigated. Numerous solutions were proposed by previous researchers for eliminating the hydraulic problems encountered in pumping intakes. But in many cases, subsurface swirl, which causes unbalanced load on the pump impeller in prototypes, is not well documented in the model studies carried out. However, there are innumerable experimental and theoretical studies on surface vortex formation and the geometric conditions required at intakes to prevent the formation of surface vortices. Several studies have been made on horizontal and vertically downward intakes but very few studies are concerned with vertically upward intakes. In view of this, major concentration is made in this experimental study to understand the swirl?flow conditions under various approach?flow conditions referred to vertically upward intakes. An attempt has been made to measure the swirl angle at the bell?mouth entry of the intake. A swirl meter along with an electronic sensor, whose signals are communicated to the data?acquisition system, is used to measure the swirl angle. A recirculating flow system with a transparent Plexiglas rectangular flume is constructed and used for the experimental investigation. The inlet?flow structure is provided at one end of the flume, with vertically upward intake pipe positioned symmetrically at the other end of the flume. The floor of the flume is elevated to a height of 1.7 m for the purpose of flow visualisation and experimental observations. In cases where the approach?flow non?uniformities contribute to strong vortex formation, a curtain wall similar to a control gate, extending across and into the water surface, is provided in the approach?flow channel as a vortex suppressor. In the present investigation, the effects of vertical and longitudinal positions of this vortex suppressor on swirl angle at the pump suction bell are studied. The experimental results indicate that large swirl?flow conditions prevail near the suction bell mouth, just before the formation of detrimental surface vortex. With further increase in the intake Reynolds number and surface?vortex formation, the swirl?flow intensity shows a decreasing trend. The critical Reynolds number which marks the formation of air?entraining vortices is found to vary with sump geometric parameters such as relative intake submergence, channel aspect ratio, approach?flow angle and back?wall clearance. From the results it is inferred that the critical Reynolds number increases with increase in aspect ratio and relative intake submergence, and decreases with increase in approach?flow angle. The increase in swirl?angle magnitude is more severe when the approach?flow angle changes from 30° to 40° and it is less severe when the approach?flow angle changes from 40° to 60°.