No abstract

As importance of renewable energy sources increase with respect to energy demands, turbine efficiencies become more important. Francis turbines are one of the most common turbine types in use at hydroelectric power plants.In some cases such as load rejection and high load operations, pressure fluctuations and vortex formation cause Francis turbine efficiency and endurance to decrease. To prevent these effects, a new component named Vortex Preventing Element (VPE) is designed. The main idea of this new design is damping the swirling slow with Vortex Preventing Element and improving the performance of suction side of runner blades. This new element is mounted between turbine runner and draft tube. CFD analyses are carried out with and without VPE. Preliminary results show that VPE with having only one spiral element provides more uniform flow through the draft tube. According to the preliminary results, the new design also provides an efficiency increment about 4%.

Hydraulic turbines are usually operating at high efficiencies around 90%. It is possible to increase the efficiency by preventing flow characteristics such as failure, cavitation and vortex rope in the draft tube. In some cases, such as partial loads or overloads, pressure pulsations and vortex rope would occur in the draft tube. These undesired events would damage the components of the turbine and that also causes the efficiency to decrease. To eliminate these artifacts, it is decided to design a new component. Vortex Preventing Element, which is designed to eliminate vortex structures and pressure fluctuations, is located at the inlet of draft tube. Computational Fluid Dynamics analyses are performed for different designs having several stage numbers of vortex preventing elements. The preliminary results showed that the one stage vortex preventing element design creates more uniform flow in the draft tube and also increases the efficiency about 3%. Since more studies about the vortex preventing element are in progress, it could be said that the vortex preventing element can handle vortex phenomena in the draft tube and effects the efficiency of the Francis turbines.