Vortex breakdown in discharge cone of the Francis Turbine

Abstract

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.