Influence of double-row guide vane cascade on performance of a Francis turbine

Abstract

In the case of small load condition, the water flowing impact angle is very large that the secondary flow and the backflow are generated between the flow passage of runner, resulting in flow separation in runner and producing inter-blade vortex when cavitation is serious. In order to improve the stability of the Francis turbine in the small load conditions, this paper presents a controlling method of double-row guide vane cascade, and its numerical simulation is carried out. In this paper, it is found that there are obvious inter-blade vortices in the runner and the vibrations in the tube are violent under the small load conditions on the original model turbine. Secondly, the relationship between the opening method of double guide vane cascade and the flow separation condition in the runner is studied, the suitable opening combination is obtained and the difference in the performance of the original turbine under the small flow condition is analysed. The results show that: Through adjusting the opening combination of outer guide vanes and inner guide vanes, it can effectively control the outflow angle of the guide vane and the runner, thus the influences of the vortex in the draft tube and the inter-blade vortex can be obviously reduced. At the same time, double-row guide vanes can effectively reduce the pressure fluctuation amplitude of the draft tube. It has provided a certain theoretical basis for improving the stability of Francis hydraulic turbine.