Effect of runner blade numbers on inter blade vortices of Francis turbine under best efficiency loading.

Abstract In this paper, the investigation of effect of change in blade number to the generation of inter-blade vortices was done in case of high head Francis turbine. Further, connection of these vortices with Rotor Stator Interaction (RSI) along with pressure fluctuation through runner blades was explored. Selection of optimal number of blades in design of Francis turbine is crucial since it directly impacts the performance of the turbine in all operating condition along with flexibility need in hybrid energy systems. This study focuses on the modified Francis-99 turbine design with the removal of the splitter blades. Eight different configurations with varying blade number ranging from minimum 14 numbers to maximum 21 numbers were formulated. It is revealed that influence of inter blade vortices on RSI is significant which also contribute to hydraulic loss and vibrations. Moreover, it is observed that inlet inter-blade vortices grow in strength with decreasing the number of blades in runner. These vortices generated at the inlet of the blade will be attached to the suction side towards shroud and persist all the way to the trailing edge, significantly disrupting the flow downstream.