Experimental investigation on a high head Francis turbine model during shutdown operation

Increased penetration of intermittent energy resources disturbs the power grid network. The frequency band of the power grid is normally controlled by automatic opening and closing of the guide vanes of hydraulic turbines. This has increased the number of shutdown cycles as compared to the defined ones for the normal operation of turbines. Turbine shutdown induced a significantly higher level of pressure fluctuations and unsteadiness in the flow field, decreasing its expected life. This paper presents experiments performed on a high head model Francis turbine during shutdown. The pressure and 2D Particle Image Velocimetry (PIV) measurements were performed to investigate the pressure fluctuations and flow instabilities in the turbine. The pressure sensors were mounted in the draft tube cone and vaneless space to measure the instantaneous pressure fluctuations. In the present study, the initial high load operating condition was considered to perform the turbine shutdown. The data were logged at the sampling frequency of 40 Hz and 5 kHz for PIV and pressure measurements, respectively. Time-resolved velocity and pressure data are presented in this paper to show the pressure fluctuations and causes of generation of unsteady flow in the draft tube.