Flow characteristics of a Francis turbine under deep part-load and various no-load conditions

In the recent years, increased use of hydraulic turbines in off-design operating conditions such as no-load and deep part-load has resulted in increased damage to the turbines. A detailed understanding of the fluctuating flow phenomena can help to identify and mitigate the potentially damaging flow structures. This paper presents a comprehensive experimental and numerical study of the flow phenomena at the inlet of a Francis turbine at four no-load operating conditions, including speed-no-load and a deep part-load operating condition. Measurements are taken using a high-frequency stereoscopic endoscopic particle image velocimetry method on radial–azimuthal planes, covering the vaneless space and a large part of the interblade channels at different spans. For the speed-no-load condition, experimental data are enriched with unsteady RANS simulation data to understand the three-dimensional behavior of the flow. The average flow phenomena, transient structures and velocity fluctuations are discussed and compared among different operating points. At all operating points, the strongest average flow circulation zone (strong enough to form a vortex only at one operating condition) consistently exhibits the highest velocity fluctuation energy. The results show that the highest velocity fluctuations, and thus the most energetic dynamic structures, are in a no-load operating point with a guide vane opening smaller than speed-no-load. Position and intensity of the interblade vortices varies not only with the guide vane opening but also with the amount of torque extracted by the runner.