Experimental study of instability characteristics in pump turbines

Experiments in a medium specific-speed pump turbine scaled model are performed to investigate instability characteristics in pump mode, namely the head-drop phenomena. Two different head-drops are captured on a pump performance curve. This paper presents the experimental investigations of the second head-drop phenomenon, with the flowrate varying from 0.6 to 0.7 times of the design flowrate in the scaled model. The low frequency signal of 0.14 times of the impeller rotating frequency in the draft tube is captured during the generation of the second head-drop. The alteration of the cavitation near the impeller inlet domain is also observed via a high-speed camera. These findings indicate that the complex vortices in the interaction area between the impeller inlet and the draft tube play a key role in the generation of the second head-drop phenomenon.