Abstract: To meet the requirements by the regulation of a new power system, pump turbines need to operate in a wider range. Low load operation of a pump turbine leads to a decrease in the performance of its units, affecting its safe and stable operation. This paper presents a study of the internal flow characteristics of a pump turbine under different operating loads, based on its prototype observations and numerical simulations. The entropy production theory is used to reveal a relationship between operating loads, energy loss, and flow structure. The results show entropy production loss is characterized by certain pulsation. At the blade inlet, low-speed zones form due to flow separation as a consequence of load decreasing that causes a decrease in the relative flow angle; At the 50% load condition, such low-speed zones appear alternately in the runner. Meanwhile, vortex structures appear along the circumferential direction at the blade inlet, blocking flow into the channel and resulting in a significant increase in entropy production loss and its proportion in the runner. At the runner outlet, the tangential velocity is closely related to the formation of a vortex rope in the draft tube, and gradually increases as the load decreases. Circumferential motions take an increasing trend after the flow enters the draft tube. In the tube, a vortex rope develops from cylindrical to spiral, and shifts to double spiral forms in the low load zone at depth. The findings would help safe and stable operation of pumped storage power stations.

Key words: pump turbine, entropy production theory, low load, vortex rope, flow field