**1. Introduction**

A reversible-pump turbine (RPT) is designed to pump water from a lower reservoir to a higher reservoir by using the surplus energy in a power grid. Furthermore, water has to go down to generate electrical energy through the RPT at peak hours or in case of an emergency. Hence, an RPT has to change working conditions between the pump mode and turbine mode [1]. In order to meet the requirements of a power grid, an RPT usually works in a low-load off-design working condition. Due to the RPT's special way of operating, it is hard to guarantee the stability of the units. Facing these challenges, many experts have made a lot of contributions. The unstable phenomena that occur in an RPT under runaway conditions (S-shaped characteristic in the turbine) are associated with fluctuations in the head and discharge of the system. This unstable behavior has been addressed by some authors [2–7] who believed that the nature of the vortex structures can be the mechanism that leads to potentially unstable characteristics.

Based on previous works, C. Gentner [8] performed a flow survey with CFD and PIV, and the results of the simulation and experiment had a great agreement, which strongly supported the earlier explanation of the mechanisms. In addition, some other authors pointed out that the reasons for the S-shaped characteristics are related to the development of rotating stall in the runner channels [9–12], or they are influenced by secondary flow in all parts of the RPT, and especially in the vaneless space [13]. Reference [14] performed

**Citation:** Ji, Q.; Wu, G.; Liao, W.; Fan, H. Flow Deflection between Guide Vanes in a Pump Turbine Operating in Pump Mode with a Slight Opening. *Energies* **2022**, *15*, 1548. https:// doi.org/10.3390/en15041548

Academic Editor: João Carlos de Campos Henriques

Received: 9 January 2022 Accepted: 16 February 2022 Published: 19 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

a depth study of the onset and development of the unsteady phenomena that cause the unstable behavior of a pump turbine during a load-rejection scenario with servomotor failure, and their work revealed that, only when the unsteady phenomena evolved in a fully developed rotating stall that was characterized by a well-defined frequency, the unstable behavior of the RPT in turbine mode could be limited or eliminated with proper design criteria that aimed to move this critical operating point at lower flow rates.

Unstable phenomena such as rotating stall always come with the vibration of the units [5–7], which is harmful to the stability of the turbine and other devices. In addition, especially during the startup or shutdown process, the resonance of the units also frequently occurs, which is usually caused by rotor–stator interference (RSI) [15], self-excitation [16], or pressure pulsation [17]. To improve the stability and guarantee the successful startup of the units, misaligned guide vanes (MGVs) have been implemented and have achieved the expected results [18].

However, a few years ago, in both the Tianhuangping pumped-storage power station [19] and Yixing pumped-storage power station [20], abnormal sounds and vibrations occurred at the distributor of the RPT, which was working in the pump mode, and the guide vane opening was very small. In order to resolve this problem, B. Nennemann [21] conducted detailed research about the abnormal phenomena in the Yixing pumped-storage power station by using a 2D periodical CFD simulation. Consequently, the author claimed that unexpected bi-stable flow conditions and a self-excited torsion-mode flutter vibration were discovered in the GVs. Moreover, the vibration problem could be successfully eliminated by modifying the shape of the vanes. To address the issue of Tianhuangping, H. G. Fan [22] studied the HT of the GVs of the RPT during the startup and shutdown processes in the turbine mode with a 2D periodical CFD simulation. His work indicated that a repeated reversal of fluid occurred when the GVs worked with a slight opening during the shutdown process in the turbine mode. This phenomenon finally resulted in a dramatic increase in the HT, which caused the vibrations.

The aforementioned research on vibrations and abnormal sounds occurring at the distributer of an RPT was all performed with 2D periodical CFD simulations; however, the flow in an RPT is much more complicated when the working conditions are far from the optimal operation point and the aperiodicity of the flow is obvious [23]. Therefore, a 2D periodical CFD simulation is imperfect for dealing with this issue, and it cannot capture all of the details of the flow behavior. Based on previous studies, this paper describes the performance of a 3D CFD simulation of the startup and shutdown processes of the unit of the Tianhuangping pumped-storage power station when working in the pump mode. In the present study, the dynamic meshing technique was implemented, and more details of the flow behaviors were captured.
