Runaway is a common phenomenon in pump-turbines for hydropower and ocean power, accompanied by strong instability, which can easily lead to accidents. This study reveals the stability during the runaway process of a pump-turbine, mainly exploring the phenomenon of guide vane rejection happening in transition conditions when dealing with hydropower or ocean power. Through model experiments and computational fluid dynamics numerical simulations, the pressure pulsation when reaching runaway was compared under different guide vane opening angles. The amplitude of pressure pulsation measured in the experiment increases with the increase in guide vane opening, but there are also local changes in size and peak. The simulation results show that when the guide vane opening angle is 12 degrees, the vortex flow in the area between the guide vane and the runner of the unit increases, leading to instability. When the opening angle of the guide vane is between 12 and 20 degrees, the vortex flow intensity does not change much and the distribution becomes uniform, resulting in a decrease in the amplitude of pressure fluctuations. The pulsation of the flow field causes a dissipation of flow energy. Relationships can be found among velocity field, vortex intensity, and entropy production. This study is of great significance for ensuring the stable operation of pump-turbines.

Francis turbines are commonly used for water energy utilization in medium- to high-head sections. The high head may cause strong flow-induced noise problems and adverse effects during operation. In order to explore the causes and specific locations of flow-induced noise, this study evaluated the flow-induced noise of a Francis turbine under different loads. By using computational fluid dynamics simulation methods and sound power level evaluation methods, flow-induced noise analysis could be performed based on turbulent flow simulations. By comparing experimental and simulated values, three different load conditions were selected located in the allowed, restricted, and prohibited regions. The results indicated that the flow state of the operating points in the allowed region was good and the flow-induced noise was low, except near the guide vane. The swirling flow in restricted and prohibited regions was strong; in particular, the flow-induced noise in the draft tube was relatively high. Overall, the local flow-induced noise of the runner blade inlet edge was the strongest, and the relationship between the flow-induced noise of the draft tube and the rotating flow was the strongest. The flow-induced noise near the guide vane was the most related to the opening angle, and the larger the opening angle, the greater the noise was. This study can assist in the diagnosis of Francis turbine noise problems and related low-noise design in engineering.

The axial-flow impellers are widely applied to industry due to their excellent hydraulic performance and simple structure, but they may be affected by their eccentricity during operation. This study compared and studied the effects of the axial-flow eccentricity of an impeller on hydraulic performance, impeller radial force, and downstream pressure pulsation of the unit. The research results indicate that impeller eccentricity has a small effect on hydraulic performance. Compared with the design conditions, the efficiency, power, and head changes caused by impeller eccentricity are all less than 1%, but the impeller eccentricity leads to a sharp increase in the radial force of the impeller. Under the design conditions, the average value of the radial force of the impeller is 31.38 N; under eccentric conditions, the average value of the radial force of the impeller increased by nine times, reaching 316.30 N. By analyzing the pressure pulsation signals decomposed by the VMD method, it is shown that the influence of eccentricity on pressure pulsation is mainly reflected in the increase in impeller frequency on pressure pulsation. Under design conditions, the corresponding amplitude of the impeller frequency is 2.6; under eccentric conditions, the amplitude corresponding to the impeller frequency increased by 100 times, reaching 274.4. This study elucidates the specific effects of axial impeller eccentricity, providing theoretical guidance for the safe and stable operation of axial-flow units, and has important engineering significance.