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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. [ABSTRACT FROM AUTHOR]

Savonius hydrokinetic turbine is a kind of turbine set which is suitable for low-velocity conditions. Unlike conventional turbines, Savonius turbines employ S-shaped blades and have simple internal structures. Therefore, there is a large space for optimizing the blade geometry. In this study, computational fluid dynamics (CFD) numerical simulation and genetic algorithm (GA) were used for the optimal design. The optimization strategies and methods were determined by comparing the results calculated by CFD with the experimental results. The weighted objective function was constructed with the maximum power coefficient Cp and the high-power coefficient range R under multiple working conditions. GA helps to find the optimal individual of the objective function. Compared the optimal scheme with the initial scheme, the overlap ratio β increased from 0.2 to 0.202, and the clearance ratio ε increased from 0 to 0.179, the blade circumferential angle γ increased from 0° to 27°, the blade shape extended more towards the spindle. The overall power of Savonius turbines was maintained at a high level over 22%, R also increased from 0.73 to 1.02. In comparison with the initial scheme, the energy loss of the optimal scheme at high blade tip speed is greatly reduced, and this reduction is closely related to the optimization of blade geometry. As R becomes larger, Savonius turbines can adapt to the overall working conditions and meet the needs of its work in low flow rate conditions. The results of this paper can be used as a reference for the hydrodynamic optimization of Savonius turbine runners. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]