Due to its unique structural features, after the structure of the water pump and flow machine meets the hydraulic design requirements, the main issues are stress control of the impeller blades and improving their fatigue resistance.
The runner of a mixed-flow pump-turbine unit consists of an upper crown, blades, and a lower ring. It has a small number of blades, but they are long and thin, typically 6-10, as seen in the 5+5 long-short blade configuration used in Jixi and Yangjiang pump-turbines. The blade wrap angle is large, and the flow passage is narrow and long. With the increasing head of pump-turbine applications, such as the 700m head sections in Dunhua and Yangjiang, the runner has become very flat, with a very high circumferential speed at the outer edge, reaching a maximum of 112.6 m/s. Due to its unique structural characteristics, after meeting the hydraulic design requirements, the main challenge for the pump-turbine structure is stress control of the runner blades and improving their fatigue resistance.
The runner of a high-head pump-turbine experiences different forces on its blades compared to those of a conventional turbine due to its unique shape. Conventional turbine runners have blades with large height and short streamline length, with each blade approximately fixed to a flat plate on its upper crown. The forces acting on the blades are primarily bending stresses caused by water pressure, which are partially offset by centrifugal force during runaway.
However, the blades of pump-turbines, especially high-head pump-turbines, are relatively long, with a large wrap angle and a long connection point with the upper crown and lower ring. Therefore, the bending force exerted on the blades by water pressure is not significant; the blade stress is mainly caused by centrifugal force. During normal operation, the blades bear a hydraulic load, and some of the centrifugal force is offset, resulting in minimal blade stress. When rotating for standby, the water pressure on the blades is low, leading to higher stress. During load shedding and speed increase, centrifugal force is the primary factor, resulting in maximum stress. Therefore, under runaway conditions, the influence of centrifugal force is particularly pronounced.
Due to the significant interference between the turbine runner blades and guide vanes, the dynamic stress on the blades is greater than that of conventional units. In recent years, the finite element method has been widely used to calculate the stress and deformation of the runner. During runner design, it is crucial to ensure a uniform stress distribution on the blades to avoid excessive stress concentration, and to maintain a sufficient interval between the runner's natural frequency and excitation frequency (hydraulic impact frequency) in the water to effectively prevent resonance. This reduces the dynamic stress of the runner to below the material's fatigue limit, ensuring that the runner does not develop any cracks, fractures, or harmful deformations under cyclically varying loads during its service life.
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