| In China,a series of large-scale inter-basin water transfer projects have been constructed,including the Eastern Route of the South-to-North Water Diversion Project,the Yangtze River Diversion to Huaihe River,the Yangtze River Diversion to Taihu Lake,and the North-to-South Water Transfer Project.These projects aim to transfer water from water-rich regions to waterdeficient plain areas.Numerous large-scale pump station hubs have been built to facilitate the efficient utilization of water resources.During non-water transfer periods,some pump station hubs can be utilized for reverse power generation by using surplus water from upstream through the reverse operation of water pumping units,thereby achieving rational development and utilization of water resources.However,it has been observed that hydraulic vibrations detrimental to the safe operation of the units may occur in the pump devices during reverse power generation,which could further lead to vibration-induced faults.Therefore,it is imperative to conduct relevant research on this issue.This study focuses on investigating the hydraulic excitation characteristics of vertical mixed-flow pump devices under reverse power generation conditions,with the Shaji Pump Station in Xuzhou City as the research object.Numerical simulation methods are employed to analyze the flow characteristics,energy characteristics,and pressure pulsation characteristics inside the vertical mixed-flow pump device.Additionally,a fluid-structure coupling method is utilized to conduct mechanical analysis of the impeller rotor.The research content and results are presented below:(1)Hydraulic performance and energy characteristics of a vertical mixed flow pump unit under reverse power generation conditions.The flow state in the discharge channel(water intake pipe)is relatively good,with the majority of the mainstream entropy production and wall entropy production concentrated at the bends of the pipe.The flow velocity distribution at the inlet and middle sections of the guide vane is uniform,with high-pressure regions mainly distributed on the suction side of the blade and low-pressure regions concentrated on the pressure side of the blade.The entropy production losses are relatively small,primarily distributed around the hub and blade rim,and the high-value regions of wall entropy production rate are mainly located at the exit of the guide vane rim,indicating significant effects of dynamic and static interference.The inlet section of the impeller is influenced by the guide vane,resulting in the presence of 7 high-pressure regions and 7 lowpressure regions.The majority of mainstream entropy production is concentrated on the suction side of the impeller blades and near the impeller exit.With an increase in the head of the generated power,the range of entropy production rate loss for the impeller gradually decreases.The flow state in the elbow-shaped inflow channel(tailrace pipe)is complex,with small high-pressure regions appearing on both sides of the inlet mouth.Adverse flow conditions are concentrated in the elbow section,with the presence of small high-pressure and low-pressure regions.The entropy production losses are relatively large.With an increase in the head of the generated power,the flow state improves,resulting in a decrease in the entropy production rate at the inlet of the elbow-shaped inflow channel and the elbow section..(2)Pressure pulsation characteristics at key positions of a vertical mixed flow pump unit under reverse power generation conditions.The pressure distribution and values at the inlet of the guide vane are relatively similar and do not show significant changes with variations in the generated power head.The pressure fluctuations in the middle of the guide vane are relatively regular,with the maximum amplitude of pressure fluctuations occurring at the rim,exhibiting a certain level of periodicity.As the generated power head increases,the amplitude of pressure fluctuations decreases,gradually approaching stability.The flow conditions at the impeller inlet are complex,resulting in a rich spectrum of frequencies,including one main frequency and multiple sub-frequencies.The overall trend of pressure fluctuations at the impeller outlet is consistent,with the amplitude decreasing radially from the rim towards the center.With an increase in the generated power head,the range of pressure fluctuations at the impeller harmonics gradually decreases.In the elbow section of the elbowshaped inflow channel,the velocity,direction,and pressure of the water flow undergo changes.Influenced by the dynamic and static interference between the impeller and the elbow-shaped inflow channel,low-frequency pulsations dominate,and the main frequencies of pressure fluctuations are within 5 times the rotational frequency.(3)The force characteristics of the impeller in a vertical mixed flow pump unit under reverse power generation conditions.The stress and strain distribution on the pressure and suction sides of the impeller exhibit a similar pattern,showing a circular symmetrical distribution.The maximum and average stress and strain of the impeller blades gradually decrease,while the minimum stress and strain increase.The deformation increases from the inside to the outside,with the maximum deformation occurring at the outer edge of the blade inlet and the minimum deformation near the hub.The application of prestress has a minor influence on the modal characteristics of the impeller.The modal frequencies tend to increase with the order of the mode.The modal frequencies of the impeller rotor in water are relatively higher than those in air.Furthermore,as the order increases,the reduction in modal frequency becomes more significant.Different vibration modes at different orders are influenced by the additional mass forces of the water in different ways. |
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