Three-dimensional numerical simulation for a prototype pump-turbine has been performed to investigate the transient characteristics and flow behaviors in the startup process. The rotational speed, which varies with runner torque and inertial of the rotating systems, is obtained by hydraulic-force coupling method. To simulate the guide vane motions, dynamic overset mesh technique is applied with good maintenance of mesh quality. Comparisons with the existing experimental results verify the validity of the current method in predicting the startup transient characteristics of a pump-turbine with good accuracy. During the startup transient, the guide vanes open according to a prescribed sequence, which brings in increasing flow rate to the unit. The runner accelerates due to growing water power energy exerted on the runner blades. When approaching the speed no-load condition, a ring-shaped flow enlacing the entire vaneless space, that we term ''water-ring,'' is found to block the through-flow along the circumference, and the pump-turbine features a prominent partial pump flow state in the runner, which equalizes the torque to no load. Thus, the runner rotational speed stops increasing and stabilizes toward rated speed. The prediction shows that the pressure fluctuation amplitude of the speed no-load can be 3.8 to 8 times that of the full load, presenting evidently stronger instability at the speed no-load.
During the starting up of the pump mode in pump turbines, the axial hydraulic force acting on the runner would develop with the guide vane opening. It causes deformation and stress on the support bracket, main shaft and runner, which influence the operation security. In this case, the axial hydraulic force of the pump turbine is studied during the starting up of pump mode. Its influences on the support bracket and main shaft are investigated in detail. Based on the prediction results of axial hydraulic force, the starting-up process can be divided into “unsteady region” and “Q flat region” with obviously different features. The mechanism is also discussed by analyzing pressure distributions and streamlines. The deformation of the support bracket and main shaft are found to have a relationship with the resultant force on the crown and band. A deflection is found on the deformation of the runner with the nodal diameter as the midline in the later stages of the starting-up process. The reason is discussed according to pressure distributions. The stress concentration of the support bracket is found on the connection between thrust seating and support plates. The stress of the runner is mainly on the connection between the crown and the blade’s leading-edge. This work will provide more useful information and strong references for similar cases. It will also help in the design of pump turbine units with more stabilized systems for reducing over-loaded hydraulic force, and in the solving of problems related to structural characteristics.
The startup process occurs frequently for pumped storage units. During this process, the rotating rate that changes rapidly and unsteady flow in runner cause the complex dynamic response of runner, sometimes even resonance. The sharp rise of stress and the large-amplitude dynamic stresses of runner will greatly shorten the fatigue life. Thus, the study of start-up process in turbine mode is critical to the safety operation. This paper introduced a method of coupling one dimensional (1D) pipeline calculation and three-dimensional computational dynamics (3D CFD) simulation to analyze transient unsteady flow in units and to obtain more accurate and reliable dynamic stresses results during start up process. According to the results, stress of the ring near fixed support increased quickly as rotating rate rose and became larger than at fillets of leading edge and band in the later stages of start-up. In addition, it was found that dynamic response can be caused by rotor stator interaction (RSI), but also could even be generated by the severe pressure fluctuation in clearance, which can also be a leading factor of dynamic stresses. This study will facilitate further estimation of dynamic stresses in complex flow and changing rotating rate cases, as well as fatigue analysis of runner during transient operation.
Load-rejection is an important process of pump-turbine unit. In this process, hydraulic characteristics would change greatly. The axial force of runner and of the entire shaft system will be impacted. This study focuses on a prototype pump-turbine unit. The axial force of this unit during load-rejection is predicted and analyzed. The hydraulic transient analysis can give a more accurate boundary condition of pumpturbine. The computational fluid dynamics simulation considering the full leakage in runner crown and shroud can have a comprehensive prediction of the axial force. Results show that the rotation speed, head and flow rate change during load-rejection process. The axial force is also influenced and changes. The law of axial force variation has a strong relationship with the flow rate variation law. The internal flow analysis found that the pressure difference between crown leakage and internal-runner near outlet is the key factor in inducing axial force. This pressure difference is strongly influenced by runner internal flow regime. During load-rejection process, flow rate strongly varies and causes the huge change of internal flow regime. This is the main reason that runner axial force changes positive and negative alternatively.
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