Prediction and Analysis of the Axial Force of Pump-Turbine during Load-Rejection Process

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 pump-turbine. 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.