Effect of Draft Tube Vortex Rope on Shaft System Mechanical Response in Pump-Turbines

During frequent peak-regulation operation, pumped storage units inevitably operate under off-design conditions. Variations in draft tube flow structures under different load conditions may alter the hydraulic loading acting on the runner and shaft system. In this study, a model pump-turbine was investigated using unsteady numerical simulations combined with finite element structural analysis. Analyses were conducted under 70%, 100%, and 105% load conditions, representing typical part-load, rated, and overload operating states. The results show that under 70% load, the draft tube flow exhibits an eccentric structure with strong circumferential pressure non-uniformity. At the rated condition (100% load), the flow field is closest to axisymmetric with the most uniform pressure distribution. Under 105% load, a concentrated central vortex forms, and the degree of non-uniformity is intermediate. The hydraulic loads under different conditions lead to significant differences in the stress and deformation characteristics of the shaft system. The maximum equivalent stress increases from 1.84 MPa at the rated condition to 3.33 MPa under overload conditions, while more pronounced flow asymmetry and pressure non-uniformity occur at part-load operation. These findings clarify the relationship between load variation, draft tube flow structures, and the mechanical response of the shaft system.