Fluid flow analysis of drooping phenomena in pump mode for a given guide vane setting of a pump-turbine model

The energy-discharge characteristics of pump-turbines in pump mode with a hump region are significantly important for operating stability. To investigate the flow characteristics, 3D steady numerical simulations are conducted for a given guide vane opening of 32 mm by solving Reynolds-averaged Navier-Stokes (RANS) equations using the shear-stress transport (SST) k-omega turbulence model. Based on the validation of computational fluid dynamics (CFD) results using experimental benchmarks, the part-load (0.45 phi (BEP)), drooping zone load (0.65 phi (BEP)), near best efficiency point (BEP) (0.90 phi (BEP)), BEP (1.00 phi (BEP)), and overload (1.24 phi (BEP)) regions are chosen to analyze how and why the fluid properties change in the runner. The causes of flow separation and spatial characteristics of flow at different load points are obtained through the analysis of flow angle and hydraulic losses. The results show that flow angle at the leading and trailing edge from the crown to the band distributes differently among these five operating points. Then, the reasons for drooping are investigated based on the Euler theory. It is found that drooping behavior comes from both the incidence/deviation effect and frictional losses. In addition, the runner losses are more consequential to drooping as shown by hydraulic loss analysis.