Influence of geometric factors at runner outlet on the hump characteristics of a pump-turbine

Abstract

In order to investigate the influence of geometric factors at runner outlet on the hump characteristic of pump-turbines, three different distribution plans of geometric factors at runner outlet are proposed and numerical simulated using the SST (shear stress transport) k-ω turbulence model. Numerical results show good agreement with the experimental data. Different distribution plans show large difference at the hump region and of which the plan with large radius and small outlet angle near the shroud can improve the hump safety margin from 4% to 5.82%. The causes are analyzed through the input energy (the Euler head) and the hydraulic loss respectively. The analysis results show that the best distribution plan can provide small absolute flow angle near the hub and the middle spanwise at the runner outlet, leading to the increase of the Euler head. Moreover, for the best distribution plan, the hydraulic loss distribute more evenly at part load operating points, resulting in the movement of the hump region to smaller discharge operating points and the increase of the hump safety margin. Detailed local entropy production rate (LEPR) analyses reveal that when the pump-turbine operates in hump region, the hydraulic loss is concentrate in vaneless region and show different variation rules in discharge varying direction.