Mechanism of runner high-pressure side on stall characteristics at typical unsteady operating points in both modes of a pump turbine

Abstract

Stall phenomenon, a classical physical phenomenon which is located in the vaneless region of a pump–turbine and accompanied by a complex vortex evolution process, is strongly related to the formation of hump unsteady region at the pump mode and S unsteady region at the turbine mode. In the present paper, a detached eddy simulation model is employed to numerically investigate the impact of runner high-pressure side (HPS) on stall characteristics at typical unsteady operating points, namely, a valley point in the hump region at the pump mode and a runaway point in the S region at the turbine mode. It is found that the stall characteristics at both investigated points are obviously changed: For the valley point, only three fixed stall cells exist in the original plan, while four additional rotating stall cells appear and rotate at the speed of 0.02nr (nr, runner rotation speed) in the optimized plan (OPT). The distinctive coexistence phenomenon of both fixed stall and rotating stall is reported for the first time and is attributed to the complex vortex evolution controlled by optimized HPS; for the runaway point, both the intensity and frequency of the stall characteristic are slightly increased in OPT. Moreover, for both operating points, the optimized HPS can effectively decrease the backflow at shroud, resulting in a significant decrease in the relative backflow rate within a complete flow period, of which 17.3% is for the valley point and 4.8% is for the runaway point. Finally, a local hydraulic loss rate (LHLR) method is adopted to investigate the hydraulic loss evolution process, and it is found that the high LHLR region in OPT is more concentrated in both circumferential direction and radial direction in the vanless region at both operating points. Based on the runner with optimized HPS proposed in the present paper, many unsteady hydraulic characteristics that is related to the stall phenomenon might be eliminated to some extent.