Internal flow phenomena of a Pump-Turbine model in turbine mode with different Thoma numbers

In the low-flow rates of off-design conditions in the pump-turbines, complicated flows can develop with vortices in the flow passages, which significantly increase the vibrations and noise, resulting in destabilizing the pump-turbine system. Under low-flow-rate conditions, an appropriate Thoma number is the most essential requirement for stable operation in turbine mode of the pump-turbine. Thus, this study elucidated to examine the internal flow and unsteady pressure phenomena in a laboratory-scale pump-turbine model operated in turbine mode with different Thoma numbers. The experimental investigations were conducted under 70% of the flow rate at the best efficiency point with different Thoma number to observe the unsteady pressures and vortex rope behavior. Then, the numerical validations were conducted by three-dimensional unsteady-state RANS analyses in a SAS-SST model. The Thoma number did not significantly affect the performance of the pump-turbine model or the unsteady flow phenomena in the vaneless and runner regions. However, the Thoma number affected the draft tube region at low flow rate condition. At low Thoma numbers, the irregular flow phenomena along the flow direction enhanced the unsteady pressure characteristics. (c) 2021 Elsevier Ltd. All rights reserved.