Numerical investigation of the wake structure and flow energy loss in the pump as a turbine with splitter blades

Pump as Turbine (PAT) systems have gained significant attention recently as they are a renewable energy source due to their cost-effectiveness and operational flexibility. However, the complex internal flow structures, particularly the wake phenomena and associated flow energy losses (FEL), remain poorly understood, limiting further optimization of PAT systems. The objective of this study is to investigate the wake structure and its correlation with the flow energy loss in a PAT operating in turbine mode. The investigation employs the detached Eddy Simulation (DDES) numerical approach to simulate the unsteady 3-D internal flow within the PAT in turbine mode. The findings showed three locations of the wake structure inside the current PAT: downstream of the stator blades, the runner's splitter blades, and the runner's main blades. The splitter wake region displays the highest flow velocity reduction at approximately 67 %, followed by the stator (65 %) and the runner main blade wake region (62 %). Additionally, the FEL values are highest at the splitter wake region (20), then the stator (19), and the runner main blade (11) at the best efficiency point. The profile of the relative velocity has a contrary relationship with the FEL pattern, as it is found that the FEL increases while the velocity coefficient decreases in all wake regions. Moreover, the high FEL and velocity gradient in the wake region collectively contribute to unsteady shedding and interaction processes, ultimately leading to FEL and reducing the turbine's performance. The findings highlight the significant impact of wake regions on energy losses for future optimization, which can enhance overall PAT performance.