Draft Tube Wake Vortex Evolution and Suppression in a Pump as Turbine with Splitter Blades Based on a Modified Burgers Vortex Model

Owing to its efficient energy recovery capability, the pump as turbine (PAT) has attracted considerable attention and has been widely applied in micro hydropower systems. However, under off-design conditions, large-scale helical vortex ropes are readily induced in the draft tube, causing severe hydraulic losses and flow instability. Because existing theoretical models do not account for the slip effect at the impeller outlet, this study combines vortex dynamics theory with numerical simulation and introduces a correction coefficient to develop a Burgers vortex-based analytical wake vortex model for a PAT with splitter blades. The model is verified by its ability to predict the peak tangential velocity and radial decay trend of the vortex core. In addition, the influence of draft tube configuration on vortex rope evolution is revealed using the Liutex vortex identification method and enstrophy analysis. The results show that the geometric curvature of the elbow draft tube induces vortex rope breakup and high energy dissipation. Finally, entropy production theory is used to quantitatively evaluate the vortex suppression benefit and hydraulic loss caused by deflector plates. The results indicate that the transverse deflector plate (TDP) provides a significantly better suppression effect than the longitudinal deflector plate (LDP) by disrupting the circumferential continuity of the vortex rope. Although increasing the insertion depth of the deflector plate improves vortex suppression, it induces non-negligible local high-entropy production on the upstream-facing surface (US). This study clarifies the physical mechanism of wake vortices in a PAT with splitter blades and provides theoretical guidance for efficient PAT operation and wake vortex control.