Flow instability of an axial flow pump-as-turbine using relative streamline coordinates

Abstract

When axial flow pumps-as-turbines (PATs) operate under off-design conditions, unstable and unsteady flow structures appear in the internal flow field, resulting in suboptimal functioning. These operating conditions not only decrease the efficiency of the hydraulic machines but also affect their mechanical reliability. This study establishes relative streamline coordinates, based on the blade's mean camber line, to investigate flow instabilities in axial flow PATs from a new perspective. Numerical simulations on an axial flow PAT were performed and validated using experimental data. The results show that flow separation is more likely to occur due to the more curved profile at the blade's suction surface, leading to considerable fluctuations in velocity along the flow direction and enstrophy amplitude near both the hub and impeller shroud. Moreover, the poor matching of the relative inflow angle of the impeller with the blade inlet angle leads to impingement losses near their leading edge, generating unstable flows and significant pressure pulsations, which induces hydraulic instability within the impeller. In addition to rotor-stator interference effects, the curvature of the blade suction surface profile and the bend structure of inlet conduit are significant factors that influence the pressure pulsation distribution of the PAT. An analysis of the enstrophy transport equation indicates that the relative vortex generation and the Reynolds stress dissipation terms play a key role in both vortex generation and dissipation, whereas the viscous term has a lower influence. These findings can serve as a reference for the optimization and efficient design of axial flow PATs.