Vortex evolution and energy loss of an axial flow Pump-As-Turbine (PAT) with the influence of upstream waves

Pump-As-Turbine (PAT) technology is currently widely used in large-scale and small rural hydropower projects. However, its energy performance is inevitably affected by upstream flow state and this aspect deserves an in-depth investigation. For this reason, the entropy production method and Omega method are used to study the distributions of both the energy loss and vortices on a PAT with upstream waves. The Mean Entropy Production Rate (MEPR) and mean rotating vorticity ΩR are redefined to quantify the energy loss rate and vortex development while removing the non-uniformity in the integration regions. Results showed that the simulated external characteristics of the analyzed PAT at different operating conditions with no waves are in good accordance with the experimental data. Macroscopically, the trend of the vortices in the PAT along the mainstream direction is consistent with the energy loss; specifically, the MEPR and ΩR values peak in the impeller and decrease as they move away from it. Furthermore, the Energy Production Rate (EPR) is high at the vortex aggregation identified by the Omega method, meaning that the gathered vortices lead to large energy losses. The enhancement of the energy loss in the analyzed PAT with upstream waves gradually decreases from inlet to outlet with a maximum growth rate of MEPR of 96 % at the inlet. Regarding the impeller, the enhancement of upstream waves in the energy loss is limited, that is, the highest growth rate of MEPR was equal to 3.6 %. In the inlet pipe, shedding vortices are generated from the attached ones by the influence of upstream waves, and the generation period of the shedding vortices is the same as the upstream wave period.