Application of an Off-Design Transient Simulation Framework for Pump-as-Turbine in OpenFOAM: Validation and Flow Analysis

Pump-as-Turbine (PaT) systems represent a cost-effective solution for hydraulic energy recovery in existing water networks and industrial processes. However, the prediction of their performance in turbine mode remains challenging, especially under off-design conditions where unsteady flow structures and internal losses strongly affect the machine efficiency. In this work, transient CFD simulations of a real industrial centrifugal pump operating as a turbine are performed using OpenFOAM and ANSYS CFX and compared with available experimental data. The investigated operating range extends from 0.7QBEP to 1.3QBEP. A mesh independence analysis is first carried out using the Grid Convergence Index method, leading to the selection of a mid-size computational mesh as a compromise between accuracy and computational cost. The transient OpenFOAM results show close agreement with the ANSYS CFX predictions over the complete operating range. Both numerical frameworks reproduce the experimental hydraulic-efficiency trend and the location of the BEP, while systematic deviations in hydraulic head and mechanical power are mainly attributed to the geometrical and physical simplifications adopted in the common computational model. The local pressure coefficient monitored at the tongue region shows that both the mean pressure level and the fluctuation amplitude increase with flow rate, indicating stronger transient behavior under high-flow conditions. Beyond the global performance comparison, the flow field is analyzed using Qcrit iso-surfaces, mean circumferential velocity, the swirl-intensity parameter Sint, relative velocity fields at the PaT operational leading edge, and volute head-loss evaluation. The results show that part-load operation is characterized by strong outlet vortical structures and high residual swirl intensity, while the BEP region corresponds to reduced outlet rotational content. Under overload conditions, the outlet swirl remains limited, but the volute head loss increases significantly, becoming a dominant contributor to the efficiency drop. The study demonstrates that PaT performance cannot be interpreted from outlet swirl alone, but results from the combined effect of residual rotational structures, tongue-region unsteadiness, impeller incidence conditions, and volute dissipation.