Hydrodynamic characteristics of a pump as turbine under periodic inflow fluctuations

In seawater reverse osmosis (SWRO) systems, the pump as turbine (PAT) recovers energy of high-pressure brine by coupling with high-pressure pumps. This can significantly reduce the system’s specific energy consumption. However, the upstream flow rate to the PAT is not constant during operation. It typically fluctuates within a certain range, thus affecting the stability of the PAT during actual operation. Therefore, this study uses computational fluid dynamics to investigate the transient flow behavior of a PAT with a fixed-axis and constant-torque connected load pump under periodic fluctuations in the incoming flow. To simulate the fluctuating flow process, user-defined functions are implemented to describe the instantaneous flow variations and capture the transient hydrodynamic characteristics at various fluctuation frequencies. The numerical simulation results are validated against experimental data. The results indicate that when the PAT is subjected to periodic inflow fluctuations, the rotational speed, torque, and internal flow field of the impeller vary significantly with the flow rate. This will exert a significant impact on the efficiency and stability of PAT operation. Meanwhile, the external characteristic parameters (head, power, and efficiency) and stability of the PAT are also affected by the inflow fluctuation frequency. At higher inflow fluctuation frequency, the PAT system exhibits higher average efficiency and a small axial force fluctuation in the impeller, but the radial force fluctuation is more intense. The results obtained in this study can provide theoretical support for the anti-interference design and operational optimization of PAT energy recovery systems in SWRO systems.