Force Characteristics of Centrifugal Pump as Turbine during Start-Up Process under Gas–Liquid Two-Phase Conditions

The start-up process of a centrifugal pump as turbine (PAT) under gas–liquid two-phase conditions was simulated based on Fluent, and the evolution mechanism of the internal flow field and the variation law of force characteristics were studied in its start-up process under gas–liquid two-phase conditions. The results show that the area with high gas phase concentration corresponds to a strong vortex at the beginning of the start-up. The vortex intensity in the impeller gradually decreases with an increase in rotational speed. The gas volume fraction of the blade suction surface is more significant than that of the blade pressure surface. The higher the inlet gas volume fraction (IGVF) is, the more severely the blade load will fluctuate during the start-up process. As the rotational speed increases, the fluctuation of the blade load gradually weakens, and the maximum load is distributed near the inner edge of the blade after the rotational speed is stable. The periodic unbalanced radial force is produced in the start-up process. From the pure liquid conditions to the gas–liquid two-phase conditions with increasing IGVF, the dominant frequency amplitude of radial force shows a similar trend of decreasing first but then increasing. After the rotational speed tends to be stable, the dominant frequency of radial force is equal to the rotational frequency of the blade. With the increase in rotational speed, the dominant frequency amplitude of axial force decreases gradually. The higher the IGVF, the greater the dominant frequency amplitude of axial force at the same time.