Influence of suction vortex evolution on the hydraulic stability of a vertical centrifugal pump

Pumping stations can serve as crucial hydraulic facilities in various applications. However, their operating conditions can often lead to the presence of suction vortices in the sump. There are some serious impacts on the safe and stable operation of the pumping station. This study aims to explore the effect of the suction vortex morphology and its evolution on the stability of the centrifugal pumps under different operating conditions. A full-channel test platform was constructed for the vertical centrifugal pump. The research object was taken as the suction pipe on the horizontal side. The volume of fluid (VOF) was used to simulate the flow characteristics. A systematic analysis was made to explore the effects of the unsteady flow on the steady operation of a centrifugal pump during the evolution of the suction vortex at high flow rates. The results show that the steady duration of the suction vortex increased with the flow rate increased during testing. The shape of the vortex was greatly varied from the surface depression to the continuous suction. The strength and diameter of the suction vortex increased continuously at the high flow rates. The whole observation time was in the continuous phase of the suction vortex. Among them, the projected area remained at a high value. At the same time, the shedding of the large-scale bubble also caused the projected area to fluctuate violently. Excellent agreement was found in the development paths of the suction vortices that were captured by numerical simulations and the experimental ones. The suction vortices first appeared near the wall of the tube, then broke off into the suction tube under the action of the incoming flow after being destroyed, and flowed along the bottom of the suction tube to form two air paths and large bubble clusters at the inlet of the centrifugal pump. The air moved constantly toward the centrifugal pump at the high flow rates, as the suction vortex evolved. The content of air then reached a peak in the flow passage components during the continuous phase. As the suction vortices evolved from the development phase to the continuous phase, the void fraction rapidly increased, and the bubbles clustered in the flow channel of the impeller, leading to a sudden change in the pressure pulsation on the suction surface of the blades. The pulsation range of pressure in the continuous phase was 2 times that of the development phase. The main frequency of the pressure pulsation was shifted from f_n（rotating frequence）to 2f_n. While the amplitude of the pulsation at 2f_n increased along the flow direction. The gas action was also enhanced dramatically by the non-uniform inflow. Large-scale vortices were observed within the channel of the impeller. The greatest velocity of the gas was observed at the leading edge of the blade near the suction pipe side. In the rest of the channel, the gas velocity varied from increasing first to decreasing, leading to the increasing amplitude of the radial force on the impeller and the radial force vector to eccentric. The gas was distributed unevenly in the impeller flow passage and then converged at the outlet of the volute after rotating. The high peak-and-peak values of the pressure pulsations were found in the three specific regions (vaneless region, guide vane, and volute) of the centrifugal pump. Furthermore, the bubble moved with the main flow towards the tongue in the continuous phase of the suction vortex. The density of the gas was much lower than that of the liquid. The bubble cluster was prevented from a pressure gradient to occupy the flow channel. There was a significant increase in the turbulent pulsation. The region was then expanded into the highly turbulent kinetic energy. Consequently, there was an increase in the peak-to-peak value of the pressure pulsation at the tongue, thus affecting the operational stability of centrifugal pumps.