This paper presents a large eddy simulation of a centrifugal pump impeller during a transient condition. The flow rate is sinusoidal and oscillates between 0.25Qd (Qd indicates design load) and 0.75Qd when the rotating speed is maintained. Research shows that in one period, the inlet flow rate will twice reach 0.5Qd, and among the impeller of one moment is a stall state, but the other is a non-stall state. In the process of flow development, the evolution of low-frequency pressure fluctuation shows an obviously sinusoidal form, whose frequency is insensitive to the monitoring position and equals to that of the flow rate. However, inside the impeller, the phase and amplitude in the stall passages lag behind more and are stronger than that in the non-stall passages. Meanwhile, the strongest region of the high-frequency pressure fluctuation appears in the stall passages at the transient rising stage. The second dominant frequency in stall passages is 2.5 times to that in non-stall passages. In addition, similar to the pressure fluctuation, the evolution of the low-frequency head shows a sinusoidal form, whose phase is lagging behind that by one-third of a period in the inlet flow rate.
The flow characteristics at a centrifugal pump impeller are significantly affected by the hub inclination angle (HIA). The large eddy simulation turbulence model is employed to simulate the centrifugal pump impeller internal flow at two HIA values under design (1.0 Qd) and off-design (0.55 Qd) conditions. The reliability of the simulation results is verified by comparing the results at a 0° impeller HIA and experimental data. The results show that an increase in HIA has a positive effect on the head of the centrifugal pump impeller, which exhibited a 5% relative deviation between HIA values of 0° and 40°. Under design conditions, there is a good and similar impeller internal flow at the two HIA values. Under off-design conditions, the relative velocity of the 40° impeller HIA channel inlet is 11.7% higher than that of the 0° impeller HIA, which reduces the flow attack angle and effectively suppresses stall. Moreover, for the 0° impeller HIA, fluctuation caused by the stall cells on the blade suction surface is observed, whereas such a phenomenon is not observed for the 40° impeller HIA.
This study uses large eddy simulation to investigate the flow characteristics of a centrifugal pump impeller with sinusoidal flow rate and constant rotational speed. Five different oscillation frequencies ( f/f0 = 1.0, 1.2, 1.5, 2.0, and 3.0, where f0 indicates one-ninth of the rotational speed) are selected to isolate the influence of oscillation frequency on the flow characteristics. Studies show that the pressure deviation between dropping and rising stages concentrates at the impeller inlet at instantaneous 0.5 Qd ( Qd is the design load), and its value increases with the increase in oscillation frequency. The lowest period of many flow characteristics, including pressure fluctuation and head, equals to that of flow rate. The differences in pressure fluctuation between adjacent channels decreases with the increase in oscillation frequency. The first and second dominant frequencies of pressure fluctuation are mainly affected by the oscillation frequency in the nonstall passage, and they have an important relationship with the stall vortex in the stall passage. With the increase in oscillation frequency, the amplitude of lowest order head pulsation increases, and the high order head pulsation gradually weakens. The average head is approximately 2.36 m.
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