When a pump operates in part-load conditions, it is apt to form flow separations and even stall cells at the blade surfaces. In some conditions, stall cells may circumferentially propagate among the blade channels, known as rotating stall, which can affect the rotor system dynamic stability. Conventionally it is believed that the formation and circumferential propagation of stall cells is attributed to the nonuniformity of the flow state in front of the impeller inlet. In recent decades, many investigations indicate that the uneven flow field at the impeller outlet related to the asymmetric volute casing is also an important factor to induce rotating stalls. Thus the formation and propagation mechanism of rotating stalls is complex and has not been clearly understood so far. In addition, previous studies mainly focused on the rotating stalls in vaned diffusers, while it is more difficult to figure out rotating stalls in impellers for which little studies has been done. In this paper, numerical simulations are conducted for a three-blade centrifugal pump with a flow rate of 0.75Qd where Qd is the design flow rate. The SST-SAS model with a curvature correction is applied to predict the unsteady internal flows. The time-averaged pump head, efficiency and axial power agree well with the experimental results from a previous test. From the numerical results, a special rotating stall is detected in this condition. In order to verify the effect of the volute casing, a contrast simulation is also conducted without the volute casing domain. It shows that rotating stalls always occur whether there is a volute casing or not, but the distribution and motion of the stall cells are changed by the existence of the volute casing. It indicates that the nonuniform flow distribution at the impeller outlet is not an essential factor for the formation of rotating stall but accelerates the variation of stall cells. Based on the whole-flow-passage result, a stall cell in one blade channel disappears for about 2/15T (T is the duration of one revolution) when the downstream blade runs across the tongue, and the same phenomenon recurs in the upstream blade channel after 1/3T, which is unusual and worthy of investigation. The pressure fluctuations within the whole flow passage are intensified by the rotating stall more or less, especially at the middle region of pressure surface of blades, around the tongue and in the beginning of the discharge passage.

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