Unsteady flow phenomena like rotating stall frequently occur in centrifugal pumps under off-design conditions. Rotating stall could lead to flow instabilities and pressure pulsation, which affect the normal operation of pumps. The mechanism of rotating stall has not been sufficiently understood in previous researches. In this study, the impact of rotating stall in the impeller on centrifugal pump stability and pressure pulsation is numerically investigated. This paper aims to detect the unsteady flow characteristics inside the centrifugal pump by computational fluid dynamics technology, to analyze pressure pulsations caused by rotating stall and to explore the propagation mechanism of rotating stall. Unsteady numerical simulations are performed by ANSYS 16.0 to model the unsteady flow within the entire flow passage of a centrifugal pump under 0.4QBEP and 0.6QBEP working conditions. Through flow characteristics research, the generation and propagation of rotating stall are discovered. Flow separation appears near the leading edge of the pressure side and transforms into vortices, which move along the passage. Meanwhile, the stall cells rotate circumferentially in the impeller. Additionally, frequencies and amplitudes of pressure pulsations related to rotating stall are investigated by spectrum analysis. The results detect a possible characteristic frequency of rotating stall and show that the interaction between stall cells and the volute tongue could have an influence on rotor–stator interaction (RSI).

Clearance always exists between the rotating impeller shrouds and the stationary casing covers in shrouded centrifugal pumps, which affects the pump internal flow and performance. Model tests were conducted for a shrouded centrifugal pump with back blades on the front shroud, and the performance parameters were obtained for three different impeller axial positions. Adjusting the impeller axial position can change the axial size of both the front and back clearances simultaneously. The results show that a tiny variation of the axial clearance size can substantially change the pump performance. A large front clearance reduces the pump efficiency and head with little change in the shaft power. Numerical simulations for a wide range of operating conditions for the three models with different impeller axial positions using the Reynolds-Averaged Navier–Stokes (RANS) with shear stress transport (SST) k–ω turbulence model agree well with the experimental results. The numerical results show how the clearance flow interfere with the main flow as the axial clearance is varied. The change in the pump hydraulic efficiency, volumetric efficiency, and mechanical efficiency was analyzed for various clearances. The hydraulic efficiency is the lowest one of the three kinds of efficiency and changes dramatically as the flow rate increases; thus, the hydraulic efficiency plays a decisive role in the pump performance. The volumetric efficiency is most sensitive to the axial clearance, which obviously decreases as the front clearance is increased. Therefore, the volumetric efficiency is the key factor for the change of the gross efficiency as the axial clearance changes. The mechanical loss varies little with changes in both axial clearance and flow rate so the mechanical efficiency can be regarded as a constant. The effect of axial clearances on the efficiency of shrouded centrifugal pumps should be considered to enable more efficient designs.