Comparative modeling and analysis of the flow asymmetricity in a centrifugal pump impeller at partial load

Abstract

Undesirable flow regime occurs at partial-load conditions of the centrifugal pump. Flow separates at the leading edge and pulses in the blade channel with complex stall cell transfer law. The passing capability of the blade channel becomes important when rotating stall happens. In this study, the blade channel number influence on the flow stability in a centrifugal pump impeller was studied by unsteady flow simulations after numerical-experimental verification. The 5-, 6-, and 7-blade impellers were discussed under the same partial-load flow rate condition and the same rotating speed. Results show that the internal flow pattern was strongly influenced by the blade channel number. Periodic half-blockage was observed in the 5-blade impeller. Alternating stall with three stalled and three well-behaved channels existed in the 6-blade impeller. Complex aperiodic flow pattern occurred in the 7-blade impeller with the well-behaved, half-blocked, and fully stalled passages were all observed with stall cell transfer. The different flow regime caused different pressure pulsations. In the 5-blade impeller, the inter-channel flow frequencies, which were induced by the fluid extruded from blocked channels flowed into other channels, dominated. In the 6-blade impeller, the pressure pulsations performed low-in-amplitude and high-in-frequency. The flow regime was stable even under the rotating stall. In the 7-blade impeller, the rotating stall frequency dominated. The inter-channel flow frequencies were also obvious. The stable rotating stall pattern does not strongly influence the pressure pulsation and impeller axial and radial forces. The transferring stall cell induces extra mild pressure pulsation and impeller forces. The inter-channel flow adds strong pressure pulsation and impeller forces. When centrifugal pumps are operating at partial-load conditions, the flow characters especially the inter-channel flow caused by half-channel-blockage should be checked to avoid operation instability and security.