The Effects of Tip Clearance on the Internal Flow Characteristics of a Mixed-Flow Pump Under Near-Stall Conditions

Leakage flow interferes with the main flow movement and has a close relationship with the rotational stall phenomenon. To study the rotational stall characteristics of mixed-flow pumps under different tip clearances (rim clearances), numerical simulations of the internal flow field of the mixed-flow pump were carried out based on the SST k-ω turbulence model and hexahedral structured meshes, with the tip clearances set to 0.2 mm, 0.5 mm, and 0.8 mm respectively. The external characteristics, internal flow field under stall conditions, impeller surface pressure, and internal vorticity distribution of the mixed-flow pump were compared among the three different tip clearances. The research results show that when the tip clearance is 0.5 mm, the numerical simulation results are in good agreement with the experimental results, indicating the high reliability of the simulation. Under the three different tip clearances, the near-stall and deep-stall operating points on the external characteristic curves are consistent. When the tip clearance is 0.8 mm, the positive slope characteristic of the flow rate–head curve of the mixed-flow pump is the most obvious. From the small flow rate condition to the large flow rate condition, the influence of the tip clearance on the efficiency of the mixed-flow pump gradually increases. Under deep-stall conditions, with increasing tip clearance, the stall vortex at the flow passage outlet causes more intense disturbances to the inlet of the downstream flow passage and induces the formation of new stall vortices at the downstream passage inlet, thereby increasing internal flow losses. The increase in the tip clearance leads to changes in the morphology of the leakage vortex, a decrease in the impeller surface pressure, intensification of flow disorder, and enhancement of the leakage vortex intensity. Moreover, compared with the rated condition, the leakage vortex of the mixed-flow pump under stall conditions is more affected by the tip clearance.