Inlet pre-swirl regulation of flow separation in mixed-flow pump passages

The effects of inlet pre-swirl generated by an adjustable inlet guide vane on flow separation in a mixed-flow pump are investigated numerically under representative off-design conditions. The circumferential velocity and relative flow angle at the impeller inlet are examined to relate changes in inlet-flow matching to the observed separation patterns. At 0.5Qd, positive pre-swirl reduces the incidence mismatch and suppresses flow separation near the impeller shroud, accompanied by a relative efficiency increase of 4.8%, reduced total entropy production, and lower dominant-frequency pressure amplitudes at most monitoring points. At 1.2Qd, negative pre-swirl improves inlet-flow matching near the impeller leading edge and increases the efficiency by 13.4%. However, the reduction in the volume of vortical regions in the impeller is accompanied by an increase in the diffuser. The impeller contribution to turbulent entropy production decreases, whereas entropy production in the diffuser and outlet regions increases. Meanwhile, the dominant-frequency pressure amplitudes at both the impeller inlet and outlet increase. These results associate the suppression of local impeller separation with pre-swirl-induced changes in inlet-flow matching while showing different downstream responses under the two operating conditions. In particular, the high-flow rate results reveal a trade-off in which improved efficiency and improved impeller inlet-flow matching coexist with increased downstream entropy production and pressure fluctuations.