The performance, internal flow fields and fluid forces acted on the impellers and balance piston of three stages centrifugal pump at design and partload conditions are investigated by using URANS (Unsteady Reynolds Averaged Navier-Stokes) CFD analysis. In this study, investigations were concentrated on the details of the flow fields in the sidewall gaps of the impellers and at the impeller exit/diSuser inlet with the rotor axial offsets, and its effects on the thrust forces acting on the impeller, especially at the partload condition. From the details of flow field predicted by CFD, the mechanisms of change of axial thrust forces with the rotor axial offsets are as follows. The overwrap between the impeller shroud wall and diffuser casing wall becomes smaller by the axial offsets at the opposite side of axial offset direction, this makes the flow to leak out easier from the impeller side into the side wall gaps there. As a result, the interaction between the swirl flow in the side wall gap and reverse flow from the diffuser become stronger. The strong inward flow in the front side wall gap induced by the diffuser reverse flow works as a swirl breaker weakening the swirl flow in the gap. The pressure distributions in the side wall gaps are affected by these changes of swirl flow in the gap and the axial thrust forces working on the rotor are changed. The predicted change of axial thrust forces with the rotor axial offsets show good agreement with the experimental values.
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