Abstract
In this study, the effect of the rotor axial offset and impeller-diffuser gap (smaller gap or larger gap) on the performances and the unsteady thrust forces of a three stage centrifugal pump were experimentally investigated. The change of the gap was realized by cutting an impeller diameter. By cutting the impeller diameter (larger gap) the positive slope of the head performance curve was observed in lower flow rates than 20% of design flow rate, while it was not observed with the smaller gap. In the flow rate range with the positive slope, the distinct pressure fluctuation with low frequency (14% of impeller rotation), measured at the inlet throat of diffuser vane, could be recognized, which was found to be caused by the diffuser rotating stall with 1 cell. The unsteady radial thrust force caused by the diffuser rotating stall was observed at the same time. The rotating stall was more significantly observed in the 3rd stage, which was probably because the test pump system employed the balancing drum for the thrust balancing device; the effective flow rate is larger in the 1st and 2nd stage diffusers than the 3rd one due to the leakage flow to the balancing drum just upstream of the 3rd stage diffuser.
