The performances of an axial pump with and without inlet guide vanes (IGVs) are investigated by both experimental method and computational fluid dynamics (CFD) method. Besides the stall inception, an obvious "positive slope" phenomenon appears under off-design operating points in experiment without IGVs. The experimental data also shows that IGVs can change the performance of axial pumps and remove the "positive slope". The CFD is mainly used to reveal the mechanism of the "positive slope" phenomenon, where the simulated results are firstly validated in comparison with the experimental data. The results obtained show that the reason for this "positive slope" is due to the backflow vortex in front of the impeller, and the tip clearance is found to take an important role in forming this backflow vortex.
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