Abstract
Centrifugal pumps equipped with vaned diffusers of specific speed nq ≥ 30 ([min−1, m3 s−1, m]) are prone to performance curve instability at part-load. The present paper deals with the numerical comparison between a middle stage of a multi stage centrifugal pump equiped with a vaned diffuser (nq ≈ 35 ([min−1, m3 s−1, m]) and a vaneless diffuser respectively. For the simulations the SAS-SST turbulence model has been used. While in the vaneless case a distinct head drop does not exist, a local dent appears for the vaned diffuser at approximately 60% of the design flow rate. With the onset of the instability, pressure generation shows extensive fluctuations between different revolutions. Besides classic performance evaluation, static pressure generation is analysed at the diffuser to characterize responsible sections for the head drop. Furthermore, the progression of the axial force shows distinct differences among both configurations. The results show, that highly transient flow detachment and vortex structures in the inlet triangle section at the diffuser vane suction side and the hub occur. These vortex structures lead to increased losses in the inlet trianglular section and induce recirculation zones at the impeller outlet. Thus, inferences can be drawn between pattern switching phenomenon and axial force of the pump.
