Abstract

In this paper, the effect of the inducer tip clearance is studied to understand its impact on the cavitating and non-cavitating performance of centrifugal pumps. Helical inducers with constant pitch and with variable (progressive) pitch are considered. Computational Fluid Dynamics (CFD) simulations of a single stage pump are conducted on each inducer type to determine the cavitating (two-phase) and non-cavitating (single-phase) performance for varying inducer tip clearance. The Rayleigh-Plesset cavitation model is used to understand the bubble dynamics under the assumptions of single fluid undergoing no thermal energy transfer between each phase. Experimental tests are conducted on a pump with variable pitch inducer to determine the true performance in cavitating and non-cavitating operating conditions. Experimental results are compared to the simulations to validate the accuracy of the proposed numerical modelling. Net Positive Suction Head (NPSH) with 3% differential head drop is used as a criterion to identify the true cavitation performance of each inducer configuration. It is found that, as the inducer tip clearance increases, excessive back leakage and larger vortex recirculation occur at the tip location. These result in pressure loss within the inducer and, consequently, degrades the cavitation performance. In addition, the change in cavitation performance with tip clearance is much more evident for variable pitch inducer geometries as compared to the constant pitch case. Furthermore, the impact on the non-cavitating performance of inducer tip clearance is found to be minimal.

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