Abstract

Inducers show generally a positive influence on the performance of centrifugal pumps in the two-phase regime, since they produce more uniform mixtures and increase the pressure before the impeller. However, the effect is much more pronounced in part-load compared to overload conditions. In the present study, the air-water two-phase flow behaviour in a pump inducer was numerically investigated. The main objectives were to clarify the effect of the inducer, the effective operating range, and to examine flow mixing. Several flow conditions were studied, covering part-load, optimal, and overload pumping conditions, together with different, relevant gas volume fractions (1%, 3%, and 5%). The simulations were performed using a transient set-up and a moving-mesh approach. Two-phase air-water interactions were modeled by the Volume of Fluid (VOF) method. After checking the proper discretization in space and time, the model was validated against experimental results, revealing excellent agreement. The numerical analysis was able to explain the different effects of inducers in part-load and overload conditions. Under overload conditions, the flow separates, leading to the generation of axial vortices and to a negative pressure change across the inducer; additionally, the residence time is reduced, hindering mixing. These vortices are intensified as the gas volume fraction increases, reducing further the pressure downstream of the inducer. This is the reason why inducers can mainly be used in part-load and near optimal conditions in order to improve pumping of two-phase flows.

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