Effect of impeller shroud trimming on the hydraulic performance of centrifugal pumps with low and medium specific speeds

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Abstract
The effect of pump impeller modification by a new approach, pump impeller shroud trimming (PIST), on the flow field and pump performance is examined both numerically and experimentally. The primary purpose of this study is to investigate the hydraulic performance of single-phase centrifugal pumps, which are refined by the PIST method, with low and medium specific speeds. In this type of shroud trimming, different trimming sizes are applied only to the shroud plate of the closed impeller, while the geometries (diameters) of the hub and blades remain unchanged. This modification, which increases the clearance between the impeller and casing from the shroud side, causes desirable conditions for pumping fluid containing undissolved gas. Computational fluid dynamics software (ANSYS-CFX) is used to predict the hydraulic performance of centrifugal pumps. Two well-known turbulence models, the renormalization group (RNG) k-ϵ developed model and shear stress transport (SST) k-ω, are used to predict the flow patterns. The computational results are verified through the comparison of experimental and unsteady numerical simulations with steady numerical data. After ensuring the accuracy of the flow-field simulation approach, further numerical analysis is performed for both pumps by changing the impeller shroud diameter. The effects of the geometric changes on the performance curves, efficiency, flow field, pressure distribution inside the pump components and the radial force acting on both pump types are investigated comprehensively. The results show that the shroud trimming reduces the produced head and efficiency at the design points. Examination of the radial force, which is applied on the rotating parts, shows that shroud-trimmed impellers experience higher radial forces than closed impellers in both pump types owing to the lack of a uniform pressure distribution around the impeller outlet. Accordingly, the relevant information obtained is used to modify the existing coefficients to predict the radial forces.

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