This paper presents a design optimization study of a low specific-speed, multistage, double-case, volute pump. The goal of this study was to accomplish an increase in overall pump efficiency by at least 3.5 percentage points. To that end, each hydraulic internal component of the pump was analyzed to find opportunities for improvement. This was done by performing a combined classical hydraulic evaluation and in-depth Computational Fluid Dynamics (CFD) analysis. The hydraulic assessment gives a good overview if the component is suitable for the desired operating conditions in a relatively quick manner. It is, however, not suitable for detailed understanding of why a certain component is underperforming.

Pump internals typically have a complex three-dimensional shape, which is not taken into consideration when performing a classical analysis. Therefore, a three-dimensional CFD simulation was performed to identify in detail the potential areas for improvement of each component.

Two different strategies were used for optimizing the internal components. The series impeller geometry was optimized automatically by an objective algorithm to obtain optimum performance; other components, including first-stage impeller, series volutes and short crossovers were redesigned by manual design iterations, which were evaluated using CFD. The results show that the distribution of the cross-sectional area in the crossovers had the greatest influence on the efficiency, and due effort was undertaken to optimize this. To validate the efficiency improvement that was predicted by CFD simulations, scaled model tests were conducted on the series stages. The entire design optimization resulted in better performance in terms of efficiency, power and head for the complete flow range.

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