In this study, we aimed to maximize surge margin improvement while minimizing the impact on efficiency through a self-recirculation casing treatment. The recirculation flow path was divided into two regions: a deceleration region with a vaneless diffuser and a blade section to eliminate swirl, and an acceleration region configured as an injection part to return the flow to the main passage. By minimizing pressure losses in these regions, we hypothesized that the recirculation flow rate could be maximized, significantly enhancing surge margin. Various shapes were investigated, focusing on the expansion angle to reduce pressure losses. The specifications were determined through CFD analysis. The analysis at the design rotational speed revealed a 0.3 point decrease in efficiency but a 13.2% reduction in the mass flow rate at surge point. Experimental results confirmed similar improvements in surge margin as predicted by CFD analysis, validating our approach.

Water-surface displacement z_S is detected to investigate the evaluation of sloshing in operation of a submerged-sleeve-valve stilling well in which a sleeve valve regulates flow rate by opening/closing the sleeve. As a result, we reveal the effect of sloshing on z_S. The dominant frequency of sloshing occasionally becomes lower by about 5% than the theoretical eigen frequency due to the existence of an upstream pipe in the stilling well.