Characterization of free-surface vortex evolution and its impact on hydraulic stability of the pump sump

Free-surface vortices (FSVs) and their associated air entrainment can induce unstable flow within the sump, thereby impacting hydraulic systems’ efficiency and operational safety. An experiment studied how the submergence depth of a bellmouth affects the spatiotemporal evolution and pressure fluctuation characteristics of an FSV, using high-speed visualization and transient pressure measurements on a transparent hydraulic platform with an axial-flow pump. The results indicate that FSV evolution has four stages: newborn (stage 1), development (stage 2), fully-developed (stage 3) and dissipation (stage 4). Shorter submergence depths lead to quicker evolution periods and distinct FSV morphologies. The scale characteristics of FSV were quantitatively elucidated using the grayscale value extraction method. As the submergence depth increases, the scale of the vortex core gradually expands, and the precursor point for FSV occurrence gradually deviates from the bellmouth. Time-domain characteristics induced by FSV exhibit short-term pressure distortion, with amplitude increasing as the submergence depth decreases, especially at the minimum submergence depth. The frequency spectrum station with FSV is higher than without FSV, and the frequency bandwidth where the blade’s passing frequency is narrower, reducing harmonic energy.