Research on vortex characteristics and stall mechanism during runaway process of the large vertical centrifugal pump under power failure

In practical engineering applications, hydraulic transient processes such as startup, valve opening, and shutdown of large vertical centrifugal pumps (LVCP) are inevitable. In particular, during the power failure shutdown transition, flow instability leads to drastic changes in unit speed, flow rate, and pressure within a short period, significantly impacting the operational stability of LVCP. This study employs the shear stress transport turbulence model (SST k–ω) combined with user-defined function in Fluent to investigate the vortex evolution characteristics and stall mechanism of LVCP during the power failure transition and runaway conditions. The Liutex rigid vortex identification method accurately identifies vortex structures in hydraulic components during the runaway process, including spiral vortices induced by blade angle variations on the hub, near-wall vortices at the impeller outlet, and strip-shaped vortices in the central region of the draft tube. Pressure pulsation analysis reveals that low-frequency rotating stall vortices significantly affect both upstream and downstream regions. The dominant frequencies in the vaneless region and volute are the blade-passing frequency and its harmonics, while the draft tube exhibits a dominant low-frequency 0.48fn. The mismatch between the flow angle at the impeller inlet and the blade inlet angle generates large-scale separation vortices, and the impeller rotational motion also induces large-scale axial vortices. Analysis based on the Liutex rigid vorticity transport equations demonstrates that the curl term of pseudo lamb vector and the rigid vorticity dilatation are the primary causes of separation vortices. Coriolis not only drives the development of separation vortices into stall vortices but also serves as the main force for stall vortices propagation within the flow passage. This study provides theoretical insights into the mechanisms of flow instability caused by vortex formation and evolution during the runaway process.