Quantitative Characterisation of Spiral-flow in the Straight Outlet Conduit of a Vertical Mixed‑flow Pump

Straight outlet conduits (SOCs) are widely used in pump stations. However, their performance is limited by residual circulation from the guide-vane outlet and curvature-induced secondary flow in the outlet elbow, which can generate spiral flow, leading to bias flow and additional hydraulic loss. This study investigated spiral-flow evolution in a vertical mixed-flow pump using unsteady numerical simulations validated against model tests, together with theoretical analysis. A simplified theoretical model was established, and the main precession frequency and swirl number were introduced to quantify spiral-flow motion and intensity, respectively. The most pronounced spiral flow was observed under the low-flow condition of ۰.۸۵Qdes. The dominant flow region shifted from the upper-right side of the elbow outlet to the upper-left side of the circular-to-square transition section (CTSTS), resulting in an outlet bias flow ratio of ۰.۹۱. The outlet elbow hydraulic loss under this condition was ۱.۲۱ times the design-condition value. At the elbow inlet, the main precession frequency reached ۰.۵۵ times the rotational frequency, while swirl number at the CTSTS outlet was ۱.۶۷ times the design-condition value. These findings identify the CTSTS and the division-pier leading-edge region as critical regions for spiral-flow development, biased-flow formation, and hydraulic-loss control. The proposed framework provides a quantitative basis for evaluating spiral-flow evolution in similar low-head pump systems.

کلیدواژه ها:

Mixed-flow pump ، Outlet conduit ، Spiral flow ، Hydraulic performance ، Numerical simulation