Numerical Simulation and Model Experiment Research on the Comparative Analysis of Axial-Flow Pump Hydraulic Models for a Large Pump Device

To investigate the effects of different impeller-diameter–rotational-speed combinations on the hydraulic performance and internal flow of a large vertical axial-flow pump device under an equal-nD condition, the ZM63 axial-flow pump hydraulic model and its vertical pump device were studied. Four schemes with approximately equal nD values were established according to the available synchronous-motor speeds, and comparative investigations were conducted by combining model tests with three-dimensional steady numerical simulations. At an approximately identical nD value, the four schemes exhibited similar head scales; however, as the impeller diameter increased and the rotational speed decreased, the operating flow-rate range and high-efficiency region shifted toward larger flow rates. At identical flow coefficients, the difference in the total hydraulic loss of the inlet conduit, guide vanes, and outlet conduit among the schemes was within approximately 2.5%. Nevertheless, marked differences were observed in the local flow within the siphon outlet conduit, and increasing the impeller diameter while reducing the rotational speed decreased the low-velocity region in the curved crest section. Under the common target condition of Q = 30 m3/s and H = 4.7 m, the blade setting angles were adjusted so that all schemes satisfied the same design requirement. Scheme 1 achieved the highest pump-device efficiency of 79.81% and the lowest shaft power of 1720 kW, indicating that diameter enlargement and speed reduction do not lead to a monotonic increase in efficiency. By contrast, the outlet-conduit loss of Scheme 4 was approximately 30.1% lower than that of Scheme 2; its pressure distribution on the blade suction surface was smoother, and the local low-pressure level was alleviated, indicating improved downstream flow matching and a larger local low-pressure safety margin, while detailed cavitation performance requires separate evaluation. The results demonstrate that an equal nD value does not guarantee identical overall hydraulic performance of a pump device. Overall, diameter enlargement combined with speed reduction provides favorable hydraulic behavior, but engineering selection should be based on a comprehensive assessment of efficiency, power demand, hydraulic matching, and cavitation risk.