Navier-Stokes equations and RNG k-ε model were used to simulate the three-dimensional incompressible turbulent flow in axial-flow pump assembly discharge passages with different diffusion angles. The inlet velocity distribution obtained by measurement was used as boundary condition. Velocity distributions on cross-sections and hydraulic losses in discharge passages were computed considering different inlet circulations. Influences of inlet circulation, diffusion angle and bending angle on hydraulic characteristics of a discharge passage were analyzed. Results indicate that influences of those three factors on the hydraulic loss are much. Usually, the circulation of pump outflow is too large, so the hydraulic loss is larger than that without circulation. The hydraulic loss can decrease by 6.6%～15.9% and the pump assembly efficiency can increase by 1.2%～1.7% by decreasing inlet circulation. The hydraulic loss is the smallest with the diffusion angle 10°～14°, which is only about 1/3 of that without diffusion, and the assembly efficiency of the former is about 15% higher than that of the latter. The hydraulic loss of the passage with 90° bending angle is about 10% larger than that with 60° bending angle. Conclusions mean much to realize the optimum design of an axial-flow pump assembly.
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