A new method is put forward to model rotor-stator interactions in the first stage of a multistage centrifugal pump consisting of an impeller, a vaned diffuser and a vaned return channel. A directional loss model is utilized to model the function of a water tank behind the return channel. A periodic boundary condition between the inlet and the outlet is applied to model a closed loop. Thus no flow specification either in the inlet or outlet is required, neither is the turbulence level. Consequently it can avoid specifying unphysical stationary boundary conditions at the inlet and the outlet for transient simulations. Transient numerical results are compared with those by the conventional simulation with stationary boundary conditions (constant total pressure at the inlet and fixed mass flow at the outlet) in detail. The new model can predict the mass flow fluctuation in the pump, which reaches 0.5% of mean flow rate during one period. The mass flow fluctuation does not have a significant influence on the velocity field; however it does have some important effects on the pressure and turbulent kinetic energy fields.

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