A Closed Loop Approach to Simulate the Unsteady Three-Dimensional Flow in an Axial Flow Pump

A new method is put forward to model the flow in a highly loaded axial flow pump. A directional loss model is utilized to model the function of a valve behind the pump stator vanes. A periodic boundary condition between the inlet and the outlet of the pump is applied to model a closed loop. Thus no flow specification in either the inlet or outlet of the pump is required; also it is not necessary to give the turbulence level. By this method no pressure level inside the flow domain is given by a boundary condition. To avoid numerical instability the pressure level has to be given at least at one grid point. A given constant pressure somewhere in the loop domain is physically invalid, especially at stall condition of the pump. This is avoided by introducing a reservoir with a constant pressure boundary condition that is nearly decoupled from the pressure field inside the main pump loop by a huge flow resistance. Consequently this method can avoid specifying non-physical stationary boundary conditions at the inlet and the outlet for transient simulations. The new model can predict the mass flow fluctuations in the pump. These fluctuations are not very strong at stable operating conditions but increase in part load or stalled flow conditions. The transient numerical results obtained by the new approach are compared with those obtained by the conventional simulation with stationary boundary conditions (constant total pressure at the inlet and fixed mass flow at the outlet) and also with results of experimental investigations performed by Kosyna and Stark. The different flow structures inside the blade passages of the pump are described and compared in detail for part load, overload and design point as well as for stalled flow conditions.