FLOW-ANALYSIS IN A PUMP DIFFUSER - PART 2 - VALIDATION AND LIMITATIONS OF CFD FOR DIFFUSER FLOWS

This paper describes an investigation into the use of CFD for highly l oaded pump diffuser flows. A reliable commercial Navier-Stokes code wi th the standard k-epsilon turbulence model was used for this work. Cal culations of a simple planar two-dimensional diffuser demonstrate the ability of the k-epsilon model to predict the measured effects of bloc kage and area ratio on the diffuser static pressure recovery at low lo ading levels. At high loading levels with flow separation the k-epsilo n model underestimates the blockage caused by the recirculation in the flow separation region and overestimates the pressure recovery in the diffuser. Three steady-state calculations of a highly loaded vaned di ffuser of a medium specific speed pump have been carried out using dif ferent inlet boundary conditions to represent the pump outlet flow. Th ese are compared to LDA measurement data of the flow field and demonst rate that although the Navier-Stokes code with the standard k-epsilon turbulence model is able to predict the presence of separation in the flow, it is not yet able to accurately predict the static pressure ris e of this highly loaded pump diffuser beyond the flow separation point .