Previous experimental investigations of an open faced impeller, low specific speed, centrifugal pump showed that increasing the clearance between the impeller and the pump casing back face decreased the axial thrust generated on the impeller. This is contrary to normal convention. It was also found that if the front clearance of the pump was maintained at a constant value, the efficiency of the pump was not affected by the changing back clearance when no balance holes were present. The pump performance did vary with back clearance when balance holes were present. These results were obtained by measuring pump casing pressure distributions by Hossain (2000). In order to determine the reasons for this axial thrust dependence upon back clearance, it is necessary to determine the flow field within this small region. It is very difficult to measure the fluid flow in the small back clearance so the entire pump was simulated with varying back clearances and with and without balance holes using CFD. The CFD results are compared to the pump housing pressure distributions and to analytical analyses of flow between simple rotating disks. The dependence of the flow inside the back clearance region of the centrifugal pump has been analyzed in terms of the gap size and balance hole size. The impeller used in the analyses and experiments includes a cutaway section on the impeller. The flow inside the gap was severely affected by the gap size and, as a consequence, the pressure distribution and the axial thrust.

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