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Impact of tip clearance flow on centrifugal pump impeller performance

A computational study has been conducted to evaluate the effects of tip-clearance on centrifugal pump impeller efficiency and pressure rise and to identify specific phenomena that lead to performance decrease with increased tip-clearance. Analysis of the flow field shows that there are two principal mechanisms responsible for the performance changes with tip-clearance. First, tip-leakage flow partially mixes with the core flow when it enters the channel. Second the resulting low streamwise velocity region (compared to the core flow) is responsible for flow blockage and decreased pressure rise. The tip-vortex structure associated with the tip-clearance flow has also been examined. The tip-vortex, which dominates the secondary flow patterns, increases in size with downstream distance. Low streamwise velocity flow is embedded in the tip vortex flow region. A simple analysis of the tip-vortex center trajectory has been developed which enables prediction of the wake flow trajectory. A model for the tip-clearance related decrease in pressure rise and efficiency has been developed, which accounts for both mixing and blockage effects. The model has been assessed against CFD results as well as published results and is found to capture performance sensitivity to tip-clearance variation for the examined data. The model also provides approximate analytical expressions for efficiency sensitivity. Thesis Supervisor: Edward M. Greitzer Title: H.N. Slater Professor of Aeronautics and Astronautics Thesis Supervisor: Choon S. Tan Title: Senior Research Engineer, Gas Turbine Laboratory