The off-design performance in a centrifugal pump is investigated experimentally. The objective is to identify flow features that lead to the onset of surge as the fundamental pumping system instability. Results show that there are primarily two reasons for the onset of surge as the flow is reduced in the pump studied: (a) adverse flow in the tongue region, and (b) destabilizing effect of the pipe diffuser. The former is due to premature diffusion of the flow entering the tongue region, which is manifested by increased flow recirculation through the tongue/impeller gap and flow separation on the volute outer side-wall opposite the tongue. These effects in the tongue region flow coupled with the destabilizing behavior of the pipe diffuser lead to the eventual unstable pump operation.

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