The pressure fluctuations and flow pattern in a mixed-flow pump are numerically investigated under design and off-design conditions. The accuracy and reliability are validated by good agreement of numerical results and experimental data of pump energy performance. At design flow rate, the dominant frequencies of pressure fluctuations are impeller rotation frequency or integer multiples of it, which are mainly induced by the rotor-stator interaction. At overload condition, the dominant frequencies of pressure fluctuations are similar as those of design flow rate, but the maximum amplitudes increase to about 2.5 times of those of design flow rate. At partial load condition, the pressure fluctuations and flow pattern are complicated. The low dominant frequencies below the impeller rotation frequency are related to the process of vortex emergency, movement and disappearance.
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