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Purpose Numerically analyzed the flow characteristic and explored the hydrodynamic mechanism of the pump mode hump district formation of a Francis pump-turbine. Design/methodology/approach Numerical simulations were conducted of the entire pump-turbine flow passage under different discharge conditions by adopting the SST-CC turbulence model. The internal flow at hump district has been explained in detail combined with the model test in this paper. The unsteady flow and pressure fluctuation characteristics are analysed under five different discharge conditions in the hump and nearby region. The reason of the hump district formation is explored combined with the flow components hydraulic loss. Findings The large hydraulic loss, high relative peak-to-peak amplitudes and low dominant frequencies are on account of the disorganized internal flow condition. The formation of the hump district is concerned with the large hydraulic loss inside the draft tube, runner and guide vanes as there occurs secondary flow, backflow even vortex in the hump district. In addition, the low dominant frequencies at recording points inside the flow passage are always accompanied with the change of flow patterns and the spreading of the pressure fluctuations. Originality/value The analysis method of each flow components hydraulic loss combined with internal flow structure is adopted to explore the mechanism of pump mode hump characteristic. The flow characteristic and pressure pulse characteristics all correspond to the flow components hydraulic loss.

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Unlike the reaction turbines, the hydraulic performance of Pelton turbines varies due to the unsteady flow of the rotating buckets in time and space. This study experimentally and numerically investigates the dynamic performance of a Pelton turbine at five different operating conditions. The unsteady numerical simulations were performed with the SST turbulence model. The three-dimensional unsteady free surface sheet flow behavior in the rotating buckets was modeled by the two-phase homogeneous model. The model was used to analyze the dynamic flow patterns and the torque variations as the buckets rotated. The predicted hydraulic performance at different unit discharge rates are compared with the field test while the free surface flow patterns are compared with previous experimental and numerical results. The predicted relative efficiency agrees well with the field test results. The calculations were used to clarify the hydraulic mechanisms with unsteady free surface sheet flow in the rotating bucket.