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Complex energy conversion and energy dissipation occur in pump-turbines during the load rejection process. However, the underlying fluid mechanism is not clear. In order to solve these problems, in this study, a three-dimensional (3D) transient turbulent flow in a pump-turbine, with clearance during the load rejection process, was simulated using the method of coupling of the rigid rotor motion with flow and dynamic mesh technology. The simulated rotational speed shows good agreement with the experimental data. Most of the differences of rotational speed between simulations and experiments are very small and lower than 5%. Based on the numerical simulation, the energy conversion process, loss distribution, and flow mechanism in a pump-turbine were analyzed using the method of coupling of the entropy production analysis with the flow analysis. The results indicate that the load rejection process of a pump-turbine is an energy-dissipation process where the energy is converted among various energy forms. After load rejection, the hydraulic loss in the reverse pump process distributes primarily in the stay/guide vanes (GV), the vaneless space, and near draft tube inlet. While the hydraulic losses in the runaway process and the braking process are distributed mainly in the elbow section of the draft tube, the clearance of runner (RN), and the vaneless space, the hydraulic losses are mainly caused by viscous dissipation effects of the vortex flows, including the flow separation vortices, the shedding vortices of flow wake, the secondary flow, and the backflow.

Draft tube vortex is one of the main causes of hydraulic instability in hydraulic reaction turbines, in particular Francis turbines. A method of cavitation calculations was proposed to predict the pressure fluctuations induced by draft tube vortices in a model Francis turbine, by solving RANS equations with RNG k-turbulence model and ZGB cavitation model, with modified turbulence viscosity. Three cases with different flow rates at high head were studied. In the study case of part load, two modes of revolutions with the same rotating direction, revolution around the axis of the draft tube cone, and revolution around the core of the vortex rope, can be recognized. The elliptical shaped vortex rope causes anisotropic characteristics of pressure fluctuations around the centerline of the draft tube cone. By analyzing the phase angles of the pressure fluctuations, the role of the vortex rope as an exciter in the oscillating case can be recognized. An analysis of Batchelor instability, i.e. instability in q-vortex like flow structure, has been carried out on the draft tube vortices in these three cases. It can be concluded that the trajectory for study case with part load lies in the region of absolute instability (AI), and it lies in the region of convective instability (CI) for study case with design flow rate. Trajectory for study case with over load lies in the AI region at the inlet of the draft tube, and enters CI region near the end of the elbow.

In order to solve the problems of observing cavitation during the testing process of model turbine, a design method of transparent water lens of model turbine taper pipe was proposed and the computer-aided design software was researched and developed. The dimension of water lens was obtained by the design method, with the least known quantity of model turbine, which is based on Fermat's principle and the improved genetic algorithm. At the same time, the field test condition was also considered. Design efficiency and accuracy were improved by the software which was developed by program languages of Visual Basic and Matlab. Working strength was reduced by the water lens which had the advantages of simple structure, low cost and practicality. It is suitable to be applied and popularized in field test.