| Reversible pump-turbine plays an important role of peak modulation, frequency modulation, phase modulation and emergency reserve in the power system, as it has both the characteristics of energy storage and power generation. With the development of pumped storage units gradually toward for high head, large capacity and high speed, higher requirements for the running stability of the units are put forward. Cavitation is one of the important factors that affect the efficiency and safety of the hydraulic machinery in the process of operation. If it is serious, the disorder of flow would be caused and intense pressure fluctuation would be generated, which would lead to the hydraulic vibration and cavitation erosion damage of the flow components. In this paper, to study a series of unsteady flow problems of pump turbine induced by cavitation vortex in draft tube under partial load condition, a related research has been carried out around the complex cavitation flow characteristics in flow channel at different stages of cavitation.The transient calculation of cavitation flow was completed by CFD software based on the model pump-turbine in a pumped storage power station. Combined with the model test data, the characteristics of the internal flow and pressure fluctuation of the unit under different device cavitation coefficients were predicted effectively. The aim is to provide a reference for choosing the reasonable operating conditions. The main research contents include the following aspects:1) Firstly, based on the geometric parameters of the research object, the three-dimensional geometry model of the flow channel was completed by PRO/E, and the mesh of the flow area was divided by ICEM-CFD. Furthermore, the independence of the grid is verified. Then, the turbulence and cavitation model were reasonably selected,meanwhile boundary conditions and convergence precision were set up. Ultimately, a entire numerical calculation model of the cavitation flow characteristics of the pump-turbine was established.2) Based on prototype model test data, the characteristic operating points were selected and the corresponding parameters were given. The numerical calculation was finished in the whole flow zone of the model pump-turbine. Combined with the theoretical analysis and model test results, the feasibility of the numerical scheme was verified, and the influence of changing the device cavitation coefficient on the hydraulic performance and the internal flow state was compared and analyzed. The results showed that the generation and development of cavitation made hydraulic efficiency drop sharply and the axial force increase significantly. Cavitation vortex strip began to generate and gradually changed from fine to coarse, leading to the obstruction of some flow paths and the decrease of flow capacity. Therefore, the operation security and stability of the unit has been adversely affected.3) Multiple pressure monitoring points were arranged in the flow zone along the flow direction to record changes in static pressure of each measuring point. The pressure data was processed by fast fourier transformation in ORIGIN software, and the corresponding spectrum characteristics were obtained. The impact of spatial location and operating conditions was considered, and the pressure fluctuation characteristics of pump-turbine were analyzed under different device cavitation coefficients. On the base of these, the corresponding relation between the generation and development of cavitation and the mode of distribution and transmission of the pressure fluctuation in the whole flow zone were discussed. |
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