For Francis hydraulic turbines, unsteady flow caused by vortex ropes in the draft tube leads to a problem of stability in operation. The unsteady flow field of a model Francis hydraulic turbine was simulated under par...For Francis hydraulic turbines, unsteady flow caused by vortex ropes in the draft tube leads to a problem of stability in operation. The unsteady flow field of a model Francis hydraulic turbine was simulated under part-load operation. A sliding mesh model was used to calculate a time-accurate solution for the strong rotor-stator interactions between the runner and guide vanes, and the draft tube. Based on three-dimensional incompressible Reynolds averaged Navier-Stokes equations and on a renormalization group k-?turbulence model, spatial discretization was obtained by using the finite volume method with unstructured grid elements, and a second order fully implicit scheme was applied for time. Pressure fluctuations in the draft tube were recorded and analyzed via a fast Fourier transform calculation. The results were compared with the experimental data, and show that the vortex rope in the draft tube and the induced pressure fluctuations are well simulated.展开更多
Owing to the part-load operations for the enhancement of grid flexibility, the Francis turbine often suffers from severe low-frequency and large-amplitude hydraulic instability, which is mostly pertinent to the highly...Owing to the part-load operations for the enhancement of grid flexibility, the Francis turbine often suffers from severe low-frequency and large-amplitude hydraulic instability, which is mostly pertinent to the highly unsteady swirling vortex rope in the draft tube. The influence of disturbances in the upstream（e.g., large-scale vortex structures in the spiral casing） on the draft-tube vortex flow is not well understood yet. In the present paper, the influence of the upstream disturbances on the vortical flow in the draft tube is studied based on the vortex identification method and the analysis of several important parameters（e.g., the swirl number and the velocity profile）. For a small guide vane opening（representing the part-load condition）, the vortices triggered in the spiral casing propagate downstream and significantly affect the swirling vortex-rope precession in the draft tube, leading to the changes of the intensity and the processional frequency of the swirling vortex rope. When the guide vane opening approaches the optimum one（representing the full-load condition）, the upstream disturbance becomes weaker and thus its influences on the downstream flow are very limited.展开更多
文摘For Francis hydraulic turbines, unsteady flow caused by vortex ropes in the draft tube leads to a problem of stability in operation. The unsteady flow field of a model Francis hydraulic turbine was simulated under part-load operation. A sliding mesh model was used to calculate a time-accurate solution for the strong rotor-stator interactions between the runner and guide vanes, and the draft tube. Based on three-dimensional incompressible Reynolds averaged Navier-Stokes equations and on a renormalization group k-?turbulence model, spatial discretization was obtained by using the finite volume method with unstructured grid elements, and a second order fully implicit scheme was applied for time. Pressure fluctuations in the draft tube were recorded and analyzed via a fast Fourier transform calculation. The results were compared with the experimental data, and show that the vortex rope in the draft tube and the induced pressure fluctuations are well simulated.
基金Project supported by the National Natural Science Foundation of China(Grant No.51506051)
文摘Owing to the part-load operations for the enhancement of grid flexibility, the Francis turbine often suffers from severe low-frequency and large-amplitude hydraulic instability, which is mostly pertinent to the highly unsteady swirling vortex rope in the draft tube. The influence of disturbances in the upstream（e.g., large-scale vortex structures in the spiral casing） on the draft-tube vortex flow is not well understood yet. In the present paper, the influence of the upstream disturbances on the vortical flow in the draft tube is studied based on the vortex identification method and the analysis of several important parameters（e.g., the swirl number and the velocity profile）. For a small guide vane opening（representing the part-load condition）, the vortices triggered in the spiral casing propagate downstream and significantly affect the swirling vortex-rope precession in the draft tube, leading to the changes of the intensity and the processional frequency of the swirling vortex rope. When the guide vane opening approaches the optimum one（representing the full-load condition）, the upstream disturbance becomes weaker and thus its influences on the downstream flow are very limited.
