Study On The Inner Flow Stabilities Of Large-scale Francis Hydro-turbine Model

The growing electric power demand calls for the high rotating speed, large capacity and high water head of Francis hydro-turbine. As the increasing of the capacity per unit and rotating speed, the hydraulic design of the turbine unit is prone to high specific speed type. Besides, the problem of flow stability induced by two-phase cavitation flow has attracted much attention in the recent years. This dissertation focuses on the cavitation flow instabilities which universally exist in high water head and large capacity Francis hydro-turbine, carrying out the investigation by using inner flow field measurement, energy characteristics test and numerical simulation.In order to measure the inner flow field in the hydro-turbine by using particle image velocimetry (PIV), we estabilished the experimental model turbine setup with transparent component parts. Specifically, the transparent runner which ficilitated the visualization and met the demand of intensity was designed, the spiral casing with transparent organic window for vaneless area flow measuring, stay vanes and guide vanes, etc. were fabricated. Then, the PIV measuring and controlling system were estabilished based on the experimental circulation system, while the parameters of flow rate, rotating speed, pressure and output work were monitored. Also the phase-lock method for PIV measurement was used to analyze the inner flow evolution. Through the experimental investigation on the flow field in the vicinity of guide vanes, the channel vortex, flow separation on the blade surface and draft tube vortex rope, it was found that the flow field variation on the suction side of guide vanes was related to the blade passing frequency, and the pressure oscillation in the vaneless area was mainly induced by its periodic changing. The vortex ropes showed the low-frequency features, and the main frequency of channel vortex in the runner was proven to be the blade passing frequency. However, the flow separation in the suction side of runner blade presented the high-frequency evolution, with the guide vane passing frequency.To analyze the characteristics of channel vortex, vortex rope in the hydro-turbine, numerical prediction on the external characteristics, inner flow field, etc. were performed, by using the large eddy simulation (LES) combined with vaporizing/condensing-based cavitation model. Under various guide vanes’ openings, the results for the external characteristics with two-phase cavitation model were more consistent with experimental results than single-phase model. At the rated opening, strong rotating flow did not exist in the outlet of runner, which effectively improved the draft tube inlet flow, hereby lower the pressure fluctuation in the draft tube. While at small opening of guide vanes, double vortex ropes showed in the draft tube.To study the flow instabilities of inner flow field of hydro-turbine, the pressure fluctuations in the turbine were captured based on the simulation results, and their frequency-spectra features and evolutions were analyzed. Also the flow instability investigation by using turbulent kinetic energy equation in the curvilinear coordinates were presented. It reveals that at different flow conditions, the main frequencies of pressure fluctuations in the vaneless area are equal to the blade passing frequency, and their secondary frequencies are two times of blade passing frequency. As the increasing of guide vanes’ opening, the high-frequency components of pressure oscillation in the draft tube inlet become evident. By studying the turbulent kinetic energy distribution in the vaneless area, it was found that the normal stress production and shear stress production terms contributed the most to the flow instabilities. At small guide vanes’ opening, the augment of pressure fluctuations in the vaneless area was mainly due to the guide vanes’ wake flow, otherwise due to the ineraction between guide vanes’ wakes and runner rotation for the large guide vanes’ opening. Furthermore, wake flow directly affects the normal stress production term of turbulent kinetic energy at small opening, while the interaction by runner blade makes an increment of normal stress production term at large opening. Thus, the main factors for the flow instabilities are the normal and shear stress production term of turbulent kinetic energy, since the curvature term and three-dimensional term of turbulent kinetic energy production account for a small part in the whole.This study illuminated the flow instaiblities issues in the vaneless area, runner inside and draft tube of Francis hydro-turbine, which could be of guiding significance to the engineering design of hydro-turbine.