Influence of seal clearance of runner on internal fluid field in Francis turbine

The performance and stability of turbine are easily affected by the complex turbulence such as clearance vortex and leakage flow. For the purpose of analyzing the influence of seal clearance on the energy features and interior flow characteristics of Francis turbine, with referencing N-S equation and SST turbulence model, the three-dimensional steady flow in the full flow passage of the Francis turbine model under five schemes of clearance were calculated. Four flow rates were chosen as: 0.6 Qd, 0.8 Qd, Qd and 1.2 Qd. Five schemes of clearance size were selected of 0.4, 0.6, 0.8, 1.0 and 1.3 mm. By comparing the effects of different clearance schemes on the efficiency and volume loss of Francis turbine, the relationship between clearance and turbine performance was analyzed based on the characteristics of internal fluid field, and the calculation results showed that by increasing the size of clearance, the turbine efficiency could be reduced, and the leakage of the turbine indicated an upward trend. When the turbine operated at a small flow rate of 0.6 Qd, the clearance appeared the most obvious influence on the energy features of the turbine. With the increase of clearance, the leakage ratio increased from 0.18% to 0.54%, and the efficiency decreased by 4.1 percentage points. At the flow rate of 1.2 Qd, the clearance presented the least influence on the energy features of the turbine. The leakage ratio increased from 0.07% to 0.27%, and the efficiency decreased by 1.38 percentage points when the clearance was increased. If the turbine was operated at small flow rates of 0.6 Qd and 0.8 Qd, the clearance increased, and the internal fluid fields inside both the clearance and the draft tube deteriorated slightly. The average velocity in the clearance increased gradually, and the eddy current intensity in the inlet and outlet pressure chambers of the clearance increased little by little, the volume of vortex rope in the center of draft tube inlet was increased simultaneously. If the unit was operated at a large flow rate of 1.2 Qd, with the clearance increased from 0.4 mm to 1.3 mm, the characteristics of fluid field inside clearance and the pressure distribution on the suction surface of the runner blades, also the fluid field inside the draft tube were all improved. The area of negative pressure zone on suction surface of runner blades decreased, and the volume of the cavity vortex rope at the inlet center of the draft tube decreased, the streamline inside the draft tube also tended to be smooth. This study could provide an effective reference for the design of seal clearance of Francis turbine. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.