Numerical Simulation On The Flow Field Of Francis Hydraulic Turbine Runner's Sealing Clearance & The Relief Pipes

The vibration of the hydropower unit caused by the relief pipes of the turbine has attracted scholars' and station staff's attention. In this paper, the three-dimensional steady turbulent flow is simulated through the whole flow passage of a power plant including the relief pipes and the runner's sealing clearance using Computational Fluid Dynamics software-CFX based on N-S equation and RNG k -εturbulence model. The main research findings are as follows:The modeling process of the turbulence numerical simulation of the three-dimensional whole flow passage including the relief pipes and the runner crown's sealing clearance are analyzed. The mesh generation method of the calculation domain in the relief pipes and runner crown's sealing clearance are discussed. The transmission parameters problem of the small regional grid to large regional grid is solved through multiblock grid techniques and computational domain extension.For the Francis turbine flow field disturbance of the relief pipes, the influence of the relief pipes on the flow field of sealing clearance, the flow field before the runner blades, the flow field of the chamber between the head cover and the crown, the flow field of draft tube are analyzed, in the crown's sealing clearance value of 1.5 mm. The influence of the relief pipes' number on the leakage of runner crown's sealing clearance and the axial hydraulic thrust of the unit are also analyzed. The results show that the existence of relief pipes is beneficial to reducing the load of the thrust bearing, however, the disturbance of the runner crown's sealing clearance and the chamber between the head cover and the crown must be considered.The influence of the crown's sealing clearance value on the flow field of sealing clearance and the flow field of the chamber between the head cover and the crown are analyzed with four relief pipes under 40% rated output, 60% rated output, 80% rated output and 100% rated output working conditions. The influence of the crown's sealing clearance value on the leakage of runner crown's sealing clearance and the axial hydraulic thrust of the unit are also discussed. The concept of stable region and unstable region are presented according to the distribution of sealing clearance flow field for the simulation case.Therefore, the design of the relief pipe and crown's sealing clearance value should be considered in a comprehensive way to ensure that the unit avoids higher axial thrust, at the same time to prevent the vibration excitated by the runner crown's sealing clearance. Consequently the unit can be operated safely and stably.