Study On Internal Flow Characteristics Of Runner Clearance Of Super-high Head Francis Turbine

Stability is one of the important parameters for evaluating the comprehensive performance of a qualified and efficient turbine,while the axial hydraulic thrust of the runner and the leakage of the runner gap affecting the stability are the key indicators during the operation.For high working head and low specific speed of francis turbine,the amount of leakage determines the cubage loss of the unit.Based on the research of a super-high head Francis turbine in China,this paper makes a detailed calculation and analysis of the hydraulic thrust of the turbine runner and the leakage of the crown and ring under different working conditions.At the same time this paper carries out the sealing clearance for the runner using numerical simulation analysis.Studying the types,quantities and arrangements of gap seals,this paper analyzes the effects on the internal flow and leakage.The results show that under all calculation conditions,the hydraulic thrust acting on the bearing surface of the crown runner is not much different from the hydraulic thrust acting on the bearing surface of the lower ring gap.In short,the value of the hydraulic thrust combined force is distributed near the zero axis.It can be seen that the crown and lower ring gap structure of the runner can balance the water pressure of the crown flow channel and the lower ring gap flow channel of the runner.At the same time,under different conditions of different gap conditions,the minimum water thrust occurrence rate is not much different,and the maximum water thrust occurrence rate increases with the increase of the runner gap size.Finally,the sealing part of the gap structure is taken as the research object,and some reference conclusions are provided for the selection and designing of the sealing structure.From the perspective of arrangement,the effect of different gap distribution forms on the blockage of the fluid is not obvious.The engineering can adopt the arrangement method with low processing cost;from the perspective of the structural form of the gap,the structure forming the double vortex should be designed.In order to decrease the leakage,this structure forming can utilize the effect of eddy block and dissipating energy.The choice of the number of gap structure distribution is closely related to the internal flow.When there are less gap structures,the sealing effect of gap structure forming a single vortex in internal flow is better.Conversely,When there are more gap structures,the sealing effect of gap structure forming double vortex in internal flow is better.The theoretically calculated optimal sealing gap width value is smaller than the actual engineering width value.