Numerical Analysis Of Flow Field Improvement For Draft Tube Of Francis Turbine

Francis turbine is an important hydroelectric equipment.The complex and changeable flow pattern inside Francis turbine is one of the causes of unit vibration.As the outlet part of the turbine,the stability of flow pattern in draft tube has a positive impact on the safe and normal operation of the unit.In this paper,three-dimensional steady and unsteady simulation of a Francis model turbine is carried out based on the STT turbulence model.In view of the complex flow pattern in draft tube,three improvement measures are put forward in this paper,which are the lengthening of discharge cone,the filling of central hole of spindle,and the installation of guide plate in elbow bend section.In order to explore the effect of three improvement measures on draft tube flow pattern improvement under different turbine operating conditions,this paper chooses the optimum working conditions,such as small flow concdition,large flow condition,partial load condition and high head condition,and sets no diversion plate according to the length of discharge cone,the amount of water supply in the central hole of spindle,and whether there is installation of diversion plate in elbow section or not.The same calculation scheme.According to the selected working conditions and schemes,the CFD simulation method is used to calculate the various schemes.The stability of draft tub,e pipeline,the eddy current viscosity of draft tube,the variation of pressure fluctuation amplitude and frequency at each monitoring point of draft tube,and the output of unit improved by different draft tube under different working conditions are analyzed.The numerical results show that:(1)the influence of the lengthening of the discharge cone on the flow pattern of draft tube is not particularly obvious under the optimal and high head conditions,but only slightly improves the local flow pattern of draft tube;under the partial load and small flow conditions,th,e lengthening of the discharge cone can improve the flow pattern of draft tube and effectively reduce the pressure fluctuation amplitude caused by the eccentric eddy band in the straight cone section and elbow section.It has a positive impact on the vibration reduction of the unit,and according to the reduction of pressure fluctuation at each monitoring point,the turbine has the best effect of extending the discharge cone to the position that is level with the lower ring of the runner.(2)When considering the volume loss caused by the amount of water supply,the efficiency of the turbine decreases linearly with the increase of the amount of water supply under the optimal condition,which indicates that excessive amount of water supply will reduce the output of the unit.By analyzing the cross-section pressure of draft tube under different water supply,eddy current viscosity of each monitoring surface and eddy belt of draft tube cavity,it is concluded that the reasonable water supply amount is different in different working conditions.The reasonable water supply amount in many working conditions should exceed 2.0%Q,too small water supply amount can not achieve the effect of water supply,and too large water supply amount will affect the unit efficiency.Generally speaking,when the water supply quantity Qad = 2.0%Q,the water supply effect will be achieved.Fruit.From the unsteady calculation results of the two schemes of non-replenishment and replenishment Qad=2.0%Q under the optimal and partial load conditions,it is also found that the pressure fluctuation amplitude at each monitoring point of draft tube under the partial load condition decreases to varying degrees when the replenishment Qad=2.O%Q,while the pressure fluctuation amplitude is not obvious under the optimal condition.(3)Under partial load conditions,the draft tube elbow section installed with diversion plate has a steady flow effect on elbow section and diffusion section,and has a slight weakening effect on the vortices in straight cone section.The efficiency change of the turbine is not obvious and will not affect the normal unit output.To sum up,all three measures can improve the local flow pattern of draft tube;when the unit runs part of the load condition,it can adopt lengthening discharge cone and installing diversion plate,use the main shaft central hole to supplement water,it can improve the flow pattern of draft tube and reduce the amplitude of pressure fluctuation,which is beneficial to the stability of the unit.Sexuality is of great significance.