Study On Flow Characteristics Of Lower Ring Spiral Seal Of Francis Turbine Runner And Its Influence On Unit Performance

Up to now,there are many types of hydraulic turbine.Among them,Francis turbine has many advantages,such as wide range of head adaptation,excellent cavitation performance,high hydraulic transmission efficiency and so on.It is the most widely used hydraulic turbine in hydropower station.According to the operation experience of hydropower station,the clearance between the lower ring and stay ring of Francis turbine runner has a very important impact on the performance of turbine and the working stability of unit.The leakage flow caused by the clearance of the lower ring directly affects the volume loss of the unit and reduces the efficiency of the turbine.And because the flow in the gap is turbulent,it is easy to form vortex in the gap,which has a negative impact on the stable operation of the unit.Most of the traditional lower ring gap seals are contact seals,which often suffer from abrasion damage,short service life and frequent maintenance and replacement.As a non-contact seal,spiral seal can not only effectively solve the above problems and avoid the failure caused by seal damage,but also has better sealing performance than contact seal.In this paper,numerical simulation technology is used to study the influence of lower ring clearance based on screw seal on the performance of Francis turbine by computational fluid dynamics analysis method.Related work is as follows:(1)Based on the flow balance theory,the flow field distribution in the spiral seal gap is simplified,and the geometric parameters involved in the seal structure are substituted into the flow field velocity distribution.On this basis,the sealing fluid formula of the spiral seal is deduced.Then,using the optimization function of MATLAB software,taking the geometric coefficient in the sealing capacity formula of spiral seal as the optimization objective,the optimal geometric parameters of spiral seal are obtained.The optimized experimental results show that the helix angle is 21.05°,When the relative groove depth is 3 and the relative groove width is 0.5,the sealing performance is the best.(2)Based on the optimal geometric parameters of spiral seal,labyrinth seal and straight seam seal are designed.A Francis turbine is taken as the research object,and different operating points are selected according to the guide vane opening to carry out steady numerical simulation.By comparing the numerical simulation results of different working points and different clearance sealing schemes,it is found that the sealing performance of spiral seal is the best at any working point,and it plays a positive role in stabilizing the operation of the unit.Compared with labyrinth seal and straight seam seal,spiral seal can improve the efficiency of hydraulic turbine in most working conditions,but in the optimal working condition,spiral seal will have adverse effects on it.(3)According to the optimal working condition,the numerical simulation results of different clearance sealing schemes are compared and analyzed.After analyzing the internal flow characteristics of the clearance seal,it is found that the spiral seal is full of backflow vortexs,which also reduces the pressure gradient at the outlet of the clearance and the leakage flow velocity when blocking the flow channel,and greatly improves the flow pattern at the inlet and outlet of the clearance.From the point of view of energy loss,although the entropy generation of turbulent dissipation increases in the spiral seal,it reduces the leakage flow velocity and greatly reduces the entropy generation of the wall.Comprehensive analysis shows that the spiral seal has the least energy loss and the highest water energy utilization efficiency.