Effect Of Lower Ring Clearance And Skited Guide Vane On Francis Turbine Performance

With the rapid development of Francis turbine,the output of unit and the size of runner increase,which makes the efficiency and stability of turbine more and more important.The unreasonable matching between the lower ring clearance and the unit,as well as the cavitation erosion of the runner blade,will have a certain impact on the efficiency and operation stability of the turbine,but also will produce noise,blade passage vortices,reduce the service life of the unit and other phenomena,so it is of great significance to the selection of the lower ring clearance and the study of the cavitation performance of the turbine.Taking a Francis turbine model machine as the research object of numerical simulation,and using the CFX software to calculate the single-phase flow in the clearance of the lower ring and the cavitation flow in the skirted guide vane in this paper.The research contents are as follows:1.According to the basic parameters of the model machine and the two-dimensional CAD engineering drawings,the three-dimensional geometric model of the Francis turbine is built in the computational domain of the whole flow passage;on the basis of the three-dimensional geometric model,the structured grids of the flow passage components are divided;the grid independence is verified according to the curve of thie turbine efficiency varying with the grid number,and trhe total efficiency is finally determined by comparing the efficiency changes under different grid numbers.The total number of grids for numerical calculation of flow passages is 6.53 million.2.Constant numerical simulation of Francis turbine at four flow operating points is carried out to analyze the influence of lower ring clearancie on energy characteristics and internal flow pattern of Francis turbine.From the calculation results,it can be seen that the turbine efficiency decreases gradually with the increase of the size of the lower ring clearance,and the flow rate of the random group with the change range increases gradually.Under the condition of smal－flow rate,the influcnce of clearance on the internal flow pattern of turbinc is mainly concentrated in the draft tube.The clearance ouflow will aggravate the instability of draft tube flow,and the larger the clearance size,the more obvious the instability.At the optimal operating point,the influence of clearance outflow on the internal flow pattern of turbine is mainly concentrated at the runner.With the increase of clearance size,the pressure distribution on the back of the runner blade is improved significantly.Under the condition of large flow rate,the influence of clearance outflow on the internal flow pattern of turbine is mainly concentrated at the runner and draft tube.The clearance outflow can improve the pressure distribution on the back of the runner and reduce the instability of the flow in draft tube.With the increase of clearance size,the improvement effect will be more obvious.3.Cavitation flow calculation of Francis turbine under two flow operating points is carried out,analysis of the influence of skirted guide vane on the cavitation performance of Francis turbine.The calculation results show that the cavitation performance of Francis turbine can be improved by adding skirted guide vane,but the efficiency of the turbine will be affected slightly.Under different flow conditions,the influence trend of skirt edges on the cavitation performance of hydraulic turbines is consistent,but the influence trend of skirted edges on the efficiency of hydraulic turbines is not the same.Therefore,when selecting the skirted guide vane,the comprehensive effect of skirted guide vane on efficiency and cavitation performance should be considered.In summary,in the process of selecting the clearance size of the lower ring and designing the skirted guide vane structure,the change of structure will cause contradiction between turbine efficiency and internal flow field.Therefore,in the process of optimum design,it is necessary not only to analyze the influence of structural changes on the external characteristics of the turbine,but also to consider the influence of structural changes on the internal flow pattern of each flow passage component.Only in this way can a more accurate analysis conclusion be drawn,and its selection scheme will also provide a reference basis for the structural optimization of hydraulic turbines.