Study On Off Design Flow Characteristics Of Tip Clearance Leakage In The Last Stage Of Steam Turbine

During the design of the flow passage structure of the steam turbine,in order to avoid the friction between the rotor and the stator during operation,resulting in component damage,a certain gap will be left between the top of the moving b lade and the cylinder wall.Because there is a certain pressure difference between the pressure surface and suction surface of the blade,under the action of the pressure difference,some steam will flow from the high-pressure side into the low-pressure side through the gap,resulting in air leakage loss.This will not only reduce the efficiency of the unit,but also affect the safe operation of the unit.Therefore,it is of great significance to fully understand the leakage flow in the tip clearance of mov ing blades to improve the working efficiency of steam turbine.At present,there are few studies on tip clearance leakage flow of steam turbine wet steam stage.The influence of leakage flow on the performance of wet steam stage and the action mechanism of leakage flow in wet steam stage under variable working conditions cannot be accurately evaluated.In order to deeply understand the tip leakage flow characteristics of wet steam stage of steam turbine,the N-S equation and turbulence equation of the last stage flow field of LP cylinder of a 600 MW supercritical steam turbine unit are solved based on computational fluid dynamics software.The characteristics of tip clearance flow field under different working conditions are studied,and the loss and efficiency in the stage are compare d and analyzed respectively.The main work contents are as follows.Firstly,the numerical calculation of different inlet mass flow conditions is carried out,and its influence on the last stage flow and tip clearance leakage is analyzed.The results show that there are separated flows on the pressure surface and suction surface of the moving blade.It is caused by the interaction between leakage flow and main flow,channel vortex and main flow.Under different working conditions,the generation position and outflow position of separated fluid are different.The parameters such as tip velocity,Mach number and outlet steam flow angle will fluctuate obviously under the influence of leakage flow a nd leakage vortex.The inlet flow decreases,the pressure difference on both sides of the blade top decreases,the leakage decreases,and the total pressure loss decreases.In addition,when the inlet mass flow increases by 1kg/s,the relative leakage can increase by about 0.05%.Secondly,the effects of different outlet pressures on tip clearance leakage flow are analyzed,and the relative internal efficiency of the stage is compared.The results show that the distribution law of blade surface streamline d iagram is basically similar to that under variable inlet flow.However,with the increase of outlet pressure,the backflow vortex began to appear at the blade root.Through the analysis of pressure field,temperature field and humidity field at different l eaf heights,it is found that the distribution of pressure and temperature is basically similar,and the change of humidity is just the opposite.The tip Mach number increases with the decrease of outlet pressure,and the tip boundary layer thickens at hig h Mach number.At the same time,through the entropy distribution,it is found that the intensity and action range of leakage flow are increasing,and the flow loss is increasing.When the outlet pressure increases from 5k Pa to 15 k Pa,the relative clearance leakage decreases from3.67% to about 1.6%.In addition,through the analysis of the relative internal efficiency of each working condition stage,it is considered that the outlet pressure should not be greater than 12.5k Pa,and the relative internal efficiency of the stage can be maintained at more than 80%.Finally,the relationship between different tip clearance and the performance of the last stage of steam turbine is studied by changing the tip clearance of moving blade under the design condition.The relationship between leakage flow and main flow under different clearances is analyzed.The results show that when the clearance increases,the steam flow angle at the inlet and outlet of the moving blade will be affected,and it mainly changes significantly near the blade top.The main flow velocity in the moving blade channel fluctuates violently from 70% axial position.The mainstream velocity in the tip region is affected by leakage flow and leakage vortex,and its size decreases first and then increases.And the occurrence position is advanced with the increase of gap.The high entropy region is located near the moving blade outlet basin,and its range expands with the increase of clearance.The entropy of 60% leaf height and above changes significantly,and the change is more intense in the leaf top region.The change of relative leakage and efficiency with clearance is more uniform.In general,the relative leakage will increase by about0.31%-0.42% and the stage efficiency will decrease by 0.34%-0.44% when the clearance of 0.1% blade height is increased.To sum up,the conclusions of this paper can provide a certain theoretical basis and support for the improvement of steam turbine flow efficiency.