Research On Water Erosion Characteristics In Low Pressure Cylinder Of Steam Turbine Under Small Volume Flow

In order to realize more clean energy grid-connected power generation,thermal power units often need to operate under depth peak load cycling conditions,which lead to the increase of turbine operating conditions of small volume flow.Under small volume flow conditions,the phenomenon of adverse pressure,deflow and windage often appear in the low pressure cylinder of steam turbine.These all intensify the water erosion of the last stage blades.If the blade is damaged,it will not only lead to the decrease of units operation efficiency,but also affect seriously the safe of the units.Therefore,it is necessary to study the water erosion characteristics of the low-pressure cylinder of steam turbine under small volume flow conditions,that can provide a reliable reference for the safe operation of the units under small volume flow conditions.Firstly,in order to make the overall flow field closer to the real flow,the one side of the low pressure cylinder of a 300 MW steam turbine was taken as the research object,and the N-S equation and the SST k-ω turbulence equation was solved to analyze the flow field of the low pressure cylinder under small volume flow conditions.By analyzing the flow field of wet steam on the meridian surface,the flow field of the cascade,the surface flow of the last blades,the temperature field and the pressure change,the wet steam flow in the low-pressure cylinder under different flow conditions was investigated.The results show that when the low-pressure cylinder of the steam turbine operates under low load conditions,backpressure will occur in the last stages,that make the tip clearance vortex appear at the upper span between the last stator blade and the last rotor blade and backflow appear at the roots of the rotor blades.At the same time,the stagnant dissipation of vortex energy will lead to the temperature rise of wet steam.At the 20%turbine heat acceptance(THA)condition,the last stage blades begin to do positive work on the steam and the steam temperature rise,which is called the windage phenomenon.At the same time,under small volume flow conditions,the flow structure is extremely complicated,especially the flow in last stage.Separating vortex,attack angle vortex,reflux vortex and the tip clearance vortex appear successively.Through the calculation of the erosion index E,it was found that as the load decreases,the degree of water erosion continues to increase.After the windage phenomenon occurs,it is necessary to use water spray desuperheating device,which will change the humidity of the flow field and affect the water erosion characteristics.Secondly,the droplet jet cooling method of Euler-Lagrangian particle tracking and the Finnie erosion model was used to study the water erosion characteristics of the last stage and the effect of water spray on the water erosion characteristics under small volume flow conditions,including the motion trajectory of secondary water droplets generated by tearing and water droplets introduced by the water spray device under extremely low load conditions,the impact on the last stage blades.The research results show that: when the load is under(100%THA-30%THA),there is no windage phenomenon,and the temperature rise has little effect on the secondary water droplets.As the load decreases,the pressure decreases in the flow field and more large secondary water droplets are likely to be generated,which are more harmful to the blades.The area where water erosion occurs is located on the inlet side of the last stage rotor blades.When the load decreases,the degree of water erosion becomes more serious and the scope becomes larger.It is mainly reflected in the radial direction of the blade.At 100% THA,the range occupies about 1/3 of the blade height.As the load decreases,the range of water erosion exceeds 1/2 of the blade height,which increases the the water erosion of the blade.The water erosion range does not change significantly along the axial deirection of the blade,and it occupies about 1/2 of the blade width.When the unit operates under the ultra-low load(20%THA-5%THA)conditions,water spray cooling device is used.The effect of the steam return to the root of the rotor mixed with water droplets makes the water erosion appearing at the rotor blades root of the outlet side more serious.As the load decreases,the return vortex becomes larger,and the high-speed area decreases in the deflector ring,and it is easier for the sprayed water droplets to move to the upper part of the deflector ring,This intensifies the water erosion on the outlet side of the last stage rotor blades.The water ersion range expands gradually from the root of the outlet side to the top.