Investigations On Vibration Characteristics And Fatigue Life Of Steam Turbine Blades Induced By Wet Steam Two-phase Flow

The steam turbine achieves energy conversion through steam expansion.At the low-pressure cylinder,the steam undergoes unbalanced condensation,resulting in the formation of liquid phase water droplets,and the working medium becomes wet steam.Under the action of steam entrainment,the liquid phase water in wet steam continuously impacts the turbine blades at high speed,resulting in increased surface roughness and formation of serrated burrs.When operating in this harsh environment for a long time,the mechanical performance of the blade will change,which will affect the vibration characteristics of the blade and shorten its service life.By accurately predicting the location of secondary water droplets impinging on the rotor blade surface in the last stage wet steam of a steam turbine,the vibration characteristics and fatigue life changes of the last stage blade under the action of liquid droplets,as well as the impact of liquid droplet particles on the aerodynamic performance of the last stage blade,can provide guidance for the protection area of the last stage blade surface of the steam turbine,that is,the location of installing a local covering layer and providing reference for parameter adjustment during operation.The main work of this article is as follows:First,taking the last stage blade of a 600 MW steam turbine as the research object,fully considering the distribution information of liquid droplets in the flow field and the impact on the blade,the distribution of wet steam droplets at the inlet of the last stage was determined using the first and second water droplet prediction method of the last stage,and then the droplet impact model was determined based on three action mechanisms of the droplet wall surface,The F source code file is written in Fortran language to implement loading the droplet collision model during the calculation process.Based on a coupled droplet wall interaction model,the vibration characteristics of the last stage blades of steam turbines under different steam models were studied through unidirectional fluid solid coupling.The load distribution,force variation,and deformation of the last stage blade of a steam turbine under two typical operating conditions are analyzed.The results show that the deformation of the leading edge and trailing edge of the blade first increases and then decreases along the blade height direction,and the deformation obtained using the wet steam model is larger than that obtained using the ideal gas model.The difference between the two reaches the maximum at the leading edge of the blade at 30% THA condition.Secondly,combined with the droplet wall interaction model,the response characteristics of the last stage blades of steam turbines under two typical operating conditions with different working media were compared and studied using a bidirectional fluid solid coupling calculation method.The results show that the axial force values and fluctuation degree of the last stage blade surface under the wet steam model are higher than those using the ideal gas model;After spectrum analysis,it was found that the frequency corresponding to the wet steam model was higher than that of an ideal gas,and its corresponding amplitude was 15 Pa-26 Pa higher.At the same time,it was found that the wet steam model had a low frequency amplitude of 460 HZ-470 HZ at the leading edge of 50% leaf height and at the trailing edge of 90% leaf height,with a peak value between 22 Pa-26 Pa;The difference between the maximum displacement of the blade surface node and the maximum equivalent stress is 0.25 mm and 62 MPa,respectively.Finally,on the basis of obtaining the equivalent stress distribution and strain response characteristics of the blade,the fatigue life of the final stage blade was predicted using the SWT(Smith Watson Topper)fatigue life prediction method.It was found that under various corresponding working conditions of different steam models,the corresponding number of cycles at the front edge of the blade suction was the lowest,followed by the trailing edge of the blade pressure surface,and the corresponding number of cycles at the 90% blade height position of the blade suction surface was the highest;At the same time,the number of cycles corresponding to the wet steam model is lower than that of the ideal gas model at the same operating condition at each location,and this phenomenon is most obvious at 30% THA.