| With the increase of the power capacity,the wheel diameter and blade length of the last turbine stage increase significantly.Therefore,the centrifugal force on the disc-blade structure of turbine last stage,the bending moment of the steam flow on the blade are greatly increased.Also,the steam flowing through the last turbine stage is in the state of wet steam,which leads to the deterioration of the operating environment.Under the combined effects of various kinds of alternating loads,fatigue cracks can be easily produced in these areas,even the risk of fracture.Therefore,the safety of the disc-blade structure of turbine last stage is one of the prerequisites for safe and stable production of power plants.There has many analysis methods of the contact status between disc and blade and the fatigue crack growth between wheel rims and blade root of large capacity turbine last stage,but by using three dimensional finite element model to analyze above-mentioned problems,the defects of other methods can be overcome.Therefore,it is necessary to explore the 3D geometric modeling method and the corresponding finite element analysis for disc-blade structure of turbine last stage,it can provide guidance for reliability design of disc-blade structure and defect diagnosis of the unit in service.The 3D models of disc-blade structure of a domestic 600MW steam turbine last stage with actual sizes were built in this dissertation.The contact status between disc and blade of large capacity turbine last stage and the fatigue crack growth between wheel rim and blade root of large capacity turbine last stage were analyzed by using the methods of finite element.Firstly,a simplified modeling method of turbine blade structure was proposed according to the dynamics principle.The software Solidworks has been used to establish a three-dimension model of a certain type of domestic 600MW turbine last disc-blade,which is constructed by following components:blade,lashing wire,integral shroud and disc.Secondly,the finite element software ANSYSWorkbench has been adopted to analyze the stress distribution of turbine last disc-blade structure.It can be observed from the numerical simulation that the peak stresses are located in contact area of disc-blade,and the peak stresses are beyond yield,but under failure limit;the peak stresses of disc-blade structure increases linearly with the increase of rotational speed;the contact area of disc-blade is not only the high stress area,but also one of the biggest area of stress changes with the increase of rotational speed;the centrifugal force has more influence on the stress changes of disc-blade structure than steam flow force.The results may serve as a reference for fatigue life predictions of turbine last disc-blade structure.Finally,the software ABAQUS has been used to analyze crack growth of domestic 600MW turbine last disc-blade,the stress distribution of turbine last disc-blade structure and the fatigue crack propagation rule of the peak stress on the rim.In this dissertation,the influence of alternating steam load on the fatigue crack of the rim is considered,under the influence of alternating steam force load,the fatigue crack propagation rule of the peak stress on the rim under rotating speed of 3000rpm and 3300rpm were obtained by simulation.The quantitative relationship between the number of fatigue crack growth cycles and the length of the crack was obtained by simulation,the crack growth length under the two two kinds of rotational speed was compared after operating 1e6 cycles,and it was concluded that the crack growth is not only easily caused under over-speed condition,but also the crack propagation rate is faster than that under the rated speed condition. |
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