Safety Analsys Of The Last Stage Blade In Stream Turbineunder Small Flow Rate Conditions

With the development and progress of China’s new energy industry,more and more new energy power generation has been put into social production and use.In this process,thermal power plants need to make flexible adjustment to accommodate new energy generation.Under this demand,a large number of thermal power units will operate under the small flow rate condition for a long time.At this time,the last stage of steam turbine low pressure cylinder will also be under the small flow rate condition for a long time.The flow field environment of the last stage will become very bad,and there will be some problems such as flow separation,backflow,over-temperature of the blast,wet steam condensation and structural instability.These problems will affect the structural stability and service life of the last stage blade,resulting in a serious impact on the safety of the blade under small flow rate conditions.Therefore,it is necessary to analyze the safety of the last stage blade of steam turbine low-pressure cylinder under small flow rate condition by studying the variation law of the above influencing factors.In order to comprehensively analyze the safety of the last stage blade under small flow rate condition,this paper takes the last stage blade of 300MW unit in a power plant as research object.The three-dimensional model of the last stage was established,and the flow characteristics,the wet steam condensation characteristics and the dynamic characteristics of blade structure were studied by numerical calculation method,and the effects on the structural stability and operating life of the last stage blade were analyzed.The main research contents are as follows:(1)Combined with the actual situation of the last stage flow field under small flow rate condition and considering the actual running of blade in the wet steam area,the combination of two-phase flow model,the modified homogeneous nucleation model and the droplet growth model were adopted in this paper.The last stage flow field and condensation characteristics of wet steam under small flow rate condition were studied.And the aerodynamic characteristics and condensation characteristics of the last stage flow field were analyzed comprehensively by both steady and unsteady methods.The results show that there is a strong three-dimensional flow state in the last flow passage at small flow rate condition,and with the decrease of flow rate,the flow separation gradually increases,and there are many obvious separation zones in the limit flow line,and the power capacity decreases,and the phenomenon of excessive blast temperature and wet steam condensation occurs.When the working condition is 5%THA,the flow separation phenomenon in the moving blade passage has become very serious,which ranges from 30%-97%of blade height.The absolute and relative steam flow angles at the tip of the rotor blade inlet reach-25 degrees and-170 degrees respectively.The distribution trend of nucleation rate along the blade height is basically unchanged under all conditions.The maximum position of nucleation rate is mainly between 70%and 80%of blade height,and the maximum value reaches1.8.In addition,the droplets after condensation of wet steam will keep moving with time,and will contact and collide with the blade surface.When running for a long time,the blade safety will be seriously affected.(2)The structural side modeling of the last stage blade was carried out,and the structural dynamics analysis of the last stage blade was carried out through the fluid-structure coupling method and modal analysis method,and the structural stability of the last stage blade under small flow rate condition was studied.The variation law of stress and strain frequency mode of blade structure under small flow rate condition was analyzed.The results show that the equivalent stress of the last stage blade decreases gradually from blade heel to blade tip under small flow rate condition,and the maximum equivalent stress increases with the decrease of operating condition.The region with large equivalent stress is mainly located at the trailing edge of blade root from 0 to 20%of blade height,concentrated at the inlet and outlet edges.At the 3%THA condition,the maximum equivalent stress of the last stage blade increases sharply and reaches571.85MPa.However,the strain distribution is relatively uniform and gradually increases from blade root to blade tip.At this time,the maximum strain is 1.9643mm,which located near the tip where the girth is connected to the blade.Below the 8%THA condition,the stress on the blade will exceed the yield limit of material,so the 8%THA condition can be regarded as the minimum safety flow rate.Above the 8%THA condition,the stress on the blade is less than the yield limit of material,meeting the safety requirements of the operation of the last stage blade.(3)Combined with the flow characteristics,condensation characteristics and structural dynamic characteristics of the flow field in the last stage under small flow rate condition,the operating life of the last stage blade was predicted through the nominal stress method and stress-strain law.The results show that the operating life of the last stage blade decreases with the decrease of the flow rate under small flow rate condition.The blade life is2.142×10~9 hours at the 30%THA condition and dthe blade life is 1493.45 hours at the 5%THA condition.Above the8%THA condition,the safe operation requirements of the unit in the accommodation of new energy or the longest running period of heating can be met.