【作者】 李娜；

【导师】 阎维平；

【作者基本信息】 华北电力大学 ， 热能工程， 2013， 博士

【摘要】 超临界锅炉采用直流运行方式,具有启动速度快、变负荷能力强等突出优点,适与参与调峰运行。锅炉频繁启停和负荷变化导致部件应力波动频繁,加速寿命损耗,影响机组安全运行。本文以超临界锅炉的主要厚壁金属承压件—汽水分离器为研究对象,开展变工况运行条件下的应力变化与寿命损耗研究工作,对汽水分离器及其它高温承压金属部件的寿命管理具有重要的参考价值。本文基于汽水分离器变工况运行的动态数学模型,对启动过程的湿态、干湿态转化和干态运行过程的动态特性进行分析,并对目前国内在役的超临界锅炉汽水分离器的几种典型结构进行对比分析。本文选取某1000MW机组汽水分离器为研究对象,建立了其瞬态温度场、瞬态应力场的数学模型和有限元模型。确定汽水分离器在冷态启动、热态启动和温态启动及停炉过程等典型工况下的边界条件,施加约束条件和初始条件模拟了汽水分离器的瞬态温度场和瞬态应力场。得到了汽水分离器关键部位的温度变化规律和壁面温差变化规律,找到热应力最大值位置及变化规律；分析了机械应力的分布规律,得到机械应力与工质压力之间的对应关系和机械应力集中指数；对汽水分离器在温度载荷和压力载荷共同作用下的的总应力场进行分析,得到了总应力的分布和变化规律,为开展汽水分离器的应力监测提供了参考依据。将有限元方法和人工神经网络方法相结合,建立了基于Elman神经网络的汽水分离器动态应力软测量模型。确定工质压力和汽水分离器金属壁面温度为模型输入变量,危险点的总应力为模型输出变量。通过对样本进行训练,确定模型的结构参数。应用电厂运行数据验证了模型的预测精度和泛化能力,为汽水分离器的应力在线监测和寿命评估提供理论依据。本文基于非线性连续损伤力学理论,考虑多轴应力的影响,建立了汽水分离器的疲劳寿命、蠕变寿命及疲劳—蠕变寿命分析的非线性多轴连续损伤模型。对各种工况下的寿命损耗进行计算,并将计算结果与传统模型结果进行比较。汽水分离器的复杂应力状态会加速材料损伤的累积,使用寿命比常规模型计算结果低。分析汽水分离器使用周期内的载荷谱,利用非线性多轴连续损伤模型进行整体寿命评估。更多还原

【Abstract】 Supercritical units operate with once-through boiler with the advantage of high start-up speed and it is suitable for variable load operation. The boiler components’s stress are varied frequently under variable operating conditions and its life loss are accelerated that can affect the safe operation of the unit. The steam-water separator which is the biggest thick wall pressure components of supercritical boiler is chosen as the research object. Its stress field and life loss are calculated and analysed under variable operating conditions.In this dissertation, a comparison is conducted among several steam-water separators used in ultra supercritical boiler that are in service in our country now. Based on the variable condition dynamic mathematical model, the dynamic characteristics of steam-water separator is analysed during wet state and wet-dry state transfer state and dry state.The steam-water separator of a1000MW unit is chosed as the research object. The mathematical models of transient temperature field and transient stress field are set up, and through reasonably simplifying the structure of the separator, a three-dimensional finite element model is set up by ANSYS software. In this model, by applying boundary conditions, constraint conditions and initial conditions, it is possible to stimulate the transient temperature field, transient thermal stress field, transient mechanic stress field and transient total stress field in all kinds of operating conditions including cold start-up, hot and warm start-up, shut down, etc.. Thus the temperatures of separator inner and outer walls can be obtained and the regularities of temperature variation can be observed. And eventually the position of the maximum thermal stress and its change law can be concluded. Through analyzing the distribution of mechanical stress and its change law, proportional relationship between mechanical stress and medium pressure, and the total stress field of the separator under temperature load and pressure load act together, the position of dangerous point and the distribution and change law of total stress can be known.In this dissertation, Finite Element method is combined with Artifical Neural Network method. Using the dynamic mapping function of Elman Neural Network, a dynamic soft sensor model of steam-water separator total stress is set up. The medium pressure and the temperature of the inner and outer walls of the separator are determined as the input variables of the model while the total stress is chosed as the output variable. The train samples are trained while the model structure parameters are determined at the same time. It is verified by the measured data from power plants that this model has satisfactory prediction performance. And this work provides a theoretical basis for on-line stress monitoring and life assessment.During the operational conditions, the main failure modes of the steam-water separator are fatigue failure caused by alternating loads, creep failure caused by high temperature and failure caused by creep-fatigue interaction. Based on continuum damage mechanics and with multi-axial load taken into consideration, a nonlinear damage model of life analysis of steam-water separator is set up. The results provided by this model are compared to the ones calculated by traditional models. It turns out that the damage result from multi-axis continuum damage model is bigger than that figured out by single axis model and linear cumulative damage theory, which shows that the multi-axial complex stress state has accelerated the accumulation of material damage, thus shortening the service life of steam-water separator. The load spectrum of the steam-water separator is analysed and the service life is evaluaed with the nonlinear multi-axis damage model.更多还原