摘要
我国的一次能源以煤为主,火力发电机组总装机容量占全国总装机容量的75%以上,而火力发电机组年总发电量占全国年总发电量的80%以上,火力发电机组的安全运行对于工业发展和人民生活具有重要的意义。随着我国电网已进入到了以超高压、大容量、自动化为标志的新阶段,对发电机组自动化控制技术的发展提出了更高的要求,提高发电自动化控制和实现发电机组实时在线监测对保证机组的安全、经济运行具有重要意义。汽轮机是火力发电机组最重要的部件之一,本论文围绕汽轮机高温部件的变形和蠕变寿命开展研究。
定义了等效对流换热系数,在圆筒壁非稳态导热理论的基础上,首次推导出内、外壁均为第三类非齐次边界条件、蒸汽温度非线性变化时二维圆筒壁温度分布的解析模型,该模型具有求解精度高、可以直接应用于实时分析系统的特点。提出了复杂结构有限子结构计算方法,应用二维圆筒壁非稳态导热解析模型,解决了汽缸、转子等复杂结构的温度场解析求解问题。针对有限子结构中非稳态温度分布,定义了反映不同形状下各段汽缸膨胀的特征值——膨胀特征温度,用来表征不规则物体的变形和膨胀能力,由膨胀特征温度计算各段的变形量,使得在线监测汽轮机各级轴向胀差成为可能。
提出了胀差裕度的定义,通过胀差裕度的在线计算结果,调整机组启动过程中温升率的变化,保证汽轮机在胀差满足要求的条件下快速启动,提高了机组启动过程的安全性,给电网的安全运行提供了保障。汽轮机通流部分胀差在线监测与变温度速率控制系统实现了对汽轮机轴向胀差在线监测与闭环控制。
根据单轴及多轴蠕变实验数据,运用ANSYS用户编程特性UPFs(User Programming Features),对缺口试件高温蠕变进行研究。研究结果表明缺口喉部骨架点(Skeletal Point)应力受到结构和加载的影响,与选用的蠕变方程无关。高温蠕变引起缺口喉部应力的再分布,给出了等效骨架点应力的定义,根据金属蠕变寿命曲线,确定不同结构下多轴蠕变寿命。
引入应变阈值,结合Norton-Kachanov蠕变方程,改进了蠕变损伤模型,模型更准确描述蠕变全过程,修改有限元软件子程序,模拟了螺栓的蠕变过程。结果表明,螺栓蠕变断裂并不是在应力集中的螺纹处,而是在螺杆处,有限元计算结果与试验结果吻合较好。
In China, coal was the major one time resource, the installed gross capacity of thermal generation was over 75 percent of the installed gross capacity of the whole nation, and the gross generation of thermal generation was over 80 percent of gross generation of the whole nation. The safety operation of thermal generating units is important to industry development and the people’s livelihood. High parameter, large capacity and automatization demand not only for electrical grid but also for power generating units will become a tendency in order to increase economics. To ensure the stability of electrical grid running the Automatic Generation Control technology has to be improved. To improve automatic generation control and to realize on-line monitor have the great significance in theory and practical application. The steam turbine is one of the most important parts in thermal generating units. This work is involved in the deformation and creep life of the high temperature parts of steam turbine.
An equivalent coefficient of convective heat transfer was defined. Based on heat transfer differential equation for cylinder, two dimensional transient analytical solution along radial and axial direction with respect to the boundary conditions of outer and inner wall with third type non-homogeneous temperature boundary is achieved, which can be used in on-line temperature monitoring system for the steam turbine casing. In order to demonstrate the expansion ability of irregular finite sub-structure, a new characteristic value, called expansion characteristic temperature, is defined. And an appropriate algorithm which can be used to calculate the deformation of different parts is also proposed. Owing to such a definition, it is possible to evaluate deformation between casing and rotor.
The Axial deformation Margin was defined. The value of the axial deformation margin was calculated on-line, which was used to control the steam temperature changing rate. It ensured the faster start-up of steam turbine with allowable axial deformation. And it improved the operation safety of the grid and the power stations. The axial deformation on-line monitor and automatic changing temperature-speed control system was closed loop control.
Using UPFs (User Programming Features) of ANSYS, the creep of notched bar under high temperature was implemented based on the experimental data of uniaxial and multiaxial specimen. The stress redistribution in the throat cross section of the notch induced by high-temperature multiaxial creep and the creep control stress- Equivalent Skeletal Stress (ESS) were founded. The rupture time of the multiaxial creep was estimated according to the uniaxial creep life curve or the rupture life equation based on ESS. The relationship of the skeletal stress and the notch geometry has been established.
Creep damage model was improved by inducting strain threshold value based on Norton-Kachanov creep model, which was more exact to describe creep process. Creep behavior of bolt was simulated by reworking ABAQUS subroutine based on improved creep damage model. It is indicated that creep rupture of bolt is not taking place at screw thread where the stress concentrated, but taking place at the position of screw shaft. A good coincidence comparing the result calculated according to the theoretical model presented in this paper with finite element calculation can be observed.
引文
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