The Study On Heat Transfer And Thermal Stress Of Large Scale Turbine Rotor

Turbine is a rotating motive power machine which transforms thermal energy of steam to mechanical energy. Compared with other forms of motive power machine, turbine has many advantages, such as high power of single machine, thermally high economy, reliable operation, ability to use multiple fuels and long life. Therefore turbine is widely employed in regular fossil fuel and nuclear power plants to produce electricity. In addition, turbine can be operated in varying speed to drive feed pump and boat propeller directly. It plays a very important role in national economy.Rotor is a key part of turbine, at the same time it is also the most fragile part during operation. In the process of start-up, due to the dramatic changes of steam parameters(e.g. temperature, pressure, flow-rate), there will be a great alternating thermal stress on the rotor, which leads to occurrence of flaws due to low frequency fatigue. Therefore, thermal stress is the key factor to be controlled during start-up process. For the safety of peak regulation turbine and economic start-up, it is necessary to monitor and control the thermal stress of rotor during such processes as start-up, shut-down and load changes. Nowadays, micro-computers are widely available, so by employing computers to monitor the thermal stress of rotor on line, we can provide effective way and sound foundation for the unit to start up optimally.In view of today's techniques, it is very difficult to measure the temperatures of internal and external surfaces of rotor directly; however, according to principle of heat transfer, mathematical model is built to calculate the temperature difference and thermal stress of rotor based on the temperature, pressure, heat transfer coefficient and physical properties of metal material. The automatic turbine control (ATC) is designed to optimize the start-up of generating unit by using thermal stress of turbine rotor as the judgment of the speed of start-up.In this paper, extensive reading is done, the comprehensive and systematic discussion and description are given regarding the ATC of Westinghouse electric corporation, furthermore the key contents are deeply analyzed of temperature distribution calculation and stress calculation, the models for predicting the temperature distribution of high pressure and intermediate pressure rotors respectively are derived; the details for application is given; the relationship between the life of rotor and stress is studied. These research results provide solid foundation for the optimal control of turbine.With respect of two different type of rotor, namely T-Root Groove rotor and Side-Entry Groove rotor, the models for calculating their stresses are derived respectively. It can be seen from these modes that the stress of rotor's surface is directly proportional to effective temperature difference. So, for the specific material of rotor, the control problem of thermal stress can be cast into the control of effective temperature difference.Considering the complexity of temperature distribution calculation of actual turbine rotor due to many factors, if multi-dimensional calculation models are employed, it will certainly lead to great burden of calculation, thus it can not meet the requirement of on line calculation. So the reasonable simplification of models is necessary. In this paper, the detailed conductive heat model of rotor is presented, which affects the temperature distribution of rotor.For the operating rotor of turbine, it is comparatively difficult to measure the stress. However, by the measurements of some points and model calculation, the control of thermal stress can be achieved via controlling the effective temperature difference, thus the whole process turbine operation, including start-up, load changes, shut-up, can be realized, and furthermore optimization can be carried out.