The first method uses a numerical solution of one-dimensional (1D) heat conduction problem and determination of equivalent steam temperature for use with the Green function determined with a constant heat transfer coefficient. This chapter presents and compares two alternative methods for steam turbine components both employing the Green functions and Duhamel’s integral and considering the variations of material physical properties and heat transfer coefficient. The effectiveness of the method has been proved by an example of a three-dimensional model of a pressure vessel of nuclear reactor. relies on the solution of nonlinear heat conduction problem by using artificial parameter method and superposition rule and replacing the time-dependent heat transfer coefficient with a constant value together with a modified fluid temperature. A full inclusion of time variability of the physical properties and heat transfer coefficients proposed by Zhang et al. However, numerical solution of a multi-dimensional heat transfer model for complex geometries present in steam turbines is complicated and time-consuming, and due to this, it cannot be used in online calculations. A more important variation of the heat transfer coefficient can be taken into account by calculating the surface temperature using a reduced heat transfer model and employing Green’s function to calculate a stress response to the step change of metal surface temperature. relies on determining the weight functions for steady-state and transient operating conditions. The inclusion of temperature-dependent physical properties proposed by Koo et al. There are known approaches assuming the determination of the influence functions at constant values of these quantities. The major issue in using Green’s function and Duhamel’s integral method in modeling transient thermal stresses in steam turbine components is the time dependence of material physical properties and heat transfer coefficients affecting proper evaluation of Green’s functions and making the problem nonlinear. Such an approach has also been adopted for monitoring power boiler operation and can also be used for calculating stress intensity factors at transient thermal loads. In power generation industry, the approach based on the Duhamel integral and the Green functions has been widely used for thermal stress calculations. In order to control the fatigue damage, thermal stresses should be computed online and limited to permissible values. The flexible operation generates elevated loads and stresses in turbine components leading to material damage due to thermo-mechanical fatigue. Modern energy markets put a requirement of a high operational flexibility of power plant units. The increasing demand for a higher thermal efficiency and a higher operational flexibility of modern power generation plants results in severe mechanical and thermal loading.
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