Thermo‐mechanical analysis of a steam turbine rotor

Abstract

In power plants, high temperatures prevail during long holding times. Furthermore, power plants are often started and shut-down in order to account for gaps or oversupplies in energy production. These loading conditions induce both creep and fatigue loads. Due to their excellent thermo-mechanical properties, such as high tensile strength and elevated corrosion resistance, heat-resistant steels are established materials for power plant components. Nevertheless, these steels tend to soften under deformation, which should be accounted for by a constitutive model.

The contribution at hand analyses the thermo-mechanical behavior of a steam turbine rotor. For this purpose, a unified phase mixture model is introduced. This constitutive model accounts for rate-dependent inelasticity, hardening, as well as softening by employing an iso-strain approach with a soft and a hard constituent. While the soft constituent represents areas with a low dislocation density, such as the interior of subgrains, the hard constituent refers to regions with a high dislocation density, i.e. the subgrain boundaries. Furthermore, two internal variables are introduced: a backstress tensor of Armstrong-Frederick type and a scalar softening variable. The model results in a coupled system of three evolution equations with respect to the inelastic strain, the backstress, and the softening variable.

To allow for the analysis of real power plant components, the model is implemented into the finite element method such that the evolution equations are integrated based on the backward EULER method. The applicability of the model is demonstrated by conducting a thermo-mechanical analysis of a steam turbine rotor with complex geometry under realistic boundary conditions. In a first step, the instationary temperature field in the rotor is computed in a heat transfer analysis. Thereby, typical steam temperatures in power plants and the corresponding heat transfer coefficients are prescribed. As a next step, the structural analysis is conducted based on the phase mixture model and the obtained temperature field as input. In addition, the time-dependent rotational frequency and steam pressure are taken into account. Note that the influence of different start-up procedures such as a cold or a hot start is examined in detail. As a result, the structural analysis provides the stress-strain hystereses, which constitute the basis for further fatigue and damage assessment.