Heat transfer mechanisms in steam turbines during warm-keeping operation

A gentle warm-keeping of steam turbines (ST) enables a significant reduction of the start time of thermal power plants. As a result, the residual load can be generated more flexibly and resources can be used more sustainably. Keeping a ST warm for extended periods of power plant standstill avoids high thermal stresses during a subsequent conventional start-up process, which also reduces the lifetime consumption of the heavily loaded components. In this thesis, heat transfer mechanisms during warm-keeping of a ST with hot air were investigated. The aim of this work was to develop a calculation model with which the thermal state of a ST can be predicted during forced heating with air. As a part of this calculation model, analytical correlations were developed for the specific heat transfer mechanisms ``convection'', ``radiation'' and ``contact heat transfer''. A ST warm-keeping process with air differs significantly from a conventional steam operation. Therefore, the analyses conducted are scientifically uncharted. For the calculation of radiation heat exchange within the flow channel, the complex geometric relationships of the individual surfaces were described with view factors. These were determined numerically for defined surfaces. Convective heat transport was investigated by the use of transient numerical conjugate heat transfer (CHT) simulations of a ST segment. Based on the results, heat transfer characteristics were determined over a wide operating range and described by means of phenomenological correlations. Most of the heat is absorbed by the blades and conducted to the rotor. A thermal contact resistance (TCR) exists at the contact areas of the blade root. The TCR depends on the centrifugal force and the surface conditions. To investigate these phenomena, a novel test rig was set up. Numerous measurements were used to determine and validate an analytical TCR correlation. A hybrid calculation approach consisting of a 3D numerical finite elements model (FEM) and the analytical 1D correlations developed was chosen to calculate the thermal state of the ST.As a result, the computing time can be reduced significantly, while ensuring a high accuracy. The calculation approach was calibrated based on measurements of a cool-down process. Subsequently, ST warm-keeping processes under different operating conditions were analyzed in a case study. The results show that the thermal state of the ST can be controlled using the operating parameters and thus the ST can be kept at hot start conditions for any period of time