Effect of the Biot Number on Metal Temperature of Thermal-Barrier-Coated Turbine Parts—Real Engine Measurements

For numerous hot gas parts (e.g., blades or vanes) of a gas turbine, thermal barrier coating (TBC) is used to reduce the metal temperature to a limit that is acceptable for the component and the required lifetime. However, the ability of the TBC to reduce the metal temperature is not constant, it is a function of Biot and Reynolds number. This behavior might lead to a vane's or blade's metal temperature increase at a lower load relative to a reference load condition of the gas turbine (i.e., at lower operating Reynolds number). A measurement campaign has been performed to evaluate metal temperature measurements on uncoated and coated turbine parts in Alstom's GT26 test power plant in Switzerland. Therefore, the impact of varying Reynolds number on the ability of the TBC to protect the turbine components was evaluated. This paper reports on engine-run validation, including details on the application of temperature sensors on thermal-barrier-coated parts. Different methods for the application of thermocouples that were taken into account during the development of the application process are shown. Measurement results for a range of Reynolds number are given and compared to model predictions. Focus of the evaluation is on the measurements underneath the TBC. The impact of different Reynolds number on the ability of the TBC to protect the parts against the hot gas is shown. TBC coated components show under certain circumstances higher metal temperatures at lower load compared to a reference load condition. The measurement values obtained from real engine tests can be confirmed by 1D-model predictions that explain the dependency of the TBC effect on Biot and Reynolds number.