Development of multiscale thermal model for in-service performance of thermal barrier coatings.

摘要

In the turbine industry, there is a growing tendency to use higher turbine inlet temperatures to improve the efficiency of the turbine engine. Consequently, the heat load on the turbine component increase, especially in the high pressure side of the turbine. The heat load is due to the exposure of the turbine parts to the enormous heat flux emanating from the combustion gases. In order to improve the life expectancy of the turbine blades and increase its efficiency as well, more effective cooling methods, in conjunction with the use of the thermal barrier coatings (TBCs), must be employed to ensure the homogenization of substrate temperature distribution. This cooling requirement applies to the turbines for all applications-air, land and sea. At high temperatures, both conduction and radiation are important in determining the temperature distribution in the metal substrates.;Design optimization of the gas turbine blade coatings at high temperatures is an important area of research. This involves prediction of temperature distribution on the blades with thermal coatings as part of the optimization process. This study presents a more realistic approach that can be used to evaluate in-service performance of thin layers of thermal barrier coatings typically found in hot sections of land based and air based turbine engines. Most thermal analysis approaches in this area of the research are based on continuum Fourier based models that assume infinite speed of heat wave propagation in materials. Due to this assumption, Fourier heat conduction equation does not give accurate results for thin film structures exposed to very high temperatures. Therefore, researchers are interested in different approaches to calculate the temperature gradient in TBC. A two part study is undertaken in this thesis work that involves a 2-dimensional (2-D) continuum modeling and a 1-dimensional (1-D) microscale modeling to investigate the thermal gradients in surface of a gas turbine blade coated with thermal barrier coatings.;In the 2-D study, the steady state heat diffusion equation is solved to determine the temperature distribution in the turbine blade using commercial computational fluid dynamics and heat transfer software (Fluent). The effects of incident radiation in the surroundings of the TBC on the metal substrate are considered. It is found that at about 1573 K free stream combustion gas temperature, the use of TBCs with radiation increases the substrate temperatures by about 15K. The application of TBCs acts like a heat suppressor, dropping the metal temperature approximately 50K. This temperature drop has the potential of increasing the life of hot gas path components by two fold. It is therefore imperative to include radiation in thermal modeling at high temperature, since neglecting it will surely underpredict the metal temperature distribution. The effect of the total heat flux through the TBC on the maximum temperature drop obtainable from the TBC was also studied. As will be expected increase in heat flux resulted in an increased temperature drop across the TBC until at about a temperature of 1473K when the temperature drop across the TBC dipped. This dip in temperature drop is attributed to the sintering of the TBC at such high temperature. In the 1-D study, one dimensional multiscale heat transfer model is developed by using Fortran 90. The hyperbolic heat conduction equation is used as the governing equation for the TBC-substrate computational domain to determine the temperature distribution in the TBC-substrate system. The results are compared with the parabolic heat conduction in the TBC-substrate system. It is found that for microscale energy transport in thin films, the hyperbolic heat conduction equation predicts more realistic temperature distribution than the parabolic heat conduction equation.;The modeling approach developed in this study when coupled with corresponding equations for stress analysis can be used to predict the thermal stress and failure life during in-service use of TBCs.