Design of Highly Reflective TBC System for Gas Turbine Application

摘要

Ceramic thermal barrier coatings (TBCs) are playing an increasingly role in advanced gas turbine engines due to their ability to sustain further increases in operating temperatures. Higher operating temperature has resulted in increased radiation in the combustion chamber and turbine section. As reported, radiation within the wavelength range of 0.3～10 μm can transmit directly through zirconia based TBCs to the metal substrate, thereby causing the temperature to increase substantially on the metallic substrate. In order to effectively reduce thermal radiation transport through TBC systems, a ceramic based multiple layered TBC system was specifically designed in this research. This multiple layered TBC system has high reflectance to radiation in the wavelength range of 0.3 ～ 5.3 μm within which more than 90% of the radiation from typical gas turbine engine falls. The multiple layered TBC system consists of a single layer ceramic material with low thermal conductivity and low refractive index, and several high reflectance multiple layered ceramic stacks, each stack designed specifically to reflect a targeted range of wavelength. A broadband reflection for the requiredwavelength range can be obtained using sufficient number of stacks. To achieve a high reflectance for each wavelength range, each stack must have multiple layers of at least two ceramic materials with alternating high and low refractive indices and the optical thickness of each layer is equal to a quarter wavelength in order to meet the condition of multiple-beam interference. The physical thicknesses of the high reflectance multiple layered structures in the present design can be tailored for different wavelength ranges so that a wide range of thermal radiation can be reflected, achieving more effective thermal radiation reduction. A one-dimensional heat transfer model has been established by the authors and radiation transfer equations are used to compute the steady state heat transfer through the designed multiple layered TBC system. Using the established model and software developed, various coating material properties and thicknesses are used as input to this model and temperature distributions within the TBC system are then computed. The temperature reduction on the metal surface was estimated to be close to 90℃ when multiple layered coating system is used. Further, substantial temperature reduction on the coating surface can also result.