Effects of Thermal Radiation on Air Plasma Spray (APS) Coated Gas Turbine Blade

Thermal barrier coatings (TBCs) are used to protect hot gas path (HGP) components such as the first two stages of turbine blades and vanes of land-based turbine engines against high temperature environment, corrosion and oxidation. The continuing thrust towards higher thermal efficiencies of gas turbines has resulted in a continuous increase of turbine inlet temperatures (TITs). This has resulted in the increase of heat load on the turbine components especially the high pressure side of the turbine necessitating the need to protect the HGP components from the heat of the exhaust gases using novel TBCs such as air plasma spray (APS) TBCs which are transparent and reflective to radiation. This paper focuses on the combined effects of radiation and conduction heat transfer in the semitransparent yttria-stabilized zirconia (YSZ) coatings used to offer thermal protection to turbine blade. The temperature distribution in the turbine blade depends on the surface convection, reflectivity and refractive index of the grey semitransparent YSZ coatings. The temperature distributions in the metal substrate and the TBC systems are determined by solving the steady state heat diffusion equation and the radiative transport equation simultaneously using ANSYS FLUENT 12.0 CFD commercial package. Preliminary results indicate that substrate metal temperature reduction of about 100K results with the use of the TBC. This temperature drop reduces the thermally activated oxidation rate of the bond coat in the TBC and so delays failure of TBC by oxidation. Furthermore, by taking into account the effect of radiation, the temperature distribution in the metal substrate with TBC exceeds the temperature distribution without radiation by about 40 K, signifying the importance of including radiation in the thermal modeling of TBCs for high temperature applications.