This paper presents a numerical solution of the hyperbolic heat conduction equation in a thermal barrier coating (TBC) structure under an imposed heat flux on the exterior of the TBC. The non-Fourier heat conduction equation is used to model the heat conduction in the TBC system that predicts the heat flux and the temperature distribution. This study presents a more realistic approach to evaluate in-service performance of thin layers of TBCs typically found in hot sections of land based and aircraft gas turbine engines. In such ultrafast heat conduction systems, the orders of magnitude of the time and space dimensions are extremely short which renders the traditional Fourier conduction law, with its implicit assumption of infinite speed of thermal propagation, inaccurate. There is, therefore, the need for an advanced modeling approach for the thermal transport phenomenon taking place in microscale systems. A hyperbolic heat conduction model can be used to predict accurately the transient temperature distribution of thermal barrier structures of turbine blades. The hyperbolic heat conduction equations are solved numerically using a new numerical scheme codenamed the mean value finite volume method (MVFVM). The numerical method yields minimal numerical dissipation and dispersion errors and captures the discontinuities such as the thermal wave front in the solution with reliable accuracy. Compared with some traditional numerical methods, the MVFVM method provides the ability to model the behavior of the single phase lag thermal wave following its reflection from domain boundary surfaces. In addition, parametric studies of properties of the substrate on the temperature and the heat flux distributions in the TBC revealed that relaxation time of the substrate material, unlike the thermal diffusivity and thermal conductivity has very little effect on the transient thermal response in the TBC. The study further showed that for thin film structures subject to short time durations of heat flux, the hyperbolic model yields more realistic results than the parabolic model.
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.
Thermal barrier coatings (TBCs) are used in gas turbine engines to achieve higher turbine inlet temperatures (TITs), improve turbine operating temperatures, reduce fuel consumption, increase components lives and thus lead to better turbine efficiency. Yttria-stabilized zirconia (YSZ), is an ideal candidate for TBCs as it has good thermal shock resistance, high thermal stability, low density, and low thermal conductivity. Traditionally, there are two main methods of fabricating TBCs: air plasma spray (APS) TBCs and electron beam physical vapor deposition (EBPVD) TBCs. It is the objective of this paper to study the effects of APS TBC microstructures in comparison with EBPVD TBCs deposited on NiCoCrAlYHf bond coated In738 substrate material for applications in advanced gas turbines. The bond coat NiCoCrAlY contains 0.25w% Hf which is expected to improve the reliability of standard (STD) and vertically cracked (VC) APS TBC material. TBC top coatings of 300 μm and 600 μm thickness for both standard and VC APS TBC and 300 μm EBPVD TBC were further investigated to determine the effect of coating thickness of TBC performance. Selected test specimens were evaluated for dry and wet thermal cyclic oxidation performance. Thermal property determination of select samples was achieved using a laser flash system that measures the thermal diffusivity and specific heat capacity from which the thermal conductivity is calculated. Lastly, select YSZ-Al2O3 composite structures were analyzed in addition to APS and EBPVD TBC microstructure, porosity, and thermal conductivity determination using a variety of analytical techniques. A laser flash system was used to measure the thermal diffusivity for all the samples. A POREMASTER 33 system was used to measure the porosity of the APS and EBPVD samples.
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