International Journal of Heat and Mass Transfer, 2014
Thermal barrier coatings (TBCs) are not the pure bulk materials, and they contain many different ... more Thermal barrier coatings (TBCs) are not the pure bulk materials, and they contain many different micro/ nano structures caused by the plasma spraying process. The microstructures in the coatings play a very important role on mechanical and thermal properties, therefore, the coating properties differ hugely from those of the corresponding bulk materials. The influence of microstructures on thermal conductivity has been investigated by many researchers, however their effect on radiative heat transfer is seldom studied. The radiative heat transfer will become increasingly important as the temperature rises, and this warrants the study on the influence of microstructures on radiative properties. In the present paper, Finite-Difference-Time-Domain (FDTD) method was employed to simulate the radiative heat transfer behaviors of TBCs with different types of microstructures. The relationship between microstructures and radiative properties of the coating is investigated. Furthermore, the parameters of microstructures such as defect size, shape coefficient, porosity and orientation angle are examined in detail. The results will help us to further understand the radiative heat transfer process across the TBCs and will provide us a theoretical guide to design TBCs with a high thermal insulation property.
In this paper the radiative properties of air plasma sprayed (APS) thermal barrier coatings are i... more In this paper the radiative properties of air plasma sprayed (APS) thermal barrier coatings are investigated by measuring the spectral normal-hemispherical reflectance and transmittance of samples with various thickness and porosities in a broad spectral band ranging from 250 nm to 15 lm. The spectral absorption and scattering coefficients are obtained by a four flux model based on the experimental data. The results show that the porous zirconia ceramics exhibits high scattering coefficient (>15,000 m À1 ) and low absorption coefficient (<500 m À1 ) in the spectral range from 0.5 to 5.5 lm. Moreover, the scattering coefficient increases with increasing porosity, while the spectral absorption coefficient is not quite sensitive to the change of porosity. In order to examine the effects of microstructure of porous zirconia coatings, a theoretical model of Mie scattering is established, and a fairly good agreement with the experimental data is achieved. The results indicate that the scattering coefficient heavily depends on the microstructures of coatings, i.e., pore size, porosity and pore distribution, and this will in turn considerably affect the thermal radiation heat transfer inside the coating.
A theoretical model on radiative properties of air plasma-sprayed (APS) 8 wt% yttria stabilized z... more A theoretical model on radiative properties of air plasma-sprayed (APS) 8 wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) is proposed and validated. Starting from analysis of microstructures, pores inside the coating are regarded as scatterers. Dependent scattering effects among scatterers are thoroughly considered in the framework of quasicrystalline approximation (QCA) and Percus-Yevick (P-Y) pair distribution function for sticky hard spherical particles, in which large nonspherical pores are treated as clusters of the elemental pores. Afterwards, the Monte Carlo (MC) method is used to solve the radiative transfer problem inside TBCs based on the radiative properties. The predicted optical responses including reflectance, transmittance and absorptance of TBC slabs with different thicknesses agree well with the experimental data. The widely used independent scattering assumption in conventional studies is also investigated and examined to be inapplicable in the semitransparent spectral region of TBCs, especially between around 3.2 and 5.6 lm. This study provides a simple and physically robust direct prediction on radiative properties of TBCs solely based on their microstructures and bulk optical properties of the material. Moreover, the present method can be easily applied to the studies on radiative properties of other semitransparent porous media.
International Journal of Heat and Mass Transfer, 2014
Thermal barrier coatings (TBCs) are not the pure bulk materials, and they contain many different ... more Thermal barrier coatings (TBCs) are not the pure bulk materials, and they contain many different micro/ nano structures caused by the plasma spraying process. The microstructures in the coatings play a very important role on mechanical and thermal properties, therefore, the coating properties differ hugely from those of the corresponding bulk materials. The influence of microstructures on thermal conductivity has been investigated by many researchers, however their effect on radiative heat transfer is seldom studied. The radiative heat transfer will become increasingly important as the temperature rises, and this warrants the study on the influence of microstructures on radiative properties. In the present paper, Finite-Difference-Time-Domain (FDTD) method was employed to simulate the radiative heat transfer behaviors of TBCs with different types of microstructures. The relationship between microstructures and radiative properties of the coating is investigated. Furthermore, the parameters of microstructures such as defect size, shape coefficient, porosity and orientation angle are examined in detail. The results will help us to further understand the radiative heat transfer process across the TBCs and will provide us a theoretical guide to design TBCs with a high thermal insulation property.
In this paper the radiative properties of air plasma sprayed (APS) thermal barrier coatings are i... more In this paper the radiative properties of air plasma sprayed (APS) thermal barrier coatings are investigated by measuring the spectral normal-hemispherical reflectance and transmittance of samples with various thickness and porosities in a broad spectral band ranging from 250 nm to 15 lm. The spectral absorption and scattering coefficients are obtained by a four flux model based on the experimental data. The results show that the porous zirconia ceramics exhibits high scattering coefficient (>15,000 m À1 ) and low absorption coefficient (<500 m À1 ) in the spectral range from 0.5 to 5.5 lm. Moreover, the scattering coefficient increases with increasing porosity, while the spectral absorption coefficient is not quite sensitive to the change of porosity. In order to examine the effects of microstructure of porous zirconia coatings, a theoretical model of Mie scattering is established, and a fairly good agreement with the experimental data is achieved. The results indicate that the scattering coefficient heavily depends on the microstructures of coatings, i.e., pore size, porosity and pore distribution, and this will in turn considerably affect the thermal radiation heat transfer inside the coating.
A theoretical model on radiative properties of air plasma-sprayed (APS) 8 wt% yttria stabilized z... more A theoretical model on radiative properties of air plasma-sprayed (APS) 8 wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) is proposed and validated. Starting from analysis of microstructures, pores inside the coating are regarded as scatterers. Dependent scattering effects among scatterers are thoroughly considered in the framework of quasicrystalline approximation (QCA) and Percus-Yevick (P-Y) pair distribution function for sticky hard spherical particles, in which large nonspherical pores are treated as clusters of the elemental pores. Afterwards, the Monte Carlo (MC) method is used to solve the radiative transfer problem inside TBCs based on the radiative properties. The predicted optical responses including reflectance, transmittance and absorptance of TBC slabs with different thicknesses agree well with the experimental data. The widely used independent scattering assumption in conventional studies is also investigated and examined to be inapplicable in the semitransparent spectral region of TBCs, especially between around 3.2 and 5.6 lm. This study provides a simple and physically robust direct prediction on radiative properties of TBCs solely based on their microstructures and bulk optical properties of the material. Moreover, the present method can be easily applied to the studies on radiative properties of other semitransparent porous media.
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