Abstract
The possibility of identifying new classes of oxides having lower thermal conductivity at high temperatures is unlikely as the values are approaching the theoretical minimum. At the same time, the drive for higher engine efficiency requires higher gas temperatures, increasing the radiative heat flux. The significance of radiative heat transfer in the selection of coating materials is examined. It is concluded that optically opaque materials will perform better as coatings under high convective flow conditions, such as on blades, by minimizing radiative heating through the coating. This can be achieved by a combination of atomic level doping and scattering.
Highlights
The possibility of identifying new classes of oxides having lower thermal conductivity at high temperatures is unlikely as the values are approaching the theoretical minimum
When the coating material is not opaque, calculations show that the radiative heat transport increases the total heat flux through the coating as well as the metal/coating interface temperature
The very high heat fluxes resulting from radiative heat transport through the coating impose additional demands on the internal cooling lowering the overall engine efficiency
Summary
The radiative heat load on the hot-section turbine components increase non-linearly with gas temperature. For coatings opaque in the visible and near infra-red, no radiative heat flux through the coating can occur and the coating’s thermal conductivity determines the heat fluxes and temperatures through the coating. When the coating material is not opaque, calculations show that the radiative heat transport increases the total heat flux through the coating as well as the metal/coating interface temperature. The very high heat fluxes resulting from radiative heat transport through the coating impose additional demands on the internal cooling lowering the overall engine efficiency. Used low thermal conductivity oxides are not effective in blocking radiative heat transport providing an opportunity to design oxides that can reduce the overall heat flow through coatings. The ability to manipulate the radiative properties of a coating, such as 8YSZ, by doping and scattering offers the possibility of reducing the radiative heat flux. The simulations are based on 8YSZ, similar findings are expected for other oxides at higher temperature, such as EBCs being considered for the protection of CMCs
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