Radiation Effects in a Semitransparent Gray Coating Heated by Convection and Cooled by Radiation

A parametric study using a one dimensional model of a semitransparent gray thermal barrier coating was performed to gain an understanding of the role thermal radiation can play in the heat transferred. Some ceramic materials are semitransparent in the wavelength ranges were thermal radiation is important. Therefore, absorption, emission, and scattering of thermal radiation can affect the heat transfer through the coating. In this paper a one dimensional layer was used to model the heat transfer process occurring in a burner test rig. The semitransparent layer is heated by a hot gas flowing over its surface. The layer and substrate are cooled by radiation to the surroundings. The back side of the substrate is insulated. The coating is assumed to be gray (absorption and scattering coefficients are not function of wavelength). An absorption coefficient of 0.3 cm -1 and scattering coefficients of 0 (no scattering) and 100 cm -1 (isotropic scattering) were used. The thickness and thermal conductivity of the layer are varied. The results show that the temperatures are affected by the properties of the semitransparent layer and the emissivity of the substrate. The substrate and surface temperatures are presented. The apparent temperature an optical pyrometer would read for the emitted energy is also given. An apparent thermal conductivity was calculated for the layer.