Radiative exchange in a parallel-plate enclosure with translucent protective coatings on its walls

Abstract

Thermal barrier and protective coatings are used for reducing temperatures in metal walls in high temperature applications, and for protection in corrosive environments. Radiative transfer is important at elevated temperatures, and hot coated surfaces that view each other can exchange radiation. Radiative exchange calculations are usually for surfaces that are opaque, and each surface has a specific radiating temperature. The analysis must be different when some or all of the surfaces have translucent coatings. Some protective coating materials such as zirconia and alumina, are partially transparent to portions of the thermal radiation spectrum. Hence, the radiative exchange process is more complex since radiation from a coated area depends on the temperatures within its coating. These temperatures are unknown, and they depend on the exchange process. The exchange analyzed here is for a parallel-plate enclosure that can have different heating conditions for the internal and external surfaces of each coated wall. Each wall has an opaque outer layer such as a metal plate, and a translucent coating on the inside. The coatings on opposite walls view each other and exchange radiation. An analytical method is developed for obtaining the temperature distribution in each coating and its opaque substrate. Illustrative results are provided for coatings with various optical thicknesses, and are compared with results for opaque coatings.