Transient Thermal Analysis of a Translucent Thermal Barrier Coating on a Metal Wall

Abstract

A common ceramic used for thermal barrier coatings is zirconia, and it is partially transparent for radiative energy at wavelengths up to approximately 5 {micro}m. Hence, at elevated temperatures such as in a turbine engine combustor, the temperature distribution in a coating can be affected by absorption of incident radiation, by internal emission, and by scattering which is large for zirconia. Transient temperatures are needed of evaluating thermal stresses, and for translucent materials numerical procedures can be used to solve the transient energy equation coupled with radiative transfer relations. For radiation, both exact and approximate equations have been used. With general external conditions of convection and radiation for a translucent layer with diffuse surfaces, it was demonstrated in Siegel (1996) that the two-flux method compares well with more exact methods for predicting transient temperature distributions and heat fluxes, including large scattering as needed in a material such as zirconia. The two-flux differential equation for the internal radiative source is solved here with a Green`s function as in Siegel (1997), including two-band spectral property variations for zirconia. Finite difference relations are developed for transient temperatures in a translucent coating on a metal substrate. Illustrative results for heating a coated wall inmore » a turbine engine combustor demonstrate transient heating behavior with translucent effects as compared with an opaque coating.« less