Optical Absorption Effects in Thermal Radiation Barrier Coating Materials

TL;DR

This paper investigates how optical absorption and scattering in thermal barrier coatings affect heat transfer in gas turbines, proposing that enhancing absorption properties can improve engine efficiency.

Contribution

It introduces a two-flux heat transfer model to analyze optical effects in coatings and identifies promising materials and conditions to reduce interface temperatures.

Findings

Absorption thickness normalized values >1 lower interface temperatures.

Spectrally selective coatings require large absorption width and length product.

Enhanced optical absorption in coatings can increase turbine efficiency.

Abstract

Future gas turbine engines will operate at higher gas temperatures and consequentially hot-section components such as blades, vanes and combustors, will be subject to higher thermal radiation fluxes than today. Current thermal barrier coating materials are translucent over the spectral region of the heat flux so future coatings will also have to provide a barrier to thermal radiation. The effects of optical absorption and scattering properties of coating materials on the temperatures and heat fluxes through coatings are explored using a two-flux heat transfer model, and promising combinations are identified that reduce the coating-alloy interface temperatures. Lower interface temperatures occur for thickness normalized absorptions of $\overline{\kappa} L$ $>$1. The effect of both a narrow and a broad band spectrally selective absorbing Gd${_2}$Zr${_2}$O$_{7}$ based coating materials are then studied. These show that large values of the product of the normalized absorption length and the spectral width of the absorption are required to significantly decrease the radiative heat transport through a coating. The results emphasize the importance of enhancing the optical absorption of the next generation barrier materials as a strategy to increase gas turbine engine efficiency by decreasing compressor bleed air cooling requirements.