Thermal Analysis in a Film Cooling Hole with Thermal Barrier Coating

Abstract

F ILMcooling and thermal barrier coatings (TBC) arewidely used cooling techniques, especially in a gas turbine, which are used to protect a material surface exposed to a high-temperature environment. Increasing the turbine inlet temperature causes the thermal expansion mismatch between the TBC and the substrate, which generates high thermal stresses and thereby TBC delamination as noted in previous reports [1,2]. In addition, TBC delamination has caused thermal cracks on the sides of film cooling holes because temperature difference between regions with and without TBC increased, as reported by the Electric Power Research Institute [3]. Furthermore, the flow temperature on the TBC surface is varied by introducing secondary cooling flow from film cooling holes. Thus, the local thermal loads on the TBC surfacewithfilm cooling holes are different from convectional cooling system. However, most previous studies [4–11] of TBC delamination have mainly focused on a convectional cooling system without a film cooling hole, and have also investigated the effects of thermal cycles and thermally grown oxide (TGO) on TBC delamination. To design hot components such as combustors, vanes, and blades andtopredict their lifetimeandsafety, it isnecessarytoestimateproper TBC thickness as well as to study the thermal cycles in a film cooling systemunderanappropriatethermalenvironment.ThinTBCresults in higher metal temperature, whereas thick TBC results in higher TBC temperature,whichcanbethesourceoftheTBCfailure.Therefore, the objective of the present paper is to determine the tolerable TBC thicknessateachmainhotflowtemperaturewithafilmcoolingsystem of normal injection. For the calculation, we conduct thermal analysis using numericalmethods [12] and experimental heat transfer data in a nondimensional form (Nusselt numberNu) from the previous studies [13,14]. The test materials of TBC and substrate are yttria-stabilized zirconia (YSZ) coating and IN738-superalloy, respectively. As results, we present maximum temperature and maximum debonding stressvaluesat theedgeof coolingholes asa functionof twovariables, that is, thickness of TBC and temperature of main flow. This work helps to design actual TBC systems and to understand mechanics of interface segregation between TBC and substrate.