Overall Cooling Performance of Convex Surfaces Protected by Combined Film Cooling and Thermal Barrier Coatings

Abstract

Abstract. While the film-cooling characteristics of surfaces coated with thermal barrier coatings (TBCs) have been extensively studied, research on the associated conjugate heat transfer under the constraint of airfoil surface curvature remains relatively scarce. This article investigates the overall cooling performance of a combined film cooling and TBC system on a typical convex surface (with a curvature radius of 60 times the film-hole diameter), utilizing advanced conjugate heat transfer measurement techniques and combining high-precision numerical simulations. The thermal conductivity of the TBC and the blowing ratio (BR) were selected as key influencing parameters. The results demonstrate that the TBC provides significant thermal protection to the convex metal surface, particularly in the region adjacent to the film-cooling holes. Depending on the film-cooling jet behavior, the overall cooling effectiveness of both the metal and the external TBC surface first decreases and then increases with rising BR, reaching a maximum at the BR of 0.5. The application of the TBC effectively reduces the surface heat flux, leading to an increase in the area-averaged overall cooling effectiveness of the metal surface by more than 16.7% (approximately 30% in the near-hole region). Nonetheless, a slight increase in internal flow loss through the film holes is observed. An increase in the thermal conductivity of the TBC reduces its thermal resistance, which in turn decreases the metal surface cooling effectiveness and increases the cooling effectiveness on the external TBC surface. In general, the presence of a TBC can reduce the sensitivity of the metal surface cooling effectiveness to variations in the BR. Nevertheless, this sensitivity is observed to increase when a TBC with lower thermal conductivity or higher thermal resistance is applied.