Effects of Hole Blockage on Endwall Film Cooling and Vane Phantom Cooling Performances of a Transonic Turbine Vane

Although the thermal-barrier coating (TBC) can protect the gas turbine components from the hot mainstream gas, it is not uncommon that the TBC partially blocks the film cooling holes during the spraying process. The film cooling hole blockage will alter the coolant flow behaviors and lead to a significant variation in film cooling performance. In this paper, a physical model of blockage holes including two key parameters of blockage ratios (B) and blockage angles (alpha), was proposed to analyze the effects of hole blockage on endwall film cooling performance, phantom cooling performance of vane pressure side surface (PS), and overall cooling performance. Based on a proposed double-coolant-temperature prediction method, the endwall film cooling effectiveness, phantom cooling effectiveness of the vane PS and area-averaged cooling effectiveness of an actual vane passage were numerically calculated and analyzed, for the common holes and blockage holes (three different blockage ratios of B and three different blockage angles of alpha) at the similarly realistic operating conditions of a gas turbine. Results indicated the hole blockage is pernicious to endwall film cooling performance, leading to a significant decrement of endwall film cooling effectiveness (up to 30% at B=0.4), where the decrement magnitudes increase with increasing the blockage ratios (B). Compared to the blockage ratios (B), the effects of blockage angle (alpha) on endwall film cooling effectiveness are secondary and slight (less than 10% in eta). Nonetheless, the hole blockage is beneficial to phantom cooling performance of the vane PS, leading to an obvious enhancement (more than 40%) in area-averaged phantom cooling effectiveness with increasing blockage ratios (B), due to the increase of the local blowing ratio (BReff). The area-averaged cooling effectiveness including endwall film cooling and phantom cooling on PS, decreases with the increase of blockage angle (alpha), and the benefits of hole blockage in overall cooling performance are negative at alpha=20deg. This suggests that the overlarge blockage ratios and short blockage lengths (correspond to the blockage position near film cooling holes exit or large blockage angle of alpha), should be utmost avoided during the spraying process of the TBC, otherwise, the film cooling scheme designs, based on the geometry of the pristine holes, may be inefficient.

Automated summary

The partial blockage of film cooling holes by thermal-barrier coating (TBC) during the spraying process has a significant impact on film cooling performance. A physical model was used to analyze the effects of hole blockage on endwall film cooling performance and overall cooling performance. Results showed that blockage significantly decreases endwall film cooling effectiveness, while its effects on phantom cooling effectiveness are negligible. Hence, large blockage ratios and positions near the holes exit should be avoided during the spraying process.