Observational analysis reveals how manufacturing variability influences adiabatic effectiveness in airfoils, suggesting improved film cooling methods.
Thermal barrier coating and film cooling are necessary for sustaining material life with turbine firing temperatures exceeding current material capabilities. Utilizing current drilling and coating processes results in manufacturing variability such as blockages and added in-hole roughness. This variability can lead to significant deviations in cooling performance by increasing the likelihood of film detachment and decreasing lateral spreading of film. While film cooling and film hole blockage effects on transonic airfoils are known, manufacturing variability as a result of various drilling and coating processes is not often studied. Avoiding this variability by implementing novel manufacturing processes could result in less variation from design-intent for meter and diffuser sections as well as increased film coverage on the airfoil. In the present study, true-scale film cooling hole coupons with industry relevant gas turbine film hole drilling processes were installed on both the pressure and suction side of an airfoil and tested in a steady transonic linear cascade. Specifically, uncoated, drill-before-coat, and drill-after-coat holes were examined to understand their effects on adiabatic film effectiveness. A foam surface was cast onto the airfoil downstream of the coupons to achieve an adiabatic surface. Using a high-definition infrared camera, spatially-resolved adiabatic film effectiveness was obtained for each coupon at various Mach numbers and freestream turbulence intensities. Drill-after-coat holes increased film effectiveness relative to drill-before-coat holes due to better film attachment and lateral spreading, while drill-before-coat holes had more blockages and misshapen diffuser sections. For all holes, increasing freestream turbulence intensity decreased cooling performance and increased lateral spreading, while increasing Mach number improved cooling performance with little effect on coolant coverage.
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Cannon-Jenkins et al. (2025) studied this question.