Determination of Cooling Parameters for a High-Speed, True-Scale, Metallic Turbine Vane

Facilities such as the Turbine Research Facility (TRF) at the Air Force Research Laboratory have been acquiring uncooled heat transfer measurements on full-scale metallic airfoils for several years. The addition of cooling flow to this type of facility has provided new capabilities and new challenges. Two primary challenges for cooled rotating hardware are that the true local film temperature is unknown, and cooled thin-walled metallic airfoils prohibit semi-infinite heat conduction calculation. Extracting true local adiabatic effectiveness and the heat transfer coefficient from measurements of surface temperature and surface heat transfer is therefore difficult. In contrast, another cooling parameter, the overall effectiveness (ϕ), is readily obtained from the measurements of surface temperature, internal coolant temperature, and mainstream temperature. The overall effectiveness is a normalized measure of surface temperatures expected for actual operating conditions and is thus an important parameter that drives the life expectancy of a turbine component. Another issue is that scaling ϕ from experimental conditions to engine conditions is dependent on the heat transfer through the part. It has been well-established that the Biot number must be matched for the experimentally measured ϕ to match ϕ at engine conditions. However, the thermal conductivity of both the metal blade and the thermal barrier coating changes substantially from low-temperature to high-temperature engine conditions and usually not in the same proportion. This paper describes a novel method of replicating the correct thermal behavior of the thermal barrier coating (TBC) relative to the metal turbine while obtaining surface temperature measurements and heat fluxes. Furthermore, this paper describes how the ϕ value obtained at the low-temperature conditions can be adjusted to predict ϕ at high-temperature engine conditions when it is impossible to match the Biot number perfectly.