Mass-Heat Coupling in Film Cooling Experiments—The Influence of Temperature and Turbulence on the Dufour Effect

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Abstract The use of foreign gases in laboratory film cooling experiments is attractive since variable density ratios can be achieved with coolant-to-freestream temperature ratios near unity, often reducing the cost and difficulty of the experimental campaign. In adiabatic effectiveness experiments employing pressure sensitive paint along with the mass transfer analogy to heat transfer, isothermal surfaces are often an experimental requirement. Furthermore, low-temperature laboratory experiments using thermal techniques often employ relatively close matches between the coolant and freestream temperatures. Using foreign gases, however, introduces off-diagonal couplings of heat and mass transport, which can produce unexpected results in film cooling experiments. In particular, the Dufour effect—also called the diffusion-thermo effect—which is the transfer of thermal energy by mass transfer processes, can manifest in surface temperatures that break the traditional bounds of thermal adiabatic effectiveness experiments: outside the upper and lower bounds of the coolant and freestream temperature. Beyond the expected confusion for the researcher, this effect can also be detrimental to those that assume that matching the coolant and freestream temperatures are the necessary and sufficient conditions to ensure isothermal surface conditions in traditional pressure sensitive paint experiments. In this work, the influence of cooling gas selection, experimental temperature, and experimental freestream turbulence conditions are explored on a simulated leading edge with compound injection from a cylindrical cooling hole. Air, argon, carbon dioxide, helium, and nitrogen coolants were analyzed due to their use in prior film cooling studies. The Dufour effect was found to be significant when using helium as the coolant, though temperature separation was also observed in argon and carbon dioxide cases. Additionally, elevated experiment temperatures generally increased temperature separation. Finally, high freestream turbulence intensity was found to reduce, but not eliminate, the Dufour effect in helium experiments.