Experimental Evaluation of Thermal and Mass Transfer Techniques to Measure Adiabatic Effectiveness With Various Coolant to Freestream Property Ratios

Experimentally evaluating gas turbine cooling schemes is generally prohibitive at engine conditions. Thus, researchers conduct film cooling experiments near room temperature and attempt to scale the results to engine conditions. An increasingly popular method of evaluating adiabatic effectiveness employs pressure sensitive paint (PSP) and the heat–mass transfer analogy. The suitability of mass transfer methods as a substitute for thermal methods is of interest in the present work. Much scaling work has been dedicated to the influence of the coolant-to-freestream density ratio (DR), but other fluid properties also differ between experimental and engine conditions. Most notably in the context of an examination of the ability of PSP to serve as a proxy for thermal methods are the properties that directly influence thermal transport. That is, even with an adiabatic wall, there is still heat transfer between the freestream flow and the coolant plume, and the mass transfer analogy would not be expected to account for the specific heat or thermal conductivity distributions within the flow. Using various coolant gases (air, carbon dioxide, nitrogen, and argon) and comparing with thermal experiments, the efficacy of the PSP method as a direct substitute for thermal measurements was evaluated on a cylindrical leading edge model with compound coolant injection. The results thus allow examination of how the two methods respond to different property variations. Overall, the PSP technique was found to overpredict the adiabatic effectiveness when compared to the results obtained from infrared (IR) thermography, but still reveals valuable information regarding the coolant flow.