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

As gas turbine engine temperatures increase, experimentally evaluating the necessary cooling schemes becomes increasingly cost prohibitive at engine conditions. Thus, researchers conduct film cooling experiments near room temperature and attempt to scale the results to engine conditions. Although thermal measurements of film cooling adiabatic effectiveness are common, an increasingly popular method of evaluating adiabatic effectiveness employs pressure sensitive paint and the heat-mass transfer analogy. Mass transfer methods are attractive because an impermeable model can be used as an analog for a perfectly adiabatic wall; however, the mass transfer analogy is imperfect. 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, but other fluid properties that differ between experimental and engine conditions have only been considered in more recent work. Most notably in the context of an examination of the ability of mass transfer methods to serve as a proxy for thermal methods are the properties that directly influence thermal transport — thermal conductivity and specific heat. 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 field. Using various coolant gases (air, carbon dioxide, nitrogen and argon) and comparing with thermal experiments, the efficacy of the pressure sensitive paint method as a direct substitute for thermal measurements was evaluated on a simulated leading edge model with compound coolant injection. The results thus allow examination of how the two methods respond to different property variations. Overall, the pressure sensitive paint technique was found to over predict the adiabatic effectiveness of a particular coolant flow when compared to the results obtained from infrared thermography, but still reveals a great deal of valuable information regarding the coolant flow structure.