For many years, there has been interest in evaluating the effect of density differences between the coolant and the freestream in terms of the cooling effectiveness. Numerous experiments have been conducted with different cooling gases or different temperature gases to evaluate the effect of the density ratio. With little agreement on the best way to scale the density ratio effect, it has become commonplace for some researchers to insist upon matching the density ratio for experimental work.
Unfortunately, the density is not the only property that differs between the various coolant gases used in experiments, and it is certainly not the only property difference between the coolant and the freestream in actual engines. In the present work, we isolate some of these effects through film cooling experiments with carefully selected and conditioned coolant gases at near identical densities but exhibiting other property differences. Most significantly, coolant specific heat varied, but subtle viscosity and thermal conductivity effects were present. Through measurements of the adiabatic effectiveness from a film cooling hole on a leading edge model, we are able to show that the specific heat effect is just as important as the density effect, providing more evidence that effects in prior research attributed to density differences, are actually a combination of density and other property differences.