Computational Fluid Dynamics Evaluations of Film Cooling Flow Scaling Between Engine and Experimental Conditions

The hostile turbine environment requires testing film cooling designs in wind tunnels that allow for appropriate instrumentation and optical access, but at temperatures much lower than in the hot section of an engine. Low-temperature experimental techniques may involve methods to elevate the coolant to freestream density ratio to match or approximately match engine conditions. These methods include the use of CO₂ or cold air for the coolant while room temperature air is used for the freestream. However, the density is not the only fluid property to differ between typical wind tunnel experiments so uncertainty remains regarding which of these methods best provide scaled film cooling performance. Furthermore, matching of both the freestream and coolant Reynolds numbers is generally impossible when either mass flux ratio or momentum flux ratio is matched. A computational simulation of a film cooled leading edge geometry at high-temperature engine conditions was conducted to establish a baseline condition to be matched at simulated low-temperature experimental conditions with a 10× scale model. Matching was performed with three common coolants used in low-temperature film cooling experiments—room temperature air, CO₂, and cold air. Results indicate that matched momentum flux ratio is the most appropriate for approximating adiabatic effectiveness for the case of room temperature air coolant, but matching the density ratio through either CO₂ or cold coolant also has utility. Cold air was particularly beneficial, surpassing the ability of CO₂ to match adiabatic effectiveness at the engine condition, even when CO₂ perfectly matches the density ratio.