AFRL Researches Use of Pulsed Coolant for Film Cooling Published Dec. 13, 2006 By Plans and Programs Directorate AFRL/XP WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- Coolant flow reduction is an important design goal in the development of advanced turbine engines. Turbine engine designers routinely use film cooling to cool engine components in the hot-gas flowpaths. Film cooling is the process of injecting coolant fluid at one or more discrete locations along surfaces regularly exposed to a harsh, high-temperature environment. The film cools and thus protects turbine engine components, enabling the engine to operate at higher turbine engine inlet temperatures and increasing its thermal efficiency. Current turbine engine designs employ a continuous coolant flow to cool turbine airfoils. AFRL researchers are working to reduce the volume of high-pressure air needed for turbine blade cooling, which will allow designers to proportionately increase the airflow available for combustion and thus increase thrust. The AFRL researchers controlled coolant pulsation by adjusting the opening and closing times of two synchronized pulsed valves, installed in the coolant supply line, to create a combination of pulsing frequency (PF) and duty cycle (DC). To conduct the study, the researchers applied hot mainstream air to the test model as they injected coolant through a cylindrical hole in the model. They released both flows onto the ambient-temperature surface of the model simultaneously in a transient mode. The goal of film cooling is to obtain both a higher film effectiveness to better protect the turbine blade/vane and a lower heat transfer coefficient to prevent the hot-gas stream from transferring heat to the turbine blade/vane. The results show that compared to the continuous coolant, the pulsed coolant provides greater film coverage, which contributes to the higher spanwise-averaged film effectiveness. The results also indicate that the pulsed coolant generates heat transfer coefficients lower than those generated by the continuous coolant in dimensionless form. This research effort demonstrates that turbine engine designers can significantly reduce film cooling flow requirements by substituting pulsed coolant flow for continuous blowing methods. Using pulsed flow, designers can reduce film cooling flow ranges by 18% to 41%--depending on DC and PF--while improving film effectiveness and lowering heat transfer coefficients. These important findings will contribute to the design of higher-performing turbine engines.