Tech Briefs

AFRL research highlights the advantages of pulsed versus continuous coolant film cooling for turbine engines.

Turbine engine designers routinely use film cooling to cool engine components in the hot-gas flowpath. Film cooling is the process of injecting coolant fluid at one or more discrete locations along a surface exposed to a harsh, high-temperature environment. The film cools and thus protects turbine engine components, enabling the engine's operation at higher turbine inlet temperatures and increasing its thermal efficiency. Current turbine engine designs employ a continuous coolant flow, typically diverting 20%- 25% of the compressor's high-pressure air to cool turbine airfoils. By reducing the volume of high-pressure air needed for turbine blade cooling, designers can proportionately increase the flow available for combustion and thus increase thrust. Therefore, coolant flow reduction is an important design goal in the development of advanced turbine engines.

Researchers recently demonstrated that pulsed jets are an effective means for controlling primary flow in lowpressure turbines. They investigated the pulsed jets, also known as vortex generator jets, with secondary flow injected perpendicular to the primary flow and found that reducing the pulsed flows to a duty cycle (DC) of 1% continued to promote reattachment of the separated primary flow. These test results indicate that pulsed flows can significantly modify the near-wall boundary layer by effecting reattachment of the separated primary flow far downstream from the location at which convection would normally separate it from the airfoil.

ImageMotivated by the pulsed flow study results, AFRL scientists initiated a research effort to determine the effects of coolant pulsing frequency (PF) and DC on heat transfer coefficient (h) and film effectiveness (η) distributions. For the test specimen, they selected a semicircular leading edge test model with an afterbody and positioned the traditional, cylindrical film hole 21.5° from the model's stagnation line. The researchers then used an infrared thermography technique to determine both the heat transfer coefficient and the film effectiveness distribution with a single transient test. (Details of this new measurement technique are accessible in an earlier AFRL Technology Horizons® article.1)