Flow path through blade with outer skin removed.
A major source of noise in commercial turbofan engines is the interaction of the fan blade wakes with the fan exit vanes (stators). These wakes can be greatly reduced by filling them with air blown out of the blade trailing edge. Extensive testing of this concept has demonstrated significant noise reductions. These tests were conducted on a low-speed, 4-ft-diameter fan using hollow blades at NASA Glenn Research Center's Aeroacoustic Propulsion Laboratory (AAPL).
The fan was designed and fabricated using new techniques and concepts developed to support design goals. The fabrication made use of considerable rapid prototyping hardware and composites. During operation, air was injected into the hollow fan shaft and flowed into the blade though radial passages that extend to the trailing edge. The blades have composite skins with internal turning vanes and a narrow slot at the trailing edge. Up to 2 percent of the fan total flow can be injected into the slots to fill the wake.
These tests measured the internal and external (far-field) noise, the steady and unsteady velocity behind the fan blades (using hotwire anemometry), the unsteady vane surface pressures, and overall performance. The blowing flow rate was varied as well as the radial extent of the slot. Early results of these tests show that the tone noise was significantly reduced especially at harmonics above the fundamental blade passing frequency. Although this was primarily a noise test, there is evidence that much of the energy in the blowing airstream was recovered, as indicated by reductions in the fan shaft torque. There were also indications from the vane unsteady pressures that broadband noise might also be reduced. We hope that future tests and further analysis of the data will lead to still greater noise reductions.
Flow path through the shaft and hub.
Fan installed in duct.
Glenn contacts: Laurence J. Heidelberg, 216-433-3859,
Laurence.J.Heidelberg@grc.nasa.gov; and Brian Fite, 216-433-3892, Eric.B.Fite@grc.nasa.gov
Sest, Inc. contact: Daniel L. Sutliff, 216-433-6290, Daniel.L.Sutliff@grc.nasa.gov
Author: Laurence J. Heidelberg
Headquarters program office: OAT
Programs/Projects: Aerospace Propulsion and Power Base Research
Last updated: June 2002
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