Low-Speed Fan Noise Reduction with Trailing EDGE Blowing

Abstract

An experimental proof-of-concept test was conducted to demonstrate rotor-stator interaction tone noise reduction through rotor trailing edge blowing. The velocity deficit from the viscous wake of the rotor blades was reduced by injecting air into the wake from a trailing edge slot. Composite hollow rotor blades with internal flow passages were designed based on Computational Fluid Dynamics codes modeling the internal flow. The hollow blade with interior guide vanes creates flow channels through which externally supplied air flows from the root of the blade to the trailing edge. The impact of the rotor wake-stator interaction on the acoustics was also predicted analytically.

The Active Noise Control Fan, located at the NASA Glenn Research Center, was used as the proof-of-concept test bed. In-duct mode and farfield directivity acoustic data were acquired at blowing rates (defined as mass flow supplied to trailing edge blowing system divided by fan mass flow) ranging from 0.5% to 2.0%. The first three blade passing frequency harmonics at fan rotational speeds of 1700 to 1900 rpm were analyzed. The acoustic tone mode power levels (PWL) in the inlet and exhaust were reduced 11.5&–0.1, 7.2&11.4, 11.8&19.1 PWL dB, respectively. The farfield tone power levels at the first three harmonics were reduced 5.4, 10.6, & 12.4 dB PWL. At selected conditions, two-component hotwire and stator vane unsteady surface pressures were acquired. These measurements show the modification of the rotor wake due to trailing edge blowing and its effect on the stator vane to illustrate the physics behind the noise reduction.