Fan Flow Control for Noise Reduction Part 1: Advanced Trailing Edge Blowing Concepts

Abstract

*† ‡ § This paper documents trailing edge blowing research performed to reduce rotor / stator interaction noise in turbofan engines, under the Ultra-Efficient Engine Technology and Quiet Aircraft Technology programs at the NASA Glenn research center. Velocity deficits are introduced into an engine's working fluid by viscous losses on fan blade surfaces. These propagate downstream where they are cut by stator vanes and produce interaction noise. The unsteady surface pressure on the vanes is coupled to acoustic duct modes that are excited to produce tonal noise at the blade passing frequency and its harmonics. The wake deficits responsible for the noise can be reduced by injecting air into the working fluid via internal passageways in the fan blades. The current technique requires an excessive amount of air to be drawn from the engine compressor for use in reducing wake deficits, resulting in a large loss of performance. The purpose of this research is to investigate new blowing configurations in order to achieve noise reduction with lesser amounts of air. Using the new configurations, air is not injected into every fan blade. Instead the application of injected air is varied circumferentially. For example, blowing air may be applied to alternating fan blades. This type of blowing configuration both reduces the amount of air used and changes the spectral shape of the tonal interaction noise. The original tones at the blade passing frequency and its harmonics are reduced and new tones are introduced between them. This change in the tonal spectral shape increases the performance of acoustic liners used in conjunction with the trailing edge blowing approach. The total noise reduction is therefore a combination of source-level reduction from wake-filling, absorption from acoustic liners, and increased performance of the liners. This paper presents numerical predictions made to estimate the sound power reductions due to these concepts, as well as experimental results taken on the ANCF rig at NASA Glenn for validation purposes. The results show that the new concepts are successful in increasing the efficiency of trailing edge blowing.