Quiet engine

Active reduction of fan noise by injecting compressed air

At the DLR Institute of Propulsion Technology, an active control system is being developed to minimise engine tones. Using active noise reduction technology, the annoying tones of engine components are suppressed by targeted superposition of an anti-sound feld. The examined concept is based on the injection of compressed air through nozzles that are integrated evenly distributed into the wall between the rotor and stator. The introduced air excites additionally alternating forces on the trailing edges of the rotor producing the required secondary noise feld to actively cancel annoying tones.

The functionality of the technology was demonstrated on the Institute’s UHBR (Ultra High Bypass Ratio) compressor-testing facility under realistic operating conditions. Compressed air was injected through two nozzle rings arranged directly one after the other, allowing two degrees of freedom from the sound feld to be controlled simultaneously. Researchers were able to demonstrate a 10-decibel reduction in the dominant components of the rotor-stator interaction tone emitted from the engine inlet. In terms of human perception, this is approximately equivalent to halving the volume level. An optimisation of the nozzle arrangement enabled the air mass required per nozzle to be minimised by more than half (compared with the frst test series). The air can be extracted from the compressed air system located in the engine.

German Aerospace Center (DLR)

Lars Enghardt · E-Mail: lars.enghardt@dlr.de · DLR.de/en