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Jet Engine Noise Reduction

Background

The noise suppression of Turbojet engines has many implications in the world today. In aircraft, the largest amount of noise exists during take-off when the engines run at maximum thrust. This means that the loudest noise is produced when the plane is closest to humans. Noise reduction is essential yet it generally cannot be accomplished at the expense of thrust. In our case, we can compromise the thrust of the engine, though it is important to note that the engine will be operated at maximum speed.

Objectives

The main objective of this project was to reduce the noise produced by a JetCat P80 miniature jet engine as much as possible. The overall system must be confined to a 18" x 4.75" x 4.75" rectangle behind the engine. Thrust was not a major concern for this particular use, though strength and robustness were important.

Solutions in Industry

Several companies have invested heavily in technology to reduce the noise output of their turbine engines. The vast majority of turbine engines are used for the amount of thrust produced, so both the pressure and the exhaust velocity must be preserved. The most common solutions available are exhaust mixing present in a turbofan engine, where a medium velocity flow of air surrounds the high velocity air which exits the main chamber of the jet engine. Both of these flows in turn mix with the ambient air. The smoother gradient of velocity present in this case helps reduce the noise generated. The second major accomplishment in the industry is the use of chevrons at the exhaust outlet. These serrated edges at the trailing edge of the jet engine also help break up the flow exiting the jet engine and promote mixing, which again helps reduce the noise.

How does this apply?

Unfortunately, the better of the two noise reduction techniques is the exterior flow produced by a Turbofan engine. Our engine is a Turbojet, which is not set up to produce this secondary flow. The chevron design also has its limitations, namely the maximum noise reduction achieved by this method is around 2.5 dB. Although this is a sizable decrease in power (a 3 dB decrease in noise means that the sound waves carry half the energy), it hardly touches the noise produced by the JetCat P80 (with a maximum noise generation of over 120 dB)! Noise levels before the silencer are capable of causing immediate hearing damage, and the exhaust with the chevrons isn't much better (30 seconds before permanent hearing damage).

Final Design