Exhaust noise is a concern with jet-powered airplanes. As hot exhaust gases collide with cool ambient air, it creates intense waves of pressurized and turbulent air, which manifests in the form of a loud roar-like noise.
Fortunately, advancements in modern aviation have led to noise-reducing solutions, such as exhaust mixers. Exhaust mixers blend core exhaust gases with bypass air. The end result is less noise and better fuel efficiency. There are two different types of exhaust mixers, however: annular and forced.
What Is an Annular Exhaust Mixer?
An annular exhaust mixer is a type of exhaust mixer that mixes core exhaust gases with bypass air in the same passage. They are known as annular exhaust mixers because the core gases and bypass air are mixed “co-annularly.”
You’ll typically find annular exhaust mixers at the back of jet engines where they form a ring around the exhaust nozzle. Hot core exhaust gases flow from the center, whereas cooler bypass air flows from the perimeter. The core exhaust gases and bypass air mix naturally before exiting through the exhaust.
What Is a Forced Exhaust Mixer?
A forced exhaust mixer is a type of exhaust mixer that uses vanes or guide plates. It’s still designed to mix hot core exhaust gases with cool bypass air. Forced exhaust mixers, however, redirect the streams so that they converge at the right time.
There are vanes or guide plates for the core exhaust gases and the bypass air. As the core exhaust gases and bypass air travel through these vanes and guide plates, they will eventually meet where they mix together.
Differences Between Annular and Forced Exhaust Mixers
Annular and forced exhaust mixers both reduce noise and improve fuel efficiency via the mixing of core exhaust gases with bypass air, but there are several key differences between them.
Annular exhaust mixers are more common than their forced counterparts. Many commercial airliners, such as the Boeing 787 and 737, use them. In comparison, forced exhaust mixers are typically found on special aircraft that require a high level of noise suppression.
One of the reasons why annular exhaust mixers are so popular is their weight. They weigh less than force exhaust mixers. Annular exhaust mixers also provide greater fuel efficiency than forced exhaust mixers. This is because annular exhaust mixers have less pressure loss, which translates into better efficiency. By using annular exhaust mixers, commercial airliners can save money on fuel expenses.
