In flight, the exhaust mixer is a feature of many turbofan engines, where air bypass (cold/slow) is mixed with exhaust core (hot/fast), before straining to atmospheric pressure through nozzle.
Video Exhaust mixer
Benefits
The mixer reduces the air velocity out of the core, and consequently, reduces the amount of noise generated. In addition, the exhaust temperature is reduced, rendering the thermal signature of the plane being lower as well. These attributes are very important in military aircraft where thermal detection or heat-seeking weapon is used.
Ideally, the mixer is also a performance benefit to boost the thrust. The exhaust drive from the jet engine is equal to the mass flow exhaust times the exhaust rate, ie, Thrust =? V , while the energy to make that impulse is given by Energy = 1/2 mv 2 . The mixer helps reduce the fastest exhaust speed of the engine core, while making the average exhaust speed faster, generating more drive with the same energy.
Maps Exhaust mixer
Mix type
- Annular - bypass and core flow put together, mixing achieved by sliding effect on the flow interface
- Forced - hot and cold launch, intertwined, forced two streams to mix
Mixing efficiency is defined in a number of ways, but simply, comparing the actual thrust gain (due to mixing) with that in the ideal case. The annular mixer has a low pressure loss, but the mixing efficiency is low. The forced mixer can have high blending efficiency, but this is offset by high pressure losses.
Mixers are very common on military turbofans, because long coaxial exhaust pipes (ie heat and cold) are avoided. Afterburning, with associated variable area nozzles, is also easier to accommodate.
Some high bypass ratios (eg Civilian) turbofan now feature mixers, the advantage is a small thrust boost, which increases specific fuel consumption. Although the mixing process is not thorough, there are benefits of jet noise as well.
References
- http://www.fluent.com/solutions/examples/x178.htm
Source of the article : Wikipedia
0 Comments