Exhaust mixer
Turbofan engines often feature an exhaust mixer, where the bypass (cold/slow) air is mixed with the core (hot/fast) exhaust gases, before exhausting to atmospheric pressure through a common (mixed flow) propelling nozzle.
Benefits
The mixer reduces the velocity of the core exiting air, and consequently, reducing the amount of noise produced. Additionally, the temperature of the exhaust is reduced, rendering the thermal signature of the aircraft to be lower as well. This attribute is critical in military aircraft where thermal detection or heat-seeking weapons are used.
Ideally, a mixer is also a performance benefit to increasing thrust. The exhaust thrust from a jet engine is equal to exhaust mass flow times exhaust velocity, ie, Thrust = mv, while the energy to make that thrust is given by Energy = 1/2mv2. A mixer helps reduce the fastest exhaust velocities from the core of the engine, while making the average exhaust velocity faster, producing more thrust with the same energy.
Mixer types
- Annular - bypass and core streams are brought together co-annularly, mixing being achieved by the shearing effect at the stream interface
- Forced - hot and cold chutes, intertwined, force the two streams to mix
Mixing efficiency is defined in a number of ways, but simply, compares the actual thrust gain (due to mixing) to that of the ideal case. Annular mixers have a low pressure loss, but a low mixing efficiency. Forced mixers can have a high mixing efficiency, but this is offset by a high pressure loss.
Mixers are extremely common on military turbofans, since long coaxial exhaust pipes (i.e. hot and cold) are avoided. Afterburning, with the associated variable area nozzle, is also easier to accommodate.
Some high bypass ratio (e.g. civil) turbofans now feature a mixer, the advantage being a small thrust gain, which improves specific fuel consumption. Although the mixing process is not thorough, there is a jet noise benefit as well.