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Exhaust mixer

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Turbofan engine component

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Pratt & Whitney JT4A turbojet with hush kit mixer andCFMi CFM56-5C turbofan with mixed exhaust under aGE Aerospace'sBoeing 707 testbed.

Exhaust mixer of aJT8D engine of aBoeing 737-200

In aviation, anexhaust mixer orflow mixer is a feature of manyturbofan engines, where slower, colder bypass air is mixed with faster, hotter coreexhaust gases, before exhausting to atmospheric pressure through a common (mixed flow) propellingnozzle.^[1]

Benefits

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Hush kit mixer on the turbojetGeneral Electric CJ805 of aConvair 880

Exhaust mixer on a mockup of aSafran Silvercrest high-bypass turbofan used on large business jets

The mixer reduces thevelocity of the core exiting air, and consequently, reducing the amount of noise produced. Additionally, the temperature of the exhaust is reduced, contributing to the overall reduction of the thermal signature of the aircraft. This attribute is critical in military aircraft where thermal detection orheat-seeking weapons are used, in particularstealth aircraft.^[2]^[3]^[4]

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, i.e.,Thrust = ṁv, while the energy to make that thrust is given byEnergy = 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.

TheIAE V2500 on theA320 family, using a mixed exhaust
TheCFM56-5A/B also powers the A320 family, but utilizing a separate exhaust nozzle

Slit shaped tail exhaustNorthrop Tacit Blue

Mixer types

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Apache AH-64 helicopter, with exhaust mixer to reduce IR signal

Annular - bypass and core streams are brought together co-annularly, mixing being achieved by theshearing 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 lowpressure 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 longcoaxial 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.