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In aviation, exhaust mixer is a feature of many
The mixer reduces the
File:Trent 700 - DSC 8123-F-WWCR - MSN 1462 (10513329855) (cropped).jpg, The Trent 700 which powers the
*Annular - bypass and core streams are brought together co-annularly, mixing being achieved by the
In aviation, exhaust mixer is a feature of many turbofan engine
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which achieves mechanic ...
s, where the bypass (cold/slow) air is mixed with the core (hot/fast) exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
es, before exhausting to atmospheric pressure through a common (mixed flow) propelling nozzle.
Benefits
The mixer reduces the velocity
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
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 or heat-seeking
Infrared homing is a passive weapon guidance system which uses the infrared (IR) light emission from a target to track and follow it seamlessly. Missiles which use infrared seeking are often referred to as "heat-seekers" since infrared is rad ...
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, i.e., ''Thrust = ṁv'', while the energy to make that thrust is given by ''Energy'' = 1/2''mv''2. 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.
Airbus A330
The Airbus A330 is a wide-body aircraft developed and produced by Airbus.
Airbus conceived several derivatives of the A300, its first airliner in the mid-1970s. Then the company began development on the A330 twinjet in parallel with the A340 ...
, has a common nozzle assembly which mixes the cold and hot flows.Lufthans Technical Training,Training Manual Airbus A330,ATA71-80 Engine RR Trent 700,page 30
File:Airbus A330-200F(F-WWYE) (4337113707).jpg, The PW4000
The Pratt & Whitney PW4000 is a family of dual-spool, axial-flow, high-bypass turbofan aircraft engines produced by Pratt & Whitney as the successor to the JT9D.
It was first run in April 1984, was FAA certified in July 1986, and was introdu ...
has separate exhaust nozzles
Mixer types
*Annular - bypass and core streams are brought together co-annularly, mixing being achieved by the shearing
Sheep shearing is the process by which the woollen fleece of a sheep is cut off. The person who removes the sheep's wool is called a '' shearer''. Typically each adult sheep is shorn once each year (a sheep may be said to have been "shorn" or ...
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
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
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
In geometry, coaxial means that several three-dimensional linear or planar forms share a common axis. The two-dimensional analog is ''concentric''.
Common examples:
A coaxial cable is a three-dimensional linear structure. It has a wire condu ...
exhaust pipes (i.e. hot and cold) are avoided. Afterburning
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat ...
, 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.
See also
* Chevron (aeronautics)References
{{Reflist *http://www.fluent.com/solutions/examples/x178.htm Gas turbines