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What is the difference between a cold-stream thrust reverser and a conventional one? It seems that all engines emit hot air, due to the combustion process.

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Turbofan engines don't push all air through the combustion chamber. Instead part of it runs along the sides.

from wikipedia

This bypass region is where the (cold) air stream is redirected from to create the reverse thrust.

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    $\begingroup$ Keep in mind that the "cold stream" isn't really cold. According to this article google.com.lb/patents/US5666802 it is about 100* Celsius (due to the heat of compression), whereas the hot stream is in the 500 - 600 deg. celsius range (due to the heat of combustion). $\endgroup$ – Skip Miller May 8 '14 at 15:56
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Not all air that flows through a turbofan engine undergoes combustion. In a turbofan, part of the air (in some engines, most of the air) is bypassed around the power turbine; that air is simply accelerated by the compressor fan and is vented, cold, out the back of the engine to provide thrust.

A cold-stream thrust reverser simply deflects this cold, bypassed air forward, whereas a "normal" thrust reverser deflects the main engine exhaust.

Wikipedia has some pictures and more description.

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    $\begingroup$ A "normal" thrust reverser deflects both of course, not just the hot stream. $\endgroup$ – Jan Hudec May 10 '14 at 0:33
  • $\begingroup$ @JanHudec: Unless, of course, your engine is a turbojet, which has only a hot stream. $\endgroup$ – Sean Jan 29 at 3:58
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Adding images to already existing answers.


Hot flow (primary flow)

These reversers use both air from the combustion in the core (purple) and air from the flow bypassing the core (blue):

enter image description here
(Source)

This design was used early on turbojets, and is still used on turbofans with a limited bypass ratio.


Cold flow (secondary or bypassed flow)

These ones use only the bypassed flow accelerated by the fan (the large rotor at the front of the engine, here in gray):

enter image description here
(Source)

Modern turbofan engines have large bypass ratios, e.g. 80% of air is bypassed. It makes sense to use reversers only on this flow. As the flow bypassing the core doesn't take part in the combustion process, it is known as the "cold flow".

(One door is open, the other is closed, this is for illustration only ; doors are always either all open or all closed).


Hybrid

Some engines had the two types together (perhaps the early Pratt & Whitney engine of the McDonnell Douglas C-17, but I'm not sure):

enter image description here
(Source)

This combination was used to get a powerful reverse thrust.


The nice colorful images have been taken from this site unfortunately only in French: L'avionnaire.

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