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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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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). –  Skip Miller May 8 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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A "normal" thrust reverser deflects both of course, not just the hot stream. –  Jan Hudec May 10 at 0:33
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