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Axial-compressor jet engines fall into two categories (and only two as far as I'm aware): turbofan and turbojet. I'm wondering if centrifugal-compressor jets also have such a distinction or something similar.

I cannot imagine how a big rotating fan-like object could both compress air centrifugally and bypass it out the rear, but I would not be surprised if some engineer found a clever way of doing it. And anyway, a centrifugal jet could be separated into two broad types based on something slightly different.

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  • $\begingroup$ I don't understand the question. Are you only interested in the bypassed air and asking if there are engines with a centrifugal fan (whatever the core solution) where the cold flow is accelerated by a centrifugal solution? $\endgroup$ – mins Sep 2 '17 at 8:46
  • $\begingroup$ @mins It's more about bypassed air, whether or not there's a fan doing it. Would be interesting to see if the centrifugal compressor could do that on its own. Centrifgual compressor jet engine with relatively high bypass ratio, 2:1 or more, if such a thing exists. $\endgroup$ – DrZ214 Sep 2 '17 at 21:13
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Yes, but the classification is not a matrix.

A turbofan by definition needs a fan, but at its core it can have a centrifugal compressor. A good example is the Pratt & Whitney Canada PW600 series. This allows a compact design, at the expense of compression ratio. It works well for low-thrust applications such as the Cessna Mustang. The advantages are the engine being small, lightweight, and having fewer parts.

enter image description here
(Google Patents) The arrows point at the two-stage compressor. The first stage is a mixed flow compressor rotor, and the second stage is a centrifugal compressor. Label 9 is the fan up ahead.

The PW617F provides a whopping 7.18 kN (1,615 lbf) of thrust and weighs under 180 kg. (The fan is slightly bigger than a human hand.)

You can watch a moving cutaway model for the JT15D turbofan with a centrifugal HPC here.

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  • $\begingroup$ Does all the air from the centrifugal compressors go into the core combustion chamber? Sorry it's hard to follow the diagram, but I see a snakey path that leads me to believe the answer is yes. $\endgroup$ – DrZ214 Sep 2 '17 at 21:15
  • $\begingroup$ @DrZ214 - Correct, check this image, you'll see centrifugal compressors are not suited for bypass. $\endgroup$ – ymb1 Sep 3 '17 at 2:05
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The fan in a turbofan engine is nothing but an air compressor, and does exactly the same as a centrifugal compressor. The axial arrangement in a turbofan allows for neat separation of bypass flow and hot flow, and this is the only (relatively simple) engineering problem that would have to be solved in a centrifugal compressor engine. Then, just blow some of the compressed air out into the atmosphere, via a suitable exhaust shape.

Axial engines allow for straight through flow, without having to bend it around corners with associated losses. They are more compact though, and therefore mostly found in turboshaft applications where jet exhaust power is negligible.

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