I would like to know how laminar/turbulent flow affects engine performance. Any additional thoughts on the topic, even if not strictly related to the main question, are welcomed.

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    $\begingroup$ Laminar. Turbulence is bad if you want the engine to keep spinning. $\endgroup$ – Lnafziger Feb 13 '16 at 15:15
  • $\begingroup$ @Lnafziger Could you explain how it exactly affects the engine? $\endgroup$ – user15037 Feb 13 '16 at 15:48
  • $\begingroup$ Thanks. I wanted a deeper answer because I was wondering if turbulent flow could help in the combustion process, not only in compression. $\endgroup$ – user15037 Feb 13 '16 at 16:59
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    $\begingroup$ Yes, induced swirl or turbulent flow can be helpful in the combustion section for a variety of reasons. If focusing on the combustion section is your intent in asking the question, you might find it helpful to specify that in your question. $\endgroup$ – J Walters Feb 13 '16 at 17:05
  • $\begingroup$ Unfortunately, I can't in a technical manner for this particular issue, but there are other people here that have more experience with his that will be able to help you! (that's also why I only posted a comment rather than an actual answer.) $\endgroup$ – Lnafziger Feb 13 '16 at 17:33

The noise within the engine will make all flow turbulent. There is not much liberty for the engine designer to choose the type of boundary layer.

Generally, a laminar boundary layer is preferable as long as the flow is accelerated, because viscous drag will be lower, and a turbulent one is better when it is decelerated, so separation can be delayed.

Outside of boundary layer phenomena, turbulence is helpful for keeping the combustion process going in the combustor and especially in the afterburner. Ahead of the combustor all engines have a diffusor to slow down the compressed air, so it has time to mix with the fuel and burn as much of it as possible. Note that the diffusor only slows down the axial movement of the air, so radial velocities will not be affected and be relatively larger relative to the axial flow at the entry of the combustion chamber. This makes the flow more turbulent, which is helpful for better fuel-air mixing.

In the afterburner, flame holders are needed to produce pockets of slow-moving, hot gas which can ignite the fuel-air mixture. Again, they cause more turbulence.

Flame holder

Flame holder (10) with annular selectors (9) of an afterburner (picture source)

At the nozzle, turbulent mixing of the hot core flow and the bypass flow reduces noise. Engine nozzles use a variety of means to speed up the mixing process.


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