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Using the convergent /divergent duct theory the low pressure cold stream bypass air would travel faster but I read on a web page that the bypass air travels much slower than core air. But again using the convergent / divergent duct theory core air is subject to higher pressures and temperatures therefore should theoretically travel slower?

Or is it because air is mixed with fuel and accelerated during combustion which makes the core speed faster and Bernoulli converging / diverging duct theory I stated above doesn't apply?

Hopefully someone can tell me if I'm thinking this incorrectly... Simple explanation please and not something over complicated.

Thanks!

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The simple way to think of it is the bypass engine is kind of part way between a pure jet and a turboprop. In a nutshell, the pure jet moves a little bit of air a lot; the turboprop's propeller moves a lot of air a little, relatively. In other words, small mass at high velocity vs large mass at low velocity, to get the same push.

A bypass jet engine is somewhere in between. It's just a fixed pitch non-geared turboprop with 30 or 40 blades, in a shroud, instead of 3 or 4 variable pitch blades in the open (geared turbofans get even closer to the "fixed pitch turboprop" thing).

The package of air being accelerated is smaller than a propeller, but much larger than a pure jet, so fan's exhaust velocity is also in between; faster than the wash of a propeller, slower than the exhaust of the core.

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    $\begingroup$ 30 - 40 might rather be 20 - 30/40. GE90-fan, for example, is 24, if I'm not mistaken. $\endgroup$ – Bram Aug 17 at 22:07
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    $\begingroup$ @Bram GE90 fan has 22 fan blades. $\endgroup$ – J.Torre Aug 19 at 1:54
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Bernoulli’s principle, as we refer to it today, states that “as the velocity of a fluid increases, the static pressure of that fluid will decrease, provided there is no energy added or energy taken away.”

(source)

As that assumption is invalidated (energy is transferred, precisely because the air is sped up), Bernoulli's principle doesn't apply. That's as simple as it gets.

A little more detail: when the fuel-air mixture is burned inside a gas turbine, it expands and drives one or more turbines. These are connected to one or more compressors, which pressurize the air that is fed to the combustion chamber. (This is why a jet engine cannot start itself.) The shaft that drives the compressors also drives the fan, which not only does some compression on the air that goes to the core, but also accelerates the air that goes past the core, through the bypass. Nevertheless - and per what you've found online - that air travels much slower than the jet exhaust, as it sees much less compression than the core air. The schematic below illustrates this.

enter image description here

This answer (from where I copied the schematic above) about what part of the engine generates most trust also discusses speeds.

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Over most of the engine length, the bypass air travels at the same speed but is much faster than the core air entering the combustor

Let's assume a typical fan engine like the CFM56 pictured below. It starts with an intake which helps to equal flow speed regardless of aircraft speed. The first stage is common for both the core and the bypass stream and increases air pressure without accelerating the air. Next, the flow is split into the core and the bypass stream. The core stream is compressed further and again the pressure rises while speed stays roughly constant. Before entering the combustor, the air is slowed down substantially so combustion is actually possible within the engine.

Cutaway view of a CFM56 fan engine

Cutaway view of a CFM56 fan engine (picture source)

In contrast to that, the bypass stream continues at roughly the speed at which it entered the fan. This is illustrated by the constant cross section of the bypass duct in the picture above. Only at its end does the duct narrow in order to convert the air pressure into speed. Doing this any earlier would mean higher friction losses.

In contrast to that, the core air is slowed down in the diffusor and picks up speed again as it is heated in the combustor. With the higher speed of sound of the heated air, it now becomes faster than the bypass air as it travels through the turbine stages. In the nozzle it is accelerated again in order to convert the remaining pressure into speed.

Therefore, the gas stream leaving the engine core is much faster than the bypass stream, but has gone through a deceleration and then an acceleration while the bypass stream never got decelerated.

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The convergent/divergent duct theory that you refer to only applies if there is no energy added. But by definition, this is what an engine does.

Adding energy to the airstream can result in higher pressure or velocity, and this is indeed influenced by the shape of the flow channel. But the hot stream has an internal energy that is seriously higher than that of the bypass flow.

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