Why does a turbofan engine force bypass air into a smaller space?

Question

I asked a question about this same concept recently but this question involves the function of the bypass air.

In this YouTube video around 1 minute in the narrator says that the bypass air is pushed by the fan into a smaller and smaller space which increases the velocity of it, but why? Why increase the exit velocity of the air beyond what the fan already does? It can't increase thrust, can it? That would seem to violate some sort of conservation law since you could theorize an infinitely narrowing space and thrust would approach infinity. So what function does it actually serve to force air into an 'ever narrowing space'?

The fan adds energy to the flow in the form of a pressure increase. It is a compressor stage, after all. That pressure is then converted into speed in the nozzle so the flow will (ideally) leave the engine at ambient pressure. The function of the narrowing space is to convert potential energy (pressure) into kinetic energy (speed) which is the basis of thrust creation. — Peter Kämpf, Mar 10 '18 at 20:08
Well if you ask why not let the fan accelerate the flow directly, the answer is, the fan doesn't work very well in a fast flow, but very well in a high pressure flow. So when the flow goes through the fans, it by design does not accelerate much, but increases in pressure a lot. Then when the flow reaches the nozzle, the nozzle converts pressure to velocity. Remember that the flow at inlet is already not far from sonic speed so you really don't want it to go any faster or the fan tips goes supersonic very easily. — user3528438, Mar 11 '18 at 04:28
@user3528438 So a propeller on a prop plane is much better at increasing the speed of the air going through right? So a follow up question to this is, which would give more thrust at low speeds: a simple propeller or a ducted fan with a nozzle smaller than the output? Considering that they are the same size and run at their optimal rpm. — TrapAlcubierreDrive, Mar 12 '18 at 17:15

score 4 · Accepted Answer · 2018-03-12T18:30:09.017

It can't increase thrust, can it?

It does increase the thrust. This is how jet propulsion works. The decreasing space is essentially a nozzle, without one the jet engine is a mere gas turbine.

You could theorize an infinitely narrowing space and thrust would approach infinity.

Not really, a very tight opening will create a huge back pressure, initially and very momentarily propelling the vehicle the opposite direction, before the gases shoot out the front stalling the engine (gases building up).

In fluid dynamics that would be a choked flow state. The mass flow out the nozzle reaches the maximum, which means inside the engine the gases will start building up (pressure increasing). If the theoretical tiny nozzle can withstand this pressure, the gases will only have one way to go, which is out the front.

So instead the nozzle is designed to decrease the pressure (by increasing the velocity) to match the free stream pressure as much as possible.

The nozzle of a jet engine is usually designed to make the exit pressure equal to free stream (NASA).

I see, thank you. Can you expand on the concept of back pressure created from too small of an exit? What is the ideal ratio between size of intake and size of exit nozzle? To keep it simple, let's just consider it as a ducted fan so we don't have to worry about the internals of the jet engine — TrapAlcubierreDrive, Mar 12 '18 at 17:10
@TrapAlcubierreDrive - You're welcome. I have expanded on that part. As for the second question, it's better to ask a new question about the factors that affect the nozzle design. — , Mar 12 '18 at 18:25

Why does a turbofan engine force bypass air into a smaller space?

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