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Old Jul 2nd 2017, 04:55 AM   #1
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The Jet Engine

Good morning everyone,

I had several questions regarding basic thermodynamics. After going to Paris Airshow (Le Bourget), lot's of questions came to my mind after looking at those beautiful Jet engines...

I have always had a very basic understanding on these engines: a fan, a compressor (High and Low Pressure), a combustor and finally turbines (that will then turn a shaft which is connected with the compressors and the fan). Thrust is produced and explained using Newton's third law of motion. However lot of questions came as I read about the new CFM LEAP engine: high by pass ratio, state of the art jet engines.

First let's start by some basic questions:

1) What are target values we need to get at exhaust regarding temperature, pressure and velocity for a jet engine? So much happens inside the jet engine that it completely confuses me...pressure goes up in the compressor, then stays constant in the combustor to finally decrease inside the turbines. So what was the point to increase it? Well...lot of strange and basic questions for all of you I suppose! So my first question is just to get familiar with some approximate target values (Temp/Pressure/Velocity) at engine exhaust to produce thrust! What are we really aiming for?

2) Let's talk about the compressor...why do we compress air? Inside the compressor, temperature and pressure increase. However velocity decreases...but how do we decrease the air's velocity? I had also read compressor helps to push the air towards the exhaust (so that air does not start flowing in the opposite direction). Any information on that as well?

3) Why do we compress air and increase it's pressure? What would happen if I would send non compressed air inside the combustors? Why does combustor efficiency increase when air is compressed?

4) In the combustors, I am really not able to understand how Pressure is kept constant....adding fuel adds energy so does it not increase the pressure?

5) Now, this gas ' gives lot of energy to the turbines so that it turns the shaft and therefore turns the compressors and the fan. Is this the reason why pressure drops in the turbines?

6) Is it not better to have air at very high pressure during exhaust to get more thrust? In all diagrams I see, pressure simply drops in the turbines. Exhaust pressure is nearly the same as pressure in the inlet...

7) Does anyone have a formula that shows me that we need to increase airspeed inside the engines to produce more thrust? For example, for lift, we have L = 1/2pV^2SCz. We can immediately see that if I increase Velocity, lift increases. Something similar for engines?

8) Now comes the real question: why is a by pass Engine much more efficient? The only thing which is happening is that air is accelerated...but at a much higher rate than it would be after coming out from the turbines right? How is air being accelerated? The fan? The engine's shape?

9) How can a by pass Engine be more fuel efficient? Now less air is going inside the combustors but we need the fan to turn at the same rpm as before. In an older jet engine (no by pass), we would simply increase fuel to increase fan's rpm. So how do we make the fan turn at a high rpm in a high by pass Engine? Does it consume more fuel?

10) Finally, in a by pass Engine, does the by passed air produce more thrust or is it the fan itself (like a propeller, it increases airflow...just like an airfoil)? Both? Is the Newton's third law still applicable for this by passed air?

Sorry for these basic questions, I only have a very basic understanding on this subject. Please don't hesitate to explain using formulas, it really might help me

Again sorry for all these questions,

Thanks!
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Old Jul 2nd 2017, 01:30 PM   #2
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To understand how jet aircraft work it is necessary to take a step back and understand momentum.

In order to propel the aircraft fowards with a momentum of the mass of the aircraft times its velocity, sufficient air must move backwards to equal this momentum.

Mass of aircraft times forward velocity = backward volume flow rate of air times density

Jet engines are configured for this.
Thus there are two air parameters to play with, flow velocity and density.
Increasing (maximising) either or both will most easily achive the required momentum.

Increasing the pressure raises the density.
Burning the fuel creates expanding gas and generates or increases the flow rate.

This is a start to understanding.
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