Thermodynamics: The Jet Engine

[TheBusFlyer]

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!
 

Edited July 2, 2017 by TheBusFlyer

[OceanBreeze]

Have you tried searching for the information you want? There is a wealth of information out there that will answer your most basic questions. After reading up on it, if you still have some specific questions then by all means come back and post them; I am sure we can help with some answers.

[exchemist]

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!

 

Indeed far too much for a single answer but on your first point, surely the principle is that the volume of hot gas expanding through through the turbine stages is far greater than the volume of air compressed in the inlet stages and so the turbine stages are arranged to let this quantity of gas accelerate out of the exhaust at high speed, extracting only enough work from the gas stream to compress the inlet air (and to run any bypass air stream.  

 

The air is compressed in order that there is enough air to burn a large amount of fuel, thereby producing as much hot gas for this process as possible.  

[OceanBreeze]

Indeed far too much for a single answer but on your first point, surely the principle is that the volume of hot gas expanding through through the turbine stages is far greater than the volume of air compressed in the inlet stages and so the turbine stages are arranged to let this quantity of gas accelerate out of the exhaust at high speed, extracting only enough work from the gas stream to compress the inlet air (and to run any bypass air stream.  

 

The air is compressed in order that there is enough air to burn a large amount of fuel, thereby producing as much hot gas for this process as possible.  

 

 

 

Yes, that is basically it. In brief, compressed air contains more energy than uncompressed air.

 

The compressed air is hotter, so it contains heat energy and so is closer to the combustion point of the fuel-air mixture.

 

Compressed air contains a much higher amount of oxygen per volume, and oxygen is needed for combustion of the fuel; the more oxygen the hotter the combustion = more energy.

 

By compressing the air, the molecules are closer together, so combustion interaction can happen more quickly and efficiently.

 

Finally, compressed air “wants” to expand again, and that is also a source of energy.

 

I should add that I wasn’t trying to brush off the questions, as this can be an interesting discussion. It is just true that most of the more basic questions can be answered with a quick search and then we can discuss the most interesting ones in detail, if the OP desires.

[billvon]

 

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?

Efficiency.  You get more energy out of compressed air than you get out of uncompressed air.  You could in theory do away with the compressor if you had a one way valve at the entrance to prevent combustion products from exiting the front - and in fact this is how pulse jets work.

 

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?

 

You burn fuel and increase the temperature of the gas.  That either increases volume or pressure per the ideal gas law - and both of those give you thrust.

 

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?

 

Pressure drops because energy is being removed from the exhaust stream.

 

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...

 

In a pure turbojet, yes, you want a high exhaust velocity.  (Which is how afterburners work BTW.)  But most jet engines today are fanjets, and the fan generates most of the thrust.  (Also high exhaust velocities are incredibly loud.)

 

 

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?

 

It is more efficient to accelerate a large mass of air by a small amount than a small mass of air by a large amount.  (Drag alone would be sufficient to explain this.)

 

 

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?

 

The two are the same.  Fans generate thrust by forcing air backwards.