How exactly does a gas turbine engine produce thrust?

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I need to correct or clarify my earlier post.

Most the thrust of most engines come from the fan.

But some of it does come from the exhaust, especially in the case of a turbojet that has no fan.

I will design a nozzle, but another group will determine what the shape of the nozzle should be on order to meet the intended performance spec of the engine. So I went to present this question to them. Thrust produced depends upon the shape of the nozzle, which is proprietary so I'll quit talking about that. But the thrust loads are transmitted through the nozzle structure.

The compressor disks pull forward on the shaft and the turbine disks pull aft. These are more or less balanced in most cases. But the lowest speed shaft is also driving the fan, which transmits large thrust loads to the thrust bearing.
 
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Most the thrust of most engines come from the fan.
For a large bypass commercial turbofan, yes. For a general gas turbine engine, no.

But some of it does come from the exhaust, especially in the case of a turbojet that has no fan.
Fan is just a term. Anything that imparts kinetic energy to a fluid stream is a compressor. However, there are many people who would contend this definition. For those who do, its just a definition, nothing more, a mere convention.


I will design a nozzle, but another group will determine what the shape of the nozzle should be on order to meet the intended performance spec of the engine. So I went to present this question to them. Thrust produced depends upon the shape of the nozzle, which is proprietary so I'll quit talking about that. But the thrust loads are transmitted through the nozzle structure.
Nozzle's shape is hardly a practical design criteria. I think I have stated this fact in this particular thread many times, nozzle is a device to match the characteristics of the turbine & compressor. Think of it as a pressure regulator, it regulates the work produced by the turbine.
Commercial engines do not even need a variable nozzle, as there operating conditions are constant for majority of the engine life.
Even for military class engines, nozzle's geometry is more or less fixed for a range of rotor speeds. Its main purpose is to open up when afterburner is switched on.

And for a fact, nozzle is a dragging component.
 
I was also trying to find the answer to the question posted by the OP. It led me to dig my training notes out and revisit some calculations. Anyway, not a diffinitive answer by any means but maybe a small insight may be found in the diagram below.

http://sphotos-h.ak.fbcdn.net/hphotos-ak-frc1/903359_10151583082451823_1971400927_o.jpg [Broken]
 
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How depressing (but not surprising) that after all these posts only one person managed to understand the question, let alone answer it! Perhaps the jet engine is an engineering enigma? This illustrates what is fundamentally wrong with education. The only thing a modern engineer can do is to throw theoretical mathematics at a problem; he, or she, can no longer visualize.
I'm afraid I too cannot answer the question; the above diagram (an Avon from RR's book) shows that the combustion chambers (though crude and draggy) act like a rocket and the cone like an aerospike. The rotor and stator blades make a net loss (the axial force on the turbine blades being more than a match for their fewer stages), and the 'propelling nozzle' is a complete misnomer (more a 'drag-limitation device'). So we are left with a combustion rocket complicated by the need to use low pressure atmospheric oxygen.
I was actually looking for an explanation of afterburning (i.e., like Ankit, what it reacts against) but it looks as if no-one on the internet (or any published work) will be able to explain it.
 
SteamKing
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I was actually looking for an explanation of afterburning (i.e., like Ankit, what it reacts against) but it looks as if no-one on the internet (or any published work) will be able to explain it.
What exactly is not clear about thrust augmentation in GT engines? You burn more fuel in the tailpipe which creates additional mass flow coming out the engine and increases the velocity of the exhaust relative to the engine. The difference in momentum of air coming into the engine and the momentum of the gases leaving the engine produces a net thrust, which drives the aircraft forward.

https://en.wikipedia.org/wiki/Afterburner

https://www.grc.nasa.gov/www/k-12/airplane/turbth.html

A jet engine creates thrust by heating and accelerating the air which it ingests thru the compressor section. The afterburner adds additional mass to the exhaust by burning raw fuel in the tail pipe after the turbine section.
 
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Thanks SteamKing but again you (and your links) have missed the point. I fear I will suffer the same frustrations as the chap who started this post.

Newton's 3rd law is simple to understand when applied to a released party balloon, or stepping off a skateboard, but where does the thrust of an afterburner act? If acting against the Turbine Exit Pressure (and its upstream surfaces) the turbine would slow and the compressor would surge. I'm sure the flame-holder can't be resisting 10,000 lb or so - in fact, by decelerating the air, I suspect its drag rises in full reheat! I used to think the solution lay entirely in the con-di nozzle of modern turbojets, until I found that older designs weren't equipped with this luxury.
 
SteamKing
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Thanks SteamKing but again you (and your links) have missed the point. I fear I will suffer the same frustrations as the chap who started this post.

Newton's 3rd law is simple to understand when applied to a released party balloon, or stepping off a skateboard, but where does the thrust of an afterburner act? If acting against the Turbine Exit Pressure (and its upstream surfaces) the turbine would slow and the compressor would surge. I'm sure the flame-holder can't be resisting 10,000 lb or so - in fact, by decelerating the air, I suspect its drag rises in full reheat! I used to think the solution lay entirely in the con-di nozzle of modern turbojets, until I found that older designs weren't equipped with this luxury.
I don't know what you mean by decelerating the air.

The whole point of the GT is to accelerate the flow through it, which is one reason it burns fuel. The gases created by the burning fuel are going to want to exit the engine that much faster, producing additional momentum and consequent thrust. The afterburner just makes additional gas come out the back end of the engine and more thrust.

You're going to have the same problem trying to figure out what a rocket pushes on when it's in outer space. Change in momentum producing thrust is an established concept. You just have to persuade yourself to accept it. But if you take the attitude that no one has explained or can explain something, that's a mighty large obstacle to understanding something yourself.
 
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RonL - thanks, I have this book but have not found the answer in it. Perhaps I need to study it again, although I'm fairly pessimistic.

SteamKing - I'm sorry for being frustrated, but if you see how unsuccessful the chap who started this post was. Many years ago I did an HND and then a degree in Aeronautical Engineering and came to the conclusion that, on many fundamental principles, very few people (often not even the lecturers) have a clue of how things actually work.
Firstly, on decelerating the air, if you read carefully I was simply referring to the flame holders.
Secondly, you have just repeated the basic concept of which we are all well aware. To make my question more obvious (and repeat my predecessor), what physical parts are taking the push (in my case for the afterburner, in his for the dry turbojet (answered partly by the RR Avon diagram))?
 
Nidum
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Most engines with reheat have variable area nozzles . Flow area is increased when reheat is in use . Increased flow area means that area of reaction surface for the exhaust gasses is greater .
 
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But what reaction surface? Tangential and forward-facing areas can only cause drag. The cone is possibly too far upstream and the rear turbine stages would slow if back pressure increased. Certainly the divergent part of a con-di nozzle acts as a rocket nozzle, but this does not feature on older and simpler afterburners.
 
russ_watters
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But what reaction surface?
There is no mystery here: The increased pressure in a gas turbine engine is generated by the compressor* and that pressure acts on all surfaces in the combustion chamber and to a lesser extent downstream of the turbine.

*and a turbofan of course also has a fan.
 
Nidum
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The cone is possibly too far upstream and the rear turbine stages would slow if back pressure increased.
There is no back pressure . The pressure distribution within the engine remains largely unchanged when reheat is engaged .
 
Nidum
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Let's start explanations at a more basic level and then work up .

(1) Air pressure rises through the compressor and then drops continuously all the way through the rest of the engine .

Ok with that ?
 
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Nidium - Point 1 is fine.
Russ - Please start back a few posts, particularly from no.29.
 
jack action
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Let's go with a simpler case: a ramjet, i.e. no turbines.

472px-Ramjet_operation.svg.png

Based on the following Brayton cycle, the pressure begins at atmospheric pressure (P1) and increases going up the cone reaching P2 at its widest part. The combustion chamber maintains the pressure at P2 (= P3), even though the area increases. The result is that the average pressure at the back of the cone is greater than the average pressure at the front of the cone, hence a forward thrust. So the cone takes all the thrust.

745px-Brayton_cycle.svg.png

If you refer to the previous image:

ge=http%3A%2F%2Fsphotos-h.ak.fbcdn.net%2Fhphotos-ak-frc1%2F903359_10151583082451823_1971400927_o.jpg


You see the same thing where 34 182 lb of thrust act in a similar fashion on the combustion chamber walls, 2 186 lb pushes on the diffuser, 19 049 lb on the compressor's blades (pressure is higher behind each subsequent stage). But the turbine takes 41 091 lb of rearward thrust on its blades to produce its required work (lower pressure after each turbine stage), and a thrust of 2 419 lb pushes forward on the exit cone and 5 587 lb pushes rearward on the convergent exit nozzle.
 
Nidum
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(2) In the compressor the air is forced in the aft direction and it's pressure rises as it passes through the sequence of compression stages .

In any one stage there is (usually) one row of moving vanes and one row of fixed vanes . Taking the two vane rows of a stage as a unit there is a lower pressure on the entry side than there is on the exit side . High pressure acting on the exit side vane area minus low pressure acting on the inlet side vane area = a net positive thrust .

Thrust is transmitted to the structure of the engine via the fixed vanes and the shaft thrust bearings .

(3) General idea is the same for turbines as it is for compressors except that pressure drops through the stages and net thrust is negative .

(4) In the combustion chamber addition of fuel produces heat which causes air to increase in volume . To pass this increased volume the exit nozzle has to be larger in area than the inlet nozzle . This means that there is a difference of reaction area between inlet side and exit side of combustion chamber . Same pressure acts on both areas so there is more thrust on the inlet reaction area than on the exit reaction area and a net positive thrust is produced .

The above may be somewhat greatly oversimplified but it is essentially what happens .
 
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russ_watters
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Nidium - Point 1 is fine.
Russ - Please start back a few posts, particularly from no.29.
I've read all of your posts. I see a lot of complaints, but not a lot of real substance, since this really is not a complicated issue. You don't seem to believe jet engines are well understood -- but obviously, they are well enough understood to build them!

Your specific question was about the afterburner, but you didn't actually say what your problem was. So I'll guess: are you thinking that the added combustion mass should increase the pressure in the combustion chamber and somehow choke off the engine? In afterburner, the exhaust nozzles open to enable that extra mass to flow with less restriction. I'm sure there is some change to the pressure profile in the engine, but the relationships between the sections stay the same (as they must for the fluid to flow from high to low pressure) and it can't change much since the afterburner is after the turbine. So in terms of the description of "how does a jet engine work?" Not much has changed.
 
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Jack - Nice repeat of previous information, although the example of the ramjet gives relief that I'm not barking up the wrong tree - as with a rocket you can see where the jet thrust is acting. All we have at the end of a jet engine is the cone and since the first German designs, that seems to be a rather neglected area with only a minor contribution. As I've said before, the turbojet appears to essentially be a combustion-chamber rocket with encumbrances at either end (even the rotor makes a net negative contribution and we can only conclude that the turbojet achieves superior efficiency by having a larger mass flow to fuel ratio than the rocket).
Nidum - (2) & (3) Fine, although the net contribution of the shaft is negative. (4) Yep, the combustion chamber is fully positive and therefore the winning component!
Russ - I joined this particular discussion because, although I've always understand the Avon diagram up to the cone, I have a great problem with the 'propelling nozzle' (a complete misnomer) and the addition of afterburning. I've phrased this to the best of my ability in my posts from 29 onwards, but let's use another analogy: If I inject fuel and ignite it halfway up a working chimney (without affecting draughting at the grate) would the chimney receive a downward thrust?
 
Nidum
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All we have at the end of a jet engine is the cone
The cone at rear of turbine and the engine casing form an annular divergent nozzle . This is exactly what is needed at this location . .

At minimum the actual ' thrust nozzle ' is just a plain parallel tube at the back end of the engine - many early engines were built this way and some still are .
 
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jack action
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Fine, although the net contribution of the shaft is negative.
You seem to suggest to remove the compressor and turbine because they produce a net negative thrust. Well, they're needed for the cycle to complete (at least at low speed), so it's a lesser of two evils.
I have a great problem with the 'propelling nozzle' (a complete misnomer) and the addition of afterburning.
The propelling nozzle increases the speed of the exhaust particles. Therefore, there is an acceleration. With acceleration, there must be a force involved somehow. That force is opposite to the acceleration. Look at it as if the gas particles are runners in a 100 m race: Both feet on the starting blocks, when the runner begins accelerating, the starting blocks take the thrust.

With an afterburner, by reheating a gas, you either increase the pressure or the velocity (i.e. there is an acceleration). In either case, it will push against the back of the engine.

I can now see how the image above might be confusing with the numbers shown for forward & rearward thrusts. I did not check them and I'm not enough of an expert in the field to comment them. But the propelling nozzle is what converts a gas turbine into a jet engine.
 
russ_watters
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I have a great problem with the 'propelling nozzle' (a complete misnomer) and the addition of afterburning... If I inject fuel and ignite it halfway up a working chimney (without affecting draughting at the grate) would the chimney receive a downward thrust?
No.

What does that have to do with your problem with afterburning?
 
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Nidum - Do you know how the cone's thrust contribution varies with afterburning?
Jack - Yes, of course I see the need for rotating blades, but only the combustion chamber receives the benefit.
Yes, indeed, we have a runner and a starting block. The afterburner runner is easy, but what component is the starting block? This is what I have been trying to ask! The turbine can not see additional backpressure and the cone may be too far upstream. By slowing the flow, the flame holders are probably still causing drag.
Incidentally, yes, a nozzle appears only to control the compressor via turbine backpressure. The only local beneficiary of this pressure can be the cone, since all parallel tubing and convergent sections add drag.
Russ - The chimney was supposed to be another simple analogy. Is an afterburner anything more than a chimney under pressure with secondary burning?

Please tell me if I'm being stupid. Here's another way to ask the question (one which my predecessor on this post used a few times): What component (apart from a divergent nozzle section) transmits the afterburner's force to the airframe?
 
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