How exactly does a gas turbine engine produce thrust?

[Groobler]

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]

Well

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.

 

[RonL]

For anyone that might want to spend some study time and get some answers, this might be of use.
A 292 page pdf, but it has everything. (the picture above reminded me of the book it is in)

http://airspot.ru/book/file/485/166837_EB161_rolls_royce_the_jet_engine_fifth_edition_gazoturbinnyy_dviga.pdf

 

[Groobler]

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]

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 .

 

[Groobler]

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]

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]

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]

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 ?

 

[Groobler]

Nidium - Point 1 is fine.
Russ - Please start back a few posts, particularly from no.29.

 

[jack action]

Let's go with a simpler case: a ramjet, i.e. no turbines.

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.

If you refer to the previous image:

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]

(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 anyone 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 .

 

[russ_watters]

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.

 

[Groobler]

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]

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 .

 

[jack action]

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]

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?

 

[Groobler]

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?

 

[Nidum]

Animation

 

[CWatters]

Also explained here in para 4.3.2.3...

https://books.google.co.uk/books?id...rner reduces the pressure drop across&f=false

 

[jack action]

The afterburner runner is easy, but what component is the starting block?

What component (apart from a divergent nozzle section) transmits the afterburner's force to the airframe?

The starting block is the rear cone behind the turbine where the thrust 2419 lb is applied. Literally, it is the frame of the engine that is pushed. I don't understand why you say «apart from a divergent nozzle section». Isn't it good enough? To me, it's like saying: «In a car, apart from the wheels, what component transmits the force to the ground?» Nothing. There are only the wheels.

The turbine can not see additional backpressure and the cone may be too far upstream.

The turbine doesn't see additional back pressure. The beauty of the system is that the energy is instantaneously transformed into an increase in velocity instead of of an increase in pressure. It is the "kick" due to the acceleration that pushes the engine. The pressure wants to increase, but before it can happen, the gas particles are already gone, so it doesn't. But the force did exist, otherwise the acceleration wouldn't be there.

 

[Groobler]

Nidum - Thanks, that's a lovely animation, but please try to understand the question.
CWatter - Likewise, an interesting book (similar to others I've read), but again failing to address the question (at least in your identified paragraph). Briefly it raised hope by admitting that an increase in back pressure would cause the admission of more fuel in the combustors (at least with a modern control system) and hence increase the work of the compressor... before confirming our belief that this is avoided. So, back to square one.

 

[russ_watters]

Russ - The chimney was supposed to be another simple analogy. Is an afterburner anything more than a chimney under pressure with secondary burning?

Well, yeah, the chimney is open on both ends, but the afterburner is open one one end and has a jet engine attached to the other! Can you explain in more detail the behavior you see that you think is similar? Are you saying that lighting the afterburner should push air backwards out the front of the engine, just like the chimney?

The turbine can not see additional backpressure...

To expand on Jack's response: the turbine doesn't need to see additional backpressure. Thrust isn't a direct function of backpressure, it is a function of mass flow and velocity. Afterburners increase mass flow and velocity.

Please tell me if I'm being stupid.

Not stupid, but this is a very strange discussion, particularly given your credentials. You are getting simple answers from multiple people and it appears just declining to accept them. I'm not sure how much more help we can be.

 

[russ_watters]

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.

That image doesn't include an afterburner. Can we say:
In an engine with an afterburner that is off, the nozzle is more constricted and thus that 5,587lb "loss" is greater. When the nozzle is opened, that "loss" goes away, but more massflow and temperature (expansion) is needed to keep the backpressure from dropping when the nozzle is opened. That is provided by the afterburner.

[In reality they would be controlled together, but looking at them in that sequence may be instructive.]

 

[Groobler]

Thanks Russ, this may well be the best explanation, that the afterburner simply reduces nozzle losses. It's not very rewarding and makes the turbojet look rather inefficient. I would like to find an aero-engine designer to confirm it.

 

[RonL]

@Groobler If you go to this link and contact Gordon, I'm pretty sure he will be willing to help with your question.

http://www.revolutionjet.com/team/

Best wishes
RonL

 

[russ_watters]

Thanks Russ, this may well be the best explanation, that the afterburner simply reduces nozzle losses. It's not very rewarding and makes the turbojet look rather inefficient. I would like to find an aero-engine designer to confirm it.

I put "loss" in quotes because in term of energy and efficiency, it isn't really a loss. Static pressure in a fixed location does not consume energy. There is, of course, an energy loss across any real nozzle, and an unused afterburner makes an engine less efficient than not having one, but that force isn't it. Sorry for the confusion.

 

[Groobler]

Russ - Thanks again. By 'loss', I was thinking anyway of the negative thrust contribution at the nozzle location, rather than the overall pressure effect.
RonL - Thanks for the contact. I'll follow him up next week when I have more time. It seems that the jet engine is poorly understood outside its thermodynamic concepts.

 

[Nidum]

It seems that the jet engine is poorly understood outside its thermodynamic concepts.

Why do you keep saying this ?

 

[Groobler]

Perhaps it's just me, Nidum. I need to have another really good think about it. As I often find in science and engineering, the maths (in this case the thermodynamics) is not intuitive and a diagram of actual forces exposes the gaping holes in established explanations.