How exactly does a gas turbine engine produce thrust?

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  • #51
jack action
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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.
 
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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.
 
  • #53
russ_watters
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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.
 
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  • #54
russ_watters
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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.
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.]
 
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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.
 
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russ_watters
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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.
 
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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.
 
  • #59
Nidum
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It seems that the jet engine is poorly understood outside its thermodynamic concepts.
Why do you keep saying this ?
 
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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.
 
  • #62
jtbell
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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!
Isn't it kind of late to be whinging about the replies to a four-year-old question? :smile:

(oops, I missed Dale's post.)
 
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russ_watters
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The topic has been adequately covered. The thread will remain closed.
 
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