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How does a jet-engine compressor compress air?

  1. Mar 12, 2007 #1
    ?????????????
    plz help me out folks
    plzzzzzzzzzzzzzzzzzz
     
    Last edited: Mar 13, 2007
  2. jcsd
  3. Mar 12, 2007 #2

    FredGarvin

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    It has various stages of rotating vanes. Air enters each stage and each stage incrementally compresses the air. The flow path the air takes gradually reduces in area as it makes its way to that latter stages. At the end of the compressor is a diffuser that slows the airstream down as well as further increases its static pressure.
     
  4. Mar 12, 2007 #3

    Astronuc

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    The rotating blades impart momentum and energy to the gas/air. Each stage adds a little more momentum/energy and gets 'pushed' into the air downstream. The opposite happens in the turbine where the heated gases impart momentum to the turbine blades. Jet engines also take advantage of the forward travel of the aircraft. In fact, ramjets simply use the high forward speed which 'rams' the air into the engine.

    See NASA's site on propulsion - http://www.grc.nasa.gov/WWW/K-12/airplane/turbine.html

    http://www.grc.nasa.gov/WWW/K-12/airplane/shortp.html
     
  5. Mar 13, 2007 #4
    Keep these four things in mind:

    1. Compressed air,high pressure, flows toward low pressure
    2. the compressor sections are in the front of the engine and the turbine in the rear
    3. the compressors are attached to the turbines via shafts
    4. Between the compressor and turbine sections is the combustion chamber where fuel is mixed with the compressed air and iginited.

    The compressor section is made up of alternating rows rotating blades and stationary vanes that are shaped like wings. Curved on one side and flat on the other and they function like wings too: Airflow creates low pressure on the curved side; high pressure on the flat. This imparts the direction of the airflow toward the rear of the engine: High pressure flows toward low pressure.

    The leading edges of these rotating blades face the leading edge of the stationary blades and as the air cascades through each pair of rotor and stationary vanes it becomes more and more compressed.

    The air enters the combustion chamber and is rapidly heated and wants to expand (but there is only so much room for expansion which is another form of compression) so it continues to the rear of the engine where the pressure is lower and it meets the turbines. The turbine are like pinwheels. The fast flowing, compressed air strikes the turbines and causes it(them) to spin.

    Add more fuel for more heat , the compressed air expands faster and the turbines spin faster. Because they are attached to the compressor section via shaft, the compressor spins faster too which compresses incoming air faster and even more.

    Air does not like to be compressed. It likes to be in its normal state so it roars past the turbines, out the back of the engine and blows the aircraft forward.
     
  6. Mar 13, 2007 #5

    Danger

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    I would expand upon that last sentence just a wee bit, since it's an area of confusion for a lot of people. The term 'blows the aircraft forward' is fine in casual conversation, for those who understand the physics involved, but it might give the wrong impression to some readers. (Remember the famous editorial scoffing at the space programme; they gleefully pointed out that a rocket couldn't possibly work in space because there was nothing for it to push against.) It's the reaction thrust of the exhaust against the engine itself that causes the propulsion, rather than the exhaust working on the outside air. Good post otherwise. :wink:
     
  7. Mar 13, 2007 #6

    FredGarvin

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    Air does not have a problem being compressed. It simply requires work to do it. Everything has a natural tendency to the dead state but that doesn't mean that there is a problem with doing it.

    Also, there is room for another mis-interpretation of what was said. The purpose of the turbines is solely to provide work to turn the compressor section. There is no thrust developed in the turbine section. The thrust of the engine is produced via Newton's 3rd law due to the acceleration imparted to the air stream and a slight P/a contribution at the exit plane of the rear nozzle.
     
  8. Mar 13, 2007 #7
    I'm sure you're right. But i guess the turbine should feel some sort of force when the gases are rushing out. And by problem i think Joe Mechanic meant that air has a tendency to oppose being compressed. And to state a fact, the air is in a compressed state even in the lower atmosphere. The higher atmosphere has a lower density and the air would have a tendency to rise upwards had gravity not been a factor.
     
  9. Mar 13, 2007 #8

    FredGarvin

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    There is a force created by the compressor section but it is restrained either by a static structure or a balance piston. That doesn't really enter the equation for the net thrust of the engine.
     
  10. Mar 13, 2007 #9
    Wow, you guys are great and correct! I didn't offer too much detail about thrust because the question was related to how the compressor worked. Also, I made the assumption that the question was being asked by a young student because of its brevity.


    I didn't mean to imply the turbines create thrust. Just that they are used to spin the compressor section via a shaft.
     
  11. Mar 14, 2007 #10
    wow

    i finally start to understand something. and joe i got ur point. thanks to u guys i learned a lot
     
    Last edited: Mar 14, 2007
  12. Mar 15, 2007 #11
    Gas turbine compressors only accelerate air. The pressure rise comes when the fast moving air is dumped into the larger volume combustion section. Its all about Bernoullis principle.
     
  13. Mar 15, 2007 #12

    FredGarvin

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    You only describe a diffuser or, in a way, a centrifugal compressor. Most engine applications, especially the large scale engines, use axial compressors that definitely compress the air after every stage of compression. When we develop a compressor, we measure the pressure ratio across each stage of compression. If you'll notice in any compressor cross section, the flow annulus decreases in size as you progress along the length of the axial compressor section. If your statement were true, the static pressure would actually decrease through the stages until you got to the diffuser. This is absolutely not what happens. The diffuser is a final stage of compression. It is not the only one though. On our engines we add a centrifugal compressor stage after the axial stages.
     
  14. Mar 15, 2007 #13

    Danger

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    I'm unfamiliar with that concept. Can you elaborate a bit? Having a bit of trouble trying to picture the blueprint in my head. :redface:
     
  15. Mar 15, 2007 #14

    FredGarvin

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    It is done on smaller engine designs. It doesn't work too terribly well for larger engines. The cost/benefit just isn't there to justify their useage. Basically a centrif is piggybacked at the end of the last axial stage. From the centrif, the flow goes to the burner.

    Here's a pretty good cut away of an example:
    http://www.ent.ohiou.edu/~me321/quiz.info/GasTurbine/compressor.jpg
     
  16. Mar 15, 2007 #15
    I forgot about axial flows getting a small rise at each stage. Its common to find centrifugal compressors on helicopter engines and APUs. The popular Allison 250 has a few stages of axial feeding a centrifugal. Centrifugal compressors require less power for starting.
     
  17. Mar 15, 2007 #16

    Danger

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    Cool picture, Fred. I'm still a bit confused, though. Dare I assume that this is something done with turboshaft engines as opposed to 'thrust' engines? I'm asking because it appears to me that the burner, and thus the exhaust, are at a right angle to the inlet flow. Is that the burner 'above' the centrifugal stage? (The shiny thing looks like it might be a fuel valve.)
     
  18. Mar 19, 2007 #17
    Search for a Rolls Royce "Nene". This is a typical thrust engine with a centrifugal compressor.
     
  19. Mar 19, 2007 #18
    hmmmmmm

    what are the other types of compressor and plz explain abut them buddys and tell me which one has the highest efficiency
     
  20. Mar 20, 2007 #19
    Thought Iwould add that large engines ejecting big,slow moving fuel-air mixtures are more efficient than smaller engines ejecting small,fast moving volumes of fuel-air.
     
  21. Mar 21, 2007 #20

    Danger

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    Thanks for the tip, Nicky. It makes sense now. I didn't realize that the airflow was realigned after compression.:redface:
     
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