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Geared turbofans for large engines

  1. Nov 29, 2007 #1

    Astronuc

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    Source: CBS MarketWatch

    Interesting developments! Certainly - if this works - it will dramatically reduce fuel consumption.

    "1,500 fewer airfoils" seems awfully large. But then I haven't counted the blades in current turbine design. I curious as to what the typical blade number is per stage.
     
  2. jcsd
  3. Nov 29, 2007 #2
    Some pictures and diagrams in this SAE article on it.

    Link:
    http://www.sae.org/aeromag/techinnovations/1298t10.htm

    And here:
    http://www.flightglobal.com/article...pratt-whitneys-obsession-with-the-geared.html

    My favorite image:
    http://www.flug-revue.rotor.com/FRHeft/FRHeft07/FRH0702/FR0702c.htm
    At six and a half feet dia., I thought that is slim by today's standards?
     

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    Last edited: Nov 29, 2007
  4. Nov 29, 2007 #3

    Astronuc

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    Thanks, kach22i. Interesting picture. The PW.UTC does not list the PW8000 and only refers to the GT (geared turbine) qualitatively.

    In the picture, the fan and HP compressor rotates in the opposite direction of the LP compressor.
     
  5. Nov 29, 2007 #4
  6. Nov 30, 2007 #5
  7. Nov 30, 2007 #6

    FredGarvin

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    CONGRATS TO ASTRONUC!!!!!

    The newest PF Mentor.

    Congrats! It's well deserved.
     
  8. Dec 18, 2007 #7

    Astronuc

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    Things are moving along.
     
  9. Dec 18, 2007 #8

    mgb_phys

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    Don't they already have multiple shaft engines alowing the different stages to run at different speeds?

    ps. that isn't Rolls-Royce's site, is this a freely available publication or something 'obtained' from RR?
     
    Last edited: Dec 18, 2007
  10. Dec 18, 2007 #9

    FredGarvin

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    Indeed they do. However, the speeds are matched based on aerodynamic and thermodynamic requirements of the components. The kind of speed variations they are talking about necessitate the inclusion of a step down gear assembly. They would never get the speed matching they need to turn the fan as slow as they need to.
     
  11. Dec 18, 2007 #10

    Astronuc

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    My thinking/conjecture -

    I haven't really looked into this, but it seems that one can signficantly vary the proportions of thrust given by the engine exhaust and by-pass fan. More thrust is desirable from the by-pass fan at low altitudes (and take-off and landing) where noise is an issue, as opposed to high altitude where a greater proportion of thrust can come from the jet exhaust.

    I think the multiple shaft engines affect the relationship between turbine and compressor, and the by-pass fan is integrated to leading compressor stages.

    Correct me if I'm wrong.
     
  12. Dec 18, 2007 #11

    mgb_phys

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    Am I right in thinking that on a high bypass engine most of the thrust comes form the front stage?
    The limit on making the fan bigger is stopping it dragging on the ground.
    With this sort of design could you have pairs of fans side-side driven by a single turbine?
     
  13. Dec 18, 2007 #12

    FredGarvin

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    You are correct that the majority of the thrust comes from the fan. The limits of size are clearance, as you stated, as well as aerodynamic. The big boys don't want their fan tips going supersonic. We have fan tip speeds that are very high and it brings about a lot of challenges, noise being one of them. Those guys have a much better time with a really big fan that would have slower speeds. A big fan puts out a lot of air.

    When you say "side-side" do you mean two fans in parallel or in series?
     
  14. Dec 18, 2007 #13

    mgb_phys

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    I meant two smaller fans side by side in parallel with only one having a turbine.
    (Looking like the twin engine nacelle on something like a B52)

    I imagine it's a headache to design an engine to run at high thrust in thick air at takeoff but also cruise efficiently at high altitude. Presumably this design could allow you to change the pressures between the fan and compressor to get a more efficient envolope for different parts of the flight.

    ps. Congratulations on the guru award.
     
  15. Dec 18, 2007 #14

    FredGarvin

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    Side by side would be one heck of a big, heavy engine. But I guess since they are playing around with gearboxes for the fans, why not? The design is really based around expected altitude cruise. That is where the majority of the time is spent, so the most dollars operation wise can be saved. Of course, at altitude thrust goes down, but so does all of the bad aspects as well. Altitude is usually your friend. Of course, you do need that take off power for Denver on a hot day.

    Wholley molley. I didn't refresh my browser. Thanks. I wouldn't have noticed it unless you pointed it out.
     
    Last edited: Dec 18, 2007
  16. Dec 18, 2007 #15

    FredGarvin

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    You're spot on. The only thing is that, at altitude, you only really would want more of a turbojet engine if you were interested in high speed. That's not a bad assumption considering everyone would want to get to their destination that much quicker. However, all things being equal, I can't see a real benefit that would outweigh the complexity of the engine to change bypass ratios in flight like that.
     
  17. Jan 26, 2008 #16
    I don't like engines to close together as a consumer because failure of one can easily transfer to the other (Concorde)
     
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