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Mechanism of fan

  1. Dec 17, 2009 #1
    hi can any body explain for me why some fans blow the air and some suck the air?.I know it depends on the form of its wing, but how?
     
  2. jcsd
  3. Dec 17, 2009 #2

    FredGarvin

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    Short/easy answer: One side of the wing/blade has high pressure, the other side has low pressure. Air gets pulled in on the low pressure side and expelled on the high side.
     
  4. Dec 17, 2009 #3

    Danger

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    To elaborate slightly regarding Fred's post:
    All fans do both. Whether you perceive one as sucking or blowing is entirely dependent upon your position in relation to it.
     
  5. Dec 17, 2009 #4
    I understood that blowing and suction depends on which side is low pressure and which is high pressure and thus we have blowing and suction is that right?
    but there is another question and that is how blades have designed that can product(please tell me a better word)different pressure in each side
     
  6. Dec 17, 2009 #5

    Danger

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    All moving surfaces displace air. If that surface is angled, the displaced air tends to go in a particular direction. That is essentially all that a fan is and does. You can make a simple one just by taping some cardboard together.
    If you're asking about specific blade design, that is an extremely complex subject. Russ might be able to give you some guidance, since he seems to field most airflow related questions, but it will of necessity be limited in scope.
     
  7. Dec 17, 2009 #6

    rcgldr

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    Its simply a matter of angle of attack and forward speed that ends up deflecting the air. You can take a flat board, angle it a bit upwards relative to it's velocity, and it will deflect the air downwards. The bottom surface of the board simply deflects the air downwards. As the upper surface of the board moves through the air, its downwards movment relative to the air causes the air to follow what would otherwise become a void (vacuum). Wiki's wording on this:

    In that case a low pressure region is generated on the upper surface of the wing which draws the air above the wing downwards towards what would otherwise be a void after the wing had passed.

    http://en.wikipedia.org/wiki/Wing

    So the forwards moving flat board ends up with low pressure above and high pressure below, coexisting with the downwards acceleration of air, drawing it downwards from above, pushing it downwards from below. The air senses this pressure differential at the speed of sound, so just before the board arrives through a volume of air, that air is initially accelerates upwards towards that low pressure area above the flat board, then is accelerated downwards as the board passes by, with a net downwards acceleration, corresponding to lift (and some forwards acceleration, corresponding to drag).

    To reduce the drag, the leading edge of that flat board can be curved downwards to somewhat follow the expected flow that is initially upwards, and then downwards as it flows accross the board. Next it's noticed that the downwards acceleration of air is in effect colliding with the trailing half of the board, so it's also curved downwards a bit, again to somewhat follow the expected flow of air. The result is a curved board that produces the same amount of lift but with less drag than the flat board. This is a crude description of the design of thin airfoils.

    To further reduce drag, the aero guys know that tear drop like shapes have low drag, so efficient wings will end up looking like elongated tear drops that are curved downwards, and it turns out that these somewhat thicker tear drop based airfoils will produce the same lift with even less drag than thin airfoils.
     
  8. Dec 18, 2009 #7
    thank you
     
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