Does cavitation cause increase in pressure?

Discussion in 'Chemistry' started by mahony123, Jul 27, 2011.

  1. mahony123 Registered Member

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    3
    Hi Guys,

    If water is in a sealed unit and some of the water is pumped out until it reached a certain negative pressure say-100kPa. At this point the water cavitates and air comes out of the water. what will happen the pressure as the air comes out?


    Thanks a million for any help
     
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  3. billvon Valued Senior Member

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    1) There's no such thing as negative pressure, only positive pressure. You can describe pressure as "amount of pressure less than atmospheric pressure" though. This is described as "gauge pressure." A gauge pressure of -100kPa will be about zero absolute pressure (i.e. vacuum.)

    2) Once the pressure drops to the boiling point of water, then the water will boil and steam will form. Thus the pressure will be maintained at that pressure until all the water is gone. At that point the pressure can drop to zero.
     
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  5. Read-Only Valued Senior Member

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    That's all correct. However, as the cavitation bubble collapses (water rushes in to fill the temporary void) pressure indeed IS increased in a very tiny area. - which I think is what the OP is asking about.

    That's precisely the effect which is behind the erosion of ship's propellers at high RPM and is also the basis for ultrasonic cleaners.
     
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  7. origin Heading towards oblivion Valued Senior Member

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    11,883
    That is not what caviation is. As Read-Only pointed out it is generally involved with a propeller on a ship, pump ectc. If the inlet pressure is low enough then a pump may sound lkle it is 'pumping gravel'. The noise is the formation of vapor bubbles (steam) and the subsequent collapse of the bubbles against the impeller due to the high pressure at the face of the impeller blade. This is classic caviation.

    In your scenario the the pressure will rapidly drop (real fast!) until the the pressure is low enough that the water begins to turn to steam at that point the pressure will remain constant until all of the water has been converted to steam. Once all of the water is converted to steam the pressure will again start to drop (assuming your evacuation equipment can remove steam as well as water).

    If this is a homework question I deserve some of the points!
     
  8. Read-Only Valued Senior Member

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    Excellent post.

    Please Register or Log in to view the hidden image!

     
  9. Trippy ALEA IACTA EST Staff Member

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    Cavitation damage in a pump:

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    This image:

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    Is from this website: http://www.nmri.go.jp/fluids/topics/highspeedcamera/english/exphighspeedcamera.html
    Which has some cool videos I intend to check out when I get home, and finally:
    Cavitation on Wikipedia
    I did not know that cavitation involved the formation of jets.
    In relation to the OP, the wikipedia article talks about inducing cavitation using sound waves (or sound half waves) so the OP almost got it right. It even seems to suggest that if you take the setup described in the OP - water with undisolved air bubbles, and lower the pressure rapidly enough, that cavitation will occur within the liquid.
     
  10. arfa brane call me arf Valued Senior Member

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    7,832
    Sorry, but that isn't true.
    In stars the positive pressure due to fusion energy is countered by the negative pressure of gravity (negative mass energy? it's a momentum product at least).

    I know that probably has bugger all to do with cavitation in fluids, but I like being pedantic.
     
  11. leopold Valued Senior Member

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    if we take a perfect vacuum at 0 psi then how can you get anything less than zero?

    edit:
    ah, you are confusing the terms "absolute" and "gauge". ( i assume you are anyway. )
     
    Last edited: Jul 28, 2011
  12. origin Heading towards oblivion Valued Senior Member

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    11,883
    Thanks!
     
  13. Bravowon Registered Senior Member

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    54
    The question could also be refering to a deaerator device use on sealed systems.

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

    In my experience, removing air will have the opposite effect to adding air. The pressure in the sealed system will reduce - but I'm not expert.
     
  14. veggiepatch Registered Senior Member

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    37
    Try the physics section...not chemistry.
     
  15. mahony123 Registered Member

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    3
    Thanks for all your replys, they are excellent.

    Sorry for the delay in getting back

    I am a geotechnical engineer and it is a university project

    The pressure does not probably reach low enough pressure to boil. I believe air is released from crevices between the water and the apparatus. The air is coming from these crevices and forming a small pocket at the top of the apparatus. The pressure is lowest at this point at the top due to the hydrostatic pressure at the bottom of the apparatus.

    I believe now that no pressure change will occur throughout this process as I dont see a reason why it would.

    Finally I am very confused on the idea of outgassing. Would this be possible where Oxygen, Nitrogen, Carbon Dioxide could possible be outgassed form the water?

    Thanks and I apologise for my lack of proper understanding, it is at the end of the day why I'm asking you guys
     
  16. origin Heading towards oblivion Valued Senior Member

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    11,883
    If there is air in the water, then lowering the pressure will cause the air to come out of solution.

    A rather painful example is the 'bends'. If a diver were to got to 100ft under water the pressure would be about 3 bar. The air that he was breathing would be at 3 bar also. Now if he were to swim to the surface and get out onto a boat the lower pressure (1 bar) would allow the outgassing of air into his precious bodily fluids. The bubbles tend to collect in the joints causing extreme pain.
     

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