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Viscosity and pressure

  1. May 24, 2015 #1
    I made a simple diagram. My question is, in the picture, can the hydraulic resistance of the tube connecting the fluid to the vacuum chamber prevent boiling of the fluid until it reaches the nozzle? viscous.png
  2. jcsd
  3. May 24, 2015 #2
    So you're asking if the tube can keep the fluid from flowing/boiling away?

    I'd say any vapor pressure of the fluid would be enough to evaporate it all against a vacuum, over time.

    If the resistance were too high, it would just flow out more slowly, no?
  4. May 24, 2015 #3


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    I assume here that your fluid must be a liquid since it is boiling (make sure you are careful with the term fluid; gases and plasmas are fluids, too). That said, if your fluid from the tank stretches all the way up to the nozzle already such that the only surface exposed to vacuum is the nozzle and that condition was maintained, then I don't see why you would observe boiling all the way back in the tank. You would probably observe boiling only as deep as it takes before hydrostatic pressure is greater than the vapor pressure. I don't see fluid resistance as being relative here. If the fluid is just sitting in the tank with vacuum in that entire tube before it, then yes, I would imagine the tank liquid would boil.
  5. May 24, 2015 #4
    What you are describing is one of the 4 elements required in a heat pump or air conditioner--the expansion valve. "Expansion valve" misnamed. It's a restriction in the circuit as you've depicted in your drawing. The low pressure side is not a good vacuum, but it's fairly close to your schematic.

  6. May 24, 2015 #5
    The fluid is viscous and through heating and the establishment of a vacuum, it finally reaches the nozzle. I am trying to understand if I can prevent boiling with hydraulic resistance until it reaches the chamber? More importantly could atomization occur with ideal temperatures even though the fluid is moving slowly?
  7. May 24, 2015 #6
    So its an expanding liquid?
  8. May 24, 2015 #7
    Expanding because of heating and because of the vacuum pump
  9. May 24, 2015 #8
    I understand -- which liquid expands significantly?

    Hint: The fluid you are imagining is probably more like a gas.
  10. May 24, 2015 #9
    I'm imagining paraffin. It is both viscous and high boiling temps.
  11. May 24, 2015 #10
    And the question is then whether paraffin oil can expand significantly up that column you drew, without going into gas phase. No (at least I doubt it, given the definition of a liquid). Basically the oil will sit in the tank, giving off fumes that you vacuum away. It will continue to do so since you keep heating it. Considering you have a vacuum it will boil as well, so long as you are not so deep in the oil that the static pressure overwhelms the vapor pressure, as aformentioned.

    Feel free to correct me if you see an error in my reasoning!
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