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Air entrainment or cooling by expansion?

  1. Oct 12, 2003 #1
    I figure that you guys must know fluid dynamics better than most physicists so here's my question:

    Open your mouth wide and exhale and it's hot air you feel. Now pucker your lips and exhale and it feels cold (unless you hold your hand right up close to your lips). Is this an example of the hot air from your lungs cooling because it is compressed by passing through a small opening or an example of air entrainment?

    Air entrainment is when a small amount of rapidly moving air draws additional air along with it. It would then be the additional air (at a lower temp. than that in your mouth) that makes the air feel cool.
  2. jcsd
  3. Oct 12, 2003 #2


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    Where would that additional air come from?

    My guess would be that air coming out of your lungs isn't quite at 100% humidity so the faster it moves, the more it cools your wet lips.
  4. Oct 13, 2003 #3
    I never noticed it cooling my lips but it will cool your hand or a bite of hot apple pie on your fork.

    My physics books say it is due to the expansion of the air after it is adiabatically compressed by passing thru your puckered lips. It sounds very convincing and makes the point about expanding air becoming cooler.

    Recently, an acquintance of mine claimed that the cooling is due to the entrainment of air. He gave some good examples of entrainment such as blowing open a large bag by blowing air into it gently but steadily. Your air draws air with it from the surrounding atmosphere and fills the bag much faster than you can possibly blow it up yourself. Try this if you haven't seen it!

    So, the cool air around you will be 'guided' in the direction you blow. The volume of air is proportional to the speed of your air, that's why a wide open mouth gives hot air and puckered lips give cool air. That's the entrainment explanation anyway.

    He claimed to have learned all this in a fluid dynamics class, I subject in which I have no experience. I have no idea if one or both explanations apply, perhaps unequally.
  5. Oct 13, 2003 #4
    Interesting question. The entrainment explanation sounds a lot more plausible than the expansive cooling one. For a start, the density changes we are talking about when you blow air around with your mouth are miniscule.

    Secondly, although your lungs (to be specific - intercostal muscles and diaphragm) do work to expel the air from your mouth, you actually do more work if you pucker your lips than if you don't...because you have to drive the air faster. So it should be the other way around if this explanation applies, at least to any significant extent.

    Also, remember any work you do adiabatically compressing the air is conserved. At the very least, it would exit at the same temperature as it was when it was in quasi-thermal equilibrium in your lungs. Take a 2 chamber device, with the 2 chambers strung together in series. A-B valve is shut. A is adiabatically compressed. Temperature, density, pressure rises. A-B valve is opened. Air rushes into B. Temperature, density, pressure fall. But...if we drive all the air from A into B by pushing the piston in A forward entirely, as long as the size of A = B, with no losses, the temperature at B = the temperature at A.

    May I propose a 3rd mechanism? Forced cooling. If you blow on something with puckered lips, the air is moving faster and moisture on your skin will evaporate faster. Do it with slow air (open lips) and evaporation is slower.

    Maybe someone can test if there is any difference by blowing puckered vs. 'O'-shaped lips on a dry thermometer?
  6. Oct 13, 2003 #5
    I will go try it right now.
  7. Oct 13, 2003 #6


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    Where would this "extra air" come from?

    And what is wrong with evaporation?
  8. Oct 13, 2003 #7
    Just try this:

    1. Hold your palm three inches from your puckered lips and blow. Cool, eh?

    2. Again puckered and blowing, now move your palm slowly until it nearly touches your lips. Getting warmer!
  9. Oct 14, 2003 #8
    Eh...my understanding of mmwave's question was just that. Was he talking about something else? [?]

    You may be reading into my example a little too literally. There isn't a mechanism for any significant rarefaction of the expelled air; you won't see density changes very far from ambient values.

    TBH, there are not may instances where expansive cooling would happen. From a thermodynamic point of view, you get this when a fluid does work against its boundaries adiabatically (strictly some heat input is allowed, as long as dW - dQ > 0). For net work to be done against the boundaries the internal pressure must be greater than the external one. The best example would be a balloon in space - as it expands because the internal pressure > 'vacuum', the gas inside cools.
  10. Oct 14, 2003 #9
    Sorry, Tyro, I blew it!
  11. Oct 15, 2003 #10
    LOL. Nice pun :smile:

    Any news from mmwave about his little experiment with a thermometer?
  12. Oct 16, 2003 #11
    Sorry for the delay, I did the experiments right away but didn't have time to report back.

    Your argument that there is little chance for significant expansion sounds reasonable.

    Ok, dry themometer I did the following:

    Wide open mouth raised temp. significantly and quickly (so we know the response time of the themometer is quick enough to do the next experiment.

    Puckered lips 1 to 6 inches away - No change in temperature.

    (This too suggests that there is no adiabatic cooling of the air by expansion. I believe if the air were cooled, the thermometer would say so.)

    Fan a dry themometer with a piece of stiff cardboard - No change in temp. This seems obvious but I did it for completeness.

    Conclusion: Fan a dry thermo. or blow on it with puckered lips and get no change in temp. so temp of the air is not changing significantly.

    Blowing on your hand or fanning your hand you feeling cooling but that must be due to evaporative cooling. I would have tried a wet bulb thermometer but the portable ones didn't have enough resolution to bother with and water was a long way away from the dry thermometer.
  13. Oct 25, 2003 #12
    I would come to a different conclusion.

    I think your air entrainment explanation would be the obvious answer.

    From the air in the room.

    If you aim a fan (or a piece of waving cardboard :wink:) on a thermometer it will not drop in temperature (assuming it is not at a temperatur front) because the thermometer is sitting in the ambient room temperature and the fan is moving air that is also ambient room temperature towards it.
    Where would the temperature change come into play?

    The further from your lips you get, the cooler the air "feels".
    Also, the faster you breathe, the cooler the air "feels".
    If you pucker, but blow very slowly, it also feels warm.

    It is air speed (and volume), not puckerivity (my word, obviously).

    The air from your lungs is not being cooled by compression or any other means.
    It is simply moving from a place that is 98° to a place that is much cooler.
    The air, "carries" more air along with it (air that is room temperature), plus it disperses into the room (with MUCH more air in it).
    So, the faster it moves and the more volume you move, the more air it drags along with it.

    Not a whole lot different than a stream of warm water passing through a large container of cooler water.

    Try it with a bowl of yellow colored water and a small pitcher of blue colored water.
    Watch the green display the dynamics of it.
    The blue water you pour, and the faster it is dispensed, the larger the green cloud will be, and the less blue the blue stream will be.
  14. Oct 25, 2003 #13


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    How exactly is the air in the room going to flow backwards through your lips to cool them while air from inside your mouth is flowing out?
    Unless it is wet, which your lips are.
  15. Oct 27, 2003 #14
    Try your experiment with a tire pump instead of a fan. The compressed pumped air will leave the pump warm from the friction and quickly cool as you test farther away from the nozzle where it is allowed to expand.
  16. Oct 27, 2003 #15


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    That doesn't apply because air in a pump is compressed. Air in your lungs is not (by any meaningful amount anyway).
  17. Oct 28, 2003 #16
    Humans can produce approximately 1.5 psi Lung Pressure. An air pump is much closer to this than a fan.
  18. Oct 28, 2003 #17


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    Thats static pressure. There needs to be an abstruction to be able to generate that much pressure. Simply puckering your lips will generate virtually nothing.

    Ever play a trumpet? Even with 15 feet of tubing and a 3mm throat in the mouthpiece, you can expel an entire breath in a couple of seconds with very little force unless you press your lips very tightly together.
    Last edited: Oct 28, 2003
  19. Oct 29, 2003 #18
    Russ, you just descibed an air pump. Small amount of volume, and pressure depending on restriction.

    Also, H20 content of breath being exhaled is higher than breath being taken in, so I doubt that evaporative cooling is much of a factor (although some still may occur), it is more a factor on the intake of breath.
  20. Oct 29, 2003 #19
    I think we may be talking about two different phenomena...

    I was responding to the scenario in the first post:
    In which the poster seemed (to me at least) to be talking about the temperature differences on your hand when you blow on it at different strengths and from different distances.
    He mentioned nothing about the apparent difference of the temperature on your lips.
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