Mouth science

Originally posted by @joe-shmo
"The air pressure is greater when you whistle. I'm assuming this squeezes heat from the air in much the same way an air conditioner compresses warm gas, causing it to release heat.
But what happens to that released heat, where does it go? "


These two statements taken together indicate a misunderstanding, I was trying to correct that...nothing more. ...[text shortened]... ression, not released.

"Does whistling produce a stream of cooler air?"

I highly doubt it.

https://www.livescience.com/38685-how-air-conditioners-work.html

"it [refrigerant] absorbs heat in its liquid state, transforming into a gas. The refrigerant is then forced to return to being a liquid, expelling the heat it absorbed and thus made ready to soak up heat once again."

According to this link the refrigerant turns from liquid to gas in the house side of the unit, then is piped to the outside chamber where the gas is compressed and heat is released.
Compressing it turns the gas into a liquid. I made the mistake of saying 'gas' instead of liquid after it's been compressed.

By the way, I don't know if this makes any difference but there's more than just air being expelled when exhaling or whistling.
There is also a high amount of water vapor.

Originally posted by @lemon-lime
https://www.livescience.com/38685-how-air-conditioners-work.html

"it [refrigerant] absorbs heat in its liquid state, transforming into a gas. The refrigerant is then forced to return to being a liquid, expelling the heat it absorbed and thus made ready to soak up heat once again."

According to this link the refrigerant turns from liquid to gas in t ...[text shortened]... just air being expelled when exhaling or whistling.
There is also a high amount of water vapor.

"According to this link the refrigerant turns from liquid to gas in the house side of the unit, then is piped to the outside chamber where the gas is compressed and heat is released.
Compressing it turns the gas into a liquid. I made the mistake of saying 'gas' instead of liquid after it's been compressed."

The gas is "compressed" and "heat is released" are two different ( independent ) steps in the refrigeration process.

1) The refrigerant is a saturated liquid at a very cold temperature and atmospheric pressure.

2) Because of (1) heat is absorbed from the room by the refrigerant at constant temperature until the gas completes a phase transformation to a saturated vapor. That is although the refrigerant absorbed heat the gas is the same temperature as the liquid ( very cold). It has precisely absorbed the quantity of heat know as the Latent Heat of Evaporation.

3) Ideally, and adiabatic (without heat transfer) compression of the gas raises the temperature of the gas to above ambient temperature ( the gas is a super heated vapor - it does not compress it back into a liquid). It also does not "squeeze" out heat in this process.

4) Because of (3) the gas may discharge heat to its surroundings (at constant temperature) until it becomes a saturated liquid again ( expelling the Latent Heat of Condensation ). The liquid is warm and under pressure.

5) Next, an ideally adiabatic expansion of the liquid returns it to a saturated liquid at the initial temperature ( very cold again ).

6) The cycle repeats.

My point in all this when a gas is compressed its temperature is raised. ( and in any "real process" heat is absorbed). So when you say that the air for a whistle is compressed, it is also consequently warmed, not cooled.

can anyone explain this "phenomena" in plain language without talking about air conditioners?

Originally posted by @uzless
can anyone explain this "phenomena" in plain language without talking about air conditioners?

It happens by magic. There, does that help?

The explanation involves some fairly advanced thermodynamics. In basic terms, when a gas expands it tends to cool. When your mouth is wide open there is little scope for expansion. When your mouth is nearly closed, in the whistling shape, the emitted air will expand a little as it leaves your mouth. So it cools.

I can do some maths if that'll help.

Originally posted by @deepthought
It happens by magic. There, does that help?

The explanation involves some fairly advanced thermodynamics. In basic terms, when a gas expands it tends to cool. When your mouth is wide open there is little scope for expansion. When your mouth is nearly closed, in the whistling shape, the emitted air will expand a little as it leaves your mouth. So it cools.

I can do some maths if that'll help.

Unfortunately, I believe we disagree on the effect. I'll go first.

Things working against whistle cooling from my perspective:

For the sake of argument lets say the ambient air temp is 70F, internal body temp ( including the mouth area is 98.6F.

1) The air is warming the entire cycle of the breath.

2) The cycle of a whistle breath is much longer than the puff

3) Before it is expanded the air is first compressed, and consequential warmed.

4) Entropy is generated via head loss as the air the flow passes through the lips. Again leading to warmer air.

4) The air then undergoes an expansion and is cooled (like you state) , but should never be cooler than air rapidly undergoing only step 1. and never cooler than air before the compression stage no matter how quickly it is expelled due to the Second Law.

Just with these effects we definitely have a warmer flow in the whistle case up to this point.

But, if there is truly a cooling effect in the stream of air; the last remaining place that I can think of is evaporative cooling for the dominant mode of heat transfer, which increases with increasing flow velocity and would work to decrease the flow temperature as moisture is being evaporated from the mouth. ( so for me its a question of magnitude, but its a step in the right direction).

The rest of the effect must be physiological from increased forced convection coefficient ( for instance when he was blowing on the hand and it "feels" cool ).

Am I missing something, what are your thoughts?

Originally posted by @uzless
can anyone explain this "phenomena" in plain language without talking about air conditioners?

Patience...we are working on it...I think. And without some experimentation the best we can do is come up with a hypothesis ...I think.

It’s not an easily answered question. Here is a physics forum that hasn’t appeared to reach a concensus.

https://physics.stackexchange.com/questions/7868/why-does-the-air-we-blow-exhale-out-from-our-mouths-change-from-hot-to-cold-depe

Originally posted by @deepthought
It happens by magic. There, does that help?

The explanation involves some fairly advanced thermodynamics. In basic terms, when a gas expands it tends to cool. When your mouth is wide open there is little scope for expansion. When your mouth is nearly closed, in the whistling shape, the emitted air will expand a little as it leaves your mouth. So it cools.

I can do some maths if that'll help.

I can do some maths if that'll help.

I seriously doubt calling it magic and offering to do some maths will help. But it might help if I asked a few pointed questions and see how you answer.

First, when gas is compressed it becomes hotter. Is this because the heat becomes concentrated into a smaller area, or does the force of compression actually create more heat?

Secondly, you said "the emitted air will expand a little as it leaves your mouth. So it cools."
In this slightly cooler state isn't it possible for the air to pick up some warmth from (for example) the surface of a finger, or the sensor of a digital thermometer?

Originally posted by @lemon-lime
[b]I can do some maths if that'll help.

I seriously doubt calling it magic and offering to do some maths will help. But it might help if I asked a few pointed questions and see how you answer.

First, when gas is compressed it becomes hotter. Is this because the heat becomes concentrated into a smaller area, or does the force of compression actu ...[text shortened]... some warmth from (for example) the surface of a finger, or the sensor of a digital thermometer?[/b]

"First, when gas is compressed it becomes hotter. Is this because the heat becomes concentrated into a smaller area, or does the force of compression actually create more heat?"

The Kinetic Theory of Gases basically states the temperature of an ideal gas is proportional to the average kinetic energy of its molecules. So picture a lone ball bouncing back and forth between two walls (its container) at a constant speed. Now start moving one of the walls closer to the other ( closing the space - compressing the gas - our idealization). When the ball strikes the moving wall the wall imparts momentum on the ball and it speeds up and so its kinetic energy has increased. By the Kinetic Theory of Gases its temperature has also increased.

The heat comes from the work required to move the wall against the pressure of the gas. If that system was thermally isolated the gas remains hotter and at a higher pressure ( the pressure is proportional to the number of collisions between the gas and its walls per unit time).

Now lets allow the hot gas to reestablish thermal communication with a cool environment. The heat from the compression will be absorbed by the environment until the gas is the same temperature as the environment. As this heat transfer is happening the gas will slow down to the speed it had before the compression - the initial state ( decreasing in pressure and temperature from its hot - compressed - isolated state), to a state where the ball has the same kinetic energy as it did before the compression, but will exert a higher pressure on the walls of the container ( because of the shorter distance between the walls - more collisions per unit time).

are we going to talk about Boyle's Law at all?

Originally posted by @uzless
are we going to talk about Boyle's Law at all?

The most relevant of that series of laws ( Charles , Boyles, etc...) that you want is the Gay-Lussac Law. It Correlates Pressure to Temperature, however they are all just special cases of the Ideal Gas Law, which is kind of what I've been talking about.

Anyhow, I don't believe it holds the answer. For a gas to expand and cool it must first be compressed. And only in the limit of reality ( isentropic process) can the compression be "perfectly undone" In which case the temperature of the air after the expansion will be equal to the temperature of the air before the compression or during the process it is in contact with a heat sink and has time to spontaneously cool to the temperature it was before the compression. Only then, upon expansion will it be cooler than its surroundings. I can assure you none of these things are happening anytime between the air entering and leaving your warm body.

I think it is evaporation of water from you mouth that cools the air. The rate at which air evaporates water ( its coefficient of evaporation) is proportional to the velocity of the air. In order for the air to absorb the water it has to vaporize it. In other words the water must absorb energy from the air until the point that is goes through a change of state. The latent heat of evaporation is a fairly large quantity of energy for water so it takes a lot of energy from the air, and the water absorbs this latent heat without warming; Thus cooling the air. That process is where I believe the effect comes from.