Blowing hot and cold

[Rick Ape]

Hello everybody.

Is there anyone who can help me with a couple of questions about pressure, heat and the conservation of energy that have been niggling me for years?

It all stems from when I first read Aesop's Fable about a man who confounds a Satyr by blowing on his hands to warm them up, and then paradoxically blows on his soup to cool it down.

Both of the air flows begin their journey in the lungs with the same temperature and they travel the same route and distance, so the only variable appears to be the shape and size of the aperture; ie his lips.

Reducing the size of the aperture increases the pressure and my understanding is that an increased pressure on a gas increases the kinetic energy of its molecules which increases its temperature.

So why is the air blown through pursed lips significantly cooler than that blown through an open mouth?  It is also significantly cooler than its originating body temperature, so where does the heat go to?

In a similar vein (but I don't know if it's actually related or not) a fan creates a stream of air molecules that are moving faster than the surrounding air yet they are significantly cooler.  I thought that faster moving molecules are warmer than slower ones so why does increasing the speed decrease the temperature, and again, where does the heat go to?

I hope this all makes sense, and please be gentle - I am not a scientist.

Thank you

 

[swansont]

Your breath is cooler than the soup and warmer than the air. There is no inherent contradiction, if that’s all there was to it.

Blowing on the soup promotes evaporation, which is a cooling process. 

Blowing through an aperture means there is expansion afterwards. If the pressure and composition of a gas remains constant and it expands, the temperature goes down. (an ideal gas follows PV = nRT, so they would be proportional under that condition)

edit: see below

[Strange]

9 hours ago, swansont said:

Blowing on the soup promotes evaporation, which is a cooling process. 

There is also the effect of moving the hot moist air above the soup away, allowing more water to evaporate from the soup. I'm not sure how much that contributes.

 

[Bufofrog]

13 hours ago, Rick Ape said:

Reducing the size of the aperture increases the pressure and my understanding is that an increased pressure on a gas increases the kinetic energy of its molecules which increases its temperature.

So why is the air blown through pursed lips significantly cooler than that blown through an open mouth?  It is also significantly cooler than its originating body temperature, so where does the heat go to?

If you measure the temperature of your breath coming out of an open mouth vs pursed lips right at your mouth the temperature will be the same.  When you blow out of pursed lips the higher velocity breath entrains the surrounding air so it is much cooler even a couple inches from your mouth.

14 hours ago, Rick Ape said:

In a similar vein (but I don't know if it's actually related or not) a fan creates a stream of air molecules that are moving faster than the surrounding air yet they are significantly cooler.  I thought that faster moving molecules are warmer than slower ones so why does increasing the speed decrease the temperature, and again, where does the heat go to?

The temperature of the air from a fan is the same temperature as the surrounding air.  The reason that moving air cools you is 2 fold.  First the moving air will increase evaporation of any sweat on your skin, cooling you.  Secondly, if the ambient air temperature is less than your skin temperature the high mass flow of air will transfer more heat away from your skin due to more cool air molecules per second hitting your skin than stagnant air, cooling you.  

If the ambient air temperature is above your skin temperature and there is no sweat or moisture on your skin, then a fan will heat your skin making you hotter.  This is because the higher mass flow of air means more hot air molecules per second are hitting your skin than stagnant air, transferring more heat to you.

Heat transfer is always from hot to cold.

[J.C.MacSwell]

23 hours ago, swansont said:

Your breath is cooler than the soup and warmer than the air. There is no inherent contradiction, if that’s all there was to it.

Blowing on the soup promotes evaporation, which is a cooling process. 

Blowing through an aperture means there is expansion afterwards. If the pressure and composition of a gas remains constant and it expands, the temperature goes down. (an ideal gas follows PV = nRT, so they would be proportional under that condition)

Doesn't that particular isobaric process require an addition of a certain amount of heat, enough to to maintain the pressure as well as enough to compensate for the work done?

 

(so the temperature would rise)

[swansont]

Sorry, I mucked that up that (not sure what I was thinking) Pressure isn’t constant going through a nozzle (and V is proportional to T for the case I gave) 

As the gas moves, the pressure tends to drop (Bernoulli’s equation).

[joigus]

Feynman Lectures on Physics, volume 1, chapter 1, Section 1-3. Atomic Motion. Go to your local library and start reading Feynman now.

Then start mimicking Feynman in whatever way you can without giving up your principles, and maybe buy a pair of bongos.

Just joking. But the key word in all of this is: Feynman.

[J.C.MacSwell]

On 5/6/2020 at 4:01 PM, joigus said:

Feynman Lectures on Physics, volume 1, chapter 1, Section 1-3. Atomic Motion. Go to your local library and start reading Feynman now.

Then start mimicking Feynman in whatever way you can without giving up your principles, and maybe buy a pair of bongos.

Just joking. But the key word in all of this is: Feynman.

Surely you're joking...

...Mr. Joigus

Edited May 13, 2020 by J.C.MacSwell

[joigus]

30 minutes ago, J.C.MacSwell said:

Surely you're joking...

...Mr. Joigus

It never crossed my mind. Well, it did, but it was a virtual process.

 

[Tom Booth]

On 4/19/2020 at 6:10 PM, Rick Ape said:

...the only variable appears to be the shape and size of the aperture; ie his lips.

Reducing the size of the aperture increases the pressure and my understanding is that an increased pressure on a gas increases the kinetic energy of its molecules which increases its temperature.

So why is the air blown through pursed lips significantly cooler than that blown through an open mouth?
 

The Joule–Thomson effect, used in refrigerators and heat pumps and in the liquefaction of gases. (Linde process)

The air, held back in the mouth under increased pressure does result in higher temperature, higher kinetic energy, which quickly equalizes with the body temperature and is carried off by the blood stream, so when the air finally escapes, it can expand and reclaim the lost heat. That is to say, it is now colder due to having previously lost heat while under pressure.

the air also does a bit of work while escaping, pushing the outside air aside. So a bit of the Claude system of refrigeration as well.

[studiot]

Thermodynamics is an experimental science.

The OP was doing pretty well until he got to this

On 4/19/2020 at 11:10 PM, Rick Ape said:

so the only variable appears to be the shape and size of the aperture; ie his lips.

Not really, as a bit of simple experimentation shows.

1) Breath out through a wide open mouth and compare the thermal effect on your hands and the cup of tea.

     Pretty ineffective cooling on the tea and heating on the spread open hands.
     Slight heating on cupped hands.

2) Breath out through pursed lips

    Effective cooling on both the tea and the spread open hands
    Effective heating on cupped hands

So pursing the lips is not the controlling factor to make the difference.

 

So what is ?

Well cupping the hands seems to lead to substantially increased hand heating in all cases so lets deal with this first.
Cupping the hands forms a small, if leaky, chamber into which the exhaled air is drive, increasing its pressure. Work is done on the air increasing its internal energy.
This work is done by the chest muscles of the body as can be felt during the exhalation.
Thie work quickly degrades to heat which transfers to the chamber walls (hands) as the atmospheric pressure is reasserted.

So that is what warms the hands .

If you blow on the spread hands or the tea, evaporation is enhanced as already noted, which process carries heat away from the natural moisture on the hands or the surface of the tea.
This occurs whether the lips are pursed or not.

The difference is the cupping of the hands.

 

[Tom Booth]

I don't think using cupped hands is without flaw as an experiment  the chamber created, as you point out, causes increased pressure which modifies the conditions and changes the temperature of the air.

Try the same experiment on the open back of the hand. I don't know about you, but I can feel a distinct difference. A thin stream of air through pursed lips feels cooler than the hot breath from an open mouth.

If the hand it too close though, there will be an increase in pressure from the thin stream, much the same as with cupped hands and no noticable difference will be felt.

A more objective test might be to breath/blow on a thermometer. Or better yet, use some sort of mechanical bellows and eliminate the human subjective element altogether.

What I do know for sure is that the orifice in a vapor compression system plays a critical role in refrigeration. A "fixed orifice" in a duel heating and cooling system works both ways. "pursed lips" for cooling and "open mouth" for heating. A kind of check valve that restricts flow through a small orifice one way for cooling but opens up for unrestricted flow when the system is reversed for heating.

 

Edited May 8, 2021 by Tom Booth

[studiot]

19 minutes ago, Tom Booth said:

I don't think using cupped hands is without flaw as an experiment  the chamber created, as you point out, causes increased pressure which modifies the conditions and changes the temperature of the air.

Try the same experiment on the open back of the hand. I don't know about you, but I can feel a distinct difference. A thin stream of air through pursed lips feels cooler than the hot breath from an open mouth.

If the hand it too close though, there will be an increase in pressure from the thin stream, much the same as with cupped hands and no noticable difference will be felt.

A more objective test might be to breath/blow on a thermometer. Or better yet, use some sort of mechanical bellows and eliminate the human subjective element altogether.

What I do know for sure is that the orifice in a vapor compression system plays a critical role in refrigeration.

 

 

Thank you for your thoughts.

But I can only suggest you read very again carefully both the opening post and what I actually wrote.

What you have just written is incompatible with both (and with the scientific method as well).

 

Edited May 8, 2021 by studiot

[Tom Booth]

32 minutes ago, studiot said:

 

Thank you for your thoughts.

But I can only suggest you read very carefully both the opening post and what I actually wrote.

What you have just written is incompatible with both (and with the scientific method as well).

 

I read, and now have reread both.

I come to much the same conclusion.

The theory being investigated is does blowing through pursed lips or open mouth result in cooling/heating, or rather, does the shape of the orifice make the difference.

A scientific method would endeavor to eliminate all variables other than the variable in question: the shape of the orifice. Open or restrictive.

Lungs, tea, cupped hands, human flesh, evaporative cooling, and potentially biased and subjective human interpretations etc. are all variables that could skew the results.

Edited May 8, 2021 by Tom Booth

[studiot]

1 hour ago, Tom Booth said:

I read, and now have reread both.

I come to much the same conclusion.

The theory being investigated is does blowing through pursed lips or open mouth result in cooling/heating, or rather, does the shape of the orifice make the difference.

A scientific method would endeavor to eliminate all variables other than the variable in question: the shape of the orifice. Open or restrictive.

Lungs, tea, cupped hands, human flesh, evaporative cooling, and potentially biased and subjective human interpretations etc. are all variables that could skew the results.

 

But this is strictly speaking off topic. I admit to discussing tea rather than soup but the principle is the same.

 

On 4/19/2020 at 11:10 PM, Rick Ape said:

It all stems from when I first read Aesop's Fable about a man who confounds a Satyr by blowing on his hands to warm them up, and then paradoxically blows on his soup to cool it down.

 

So  the topic is what is the difference between blowing on your hands and blowing on your soup (or tea). ?

2 hours ago, Tom Booth said:

Lungs, tea, cupped hands, human flesh, evaporative cooling, and potentially biased and subjective human interpretations etc. are all variables that could skew the results.

So you can't dismiss the subject of the topic.

[exchemist]

It seems to me @Bufofrog has got it pretty well spot-on. Air entrainment will account for the faster stream of air being a bit cooler. There is negligible pressure drop across pursed lips, so adiabatic cooling won't be relevant. Cooling by evaporation - accelerated by rapid removal of evaporated vapour - will be very important in the soup/tea case. A cup of tea drunk outside cools a lot faster than indoors at the same temperature, if there is a bit of a breeze.   

  

 

[Tom Booth]

Off topic?

Quote

"the only variable appears to be the shape and size of the aperture; ie his lips.


So why is the air blown through pursed lips significantly cooler than that blown through an open mouth?

That was the OP's theory, plain as day. 

The general story, the fable, is not any kind of theory to be tested. A question is not a theory to be tested. Certainly other variables, like the cupped hands could and should also be isolated and tested, one at a time and in combination before any hard conclusions are drawn.

You are right to point out that there are other variables and possible contributing factors, but how something feels to the hands subjectively is not always reliable and I'm not sure how it was deduced that one method or the other of blowing on the tea was effective or not, or more or less effective.

You seem to be hastening to discard the OP's original premise without ever actually testing it or proving it wrong.

I do agree that one pet theory or conclusion should not be elevated over another, and strictly speaking, to say: "the ONLY variable appears to be the shape and size of the aperture", is overlooking the many other variables you point out, and no doubt many more that could be isolated and tested.

But a theory is not "off topic" or "unscientific", just because there are alternative theories that could also be valid.

6 minutes ago, exchemist said:

It seems to me @Bufofrog has got it pretty well spot-on. Air entrainment will account for the faster stream of air being a bit cooler. There is negligible pressure drop across pursed lips, so adiabatic cooling won't be relevant. Cooling by evaporation - accelerated by rapid removal of evaporated vapour - will be very important in the soup/tea case. A cup of tea drunk outside cools a lot faster than indoors at the same temperature, if there is a bit of a breeze.   

  

 

How do you support that conclusion? It appears to be simply opinion without documented evidence. Has anyone ever actually  measured the pressure drop? The actual temperature?

Entrainment could be a causative factor, but adding more ambient air into the stream might increase the temperature of the stream. It might also increase the impact of the stream as with the "air amplifier" type device, increasing the pressure or speeding evaporative cooling, but I think it is premature to just dismiss what the OP suggests "seems to be" the most obvious or "only" variable.

[exchemist]

32 minutes ago, Tom Booth said:

Off topic?

That was the OP's theory, plain as day. 

The general story, the fable, is not any kind of theory to be tested. A question is not a theory to be tested. Certainly other variables, like the cupped hands could and should also be isolated and tested, one at a time and in combination before any hard conclusions are drawn.

You are right to point out that there are other variables and possible contributing factors, but how something feels to the hands subjectively is not always reliable and I'm not sure how it was deduced that one method or the other of blowing on the tea was effective or not, or more or less effective.

You seem to be hastening to discard the OP's original premise without ever actually testing it or proving it wrong.

I do agree that one pet theory or conclusion should not be elevated over another, and strictly speaking, to say: "the ONLY variable appears to be the shape and size of the aperture", is overlooking the many other variables you point out, and no doubt many more that could be isolated and tested.

But a theory is not "off topic" or "unscientific", just because there are alternative theories that could also be valid.

How do you support that conclusion? It appears to be simply opinion without documented evidence. Has anyone ever actually  measured the pressure drop? The actual temperature?

Entrainment could be a causative factor, but adding more ambient air into the stream might increase the temperature of the stream. It might also increase the impact of the stream as with the "air amplifier" type device, increasing the pressure or speeding evaporative cooling, but I think it is premature to just dismiss what the OP suggests "seems to be" the most obvious or "only" variable.

You know the pressure change is not significant from the tension or lack of it in your cheeks. But I looked it up and, in pursed lip breathing exercises for medical conditions, patients typically generate a pressure of the order of 15cm H2O, i.e. ~10mmHg. I leave it to you to calculate the temperature drop from adiabatic expansion by this amount, if you think it is significant. 

[Tom Booth]

That's about 87.7 grams pressure per square inch, if my use of an online calculator can be trusted.

But I have studied this phenomenon in some depth, and from what I've read, the effect is (or can often be) cumulative. That is something rarely mentioned in the textbooks.

That is, there is a temperature drop that may be compounded over time.

The cooling effect the first instant is added to that of the second, etc.

So what may look like almost nothing on paper, in reality can be quite significant.

Some sources also say a "normal person" may produce a pressure from 1 to 2 psi. which seems significantly higher than what you site, for people with "medical conditions".

I'm thinking specifically of the "Joule-Thompson Effect". I'm not sure that is the same thing as adiabatic expansion exactly.

[exchemist]

2 hours ago, Tom Booth said:

That's about 87.7 grams pressure per square inch, if my use of an online calculator can be trusted.

But I have studied this phenomenon in some depth, and from what I've read, the effect is (or can often be) cumulative. That is something rarely mentioned in the textbooks.

That is, there is a temperature drop that may be compounded over time.

The cooling effect the first instant is added to that of the second, etc.

So what may look like almost nothing on paper, in reality can be quite significant.

Some sources also say a "normal person" may produce a pressure from 1 to 2 psi. which seems significantly higher than what you site, for people with "medical conditions".

I'm thinking specifically of the "Joule-Thompson Effect". I'm not sure that is the same thing as adiabatic expansion exactly.

Indeed. But we can forget that effect here. The J-T coefficient for N2 is 0.2K/bar and 10mmHg is ~ 0.13bar, so we're looking at cooling of 0.025K or so due to the Joule Thomson effect. The intermolecular attractions of N2 and O2 are not very strong, so air does not deviate that much from an ideal gas.

[Tom Booth]

Ok, I'll have to take your word for it, But I'm still convinced that the combined effects of JT, heat loss from compressed air held back in the mouth, work done by the gas as it expands escaping pressurization through pursed lips pushing air aside, cooling from expansion itself, and the combined synergistic effect of all these elements put together, all as a consequence of the blowing through pursed lips, or a restriction, throttle, orifice 

This cooling method is so widely used everywhere in every day life and in so many vital industrial processes I hesitate to just dismiss it as a contributing factor.

I don't think it is so much what happens outside the mouth after the air escapes, as what happens inside when the air is being compressed causing the temperature to rise, while also having time to equalize with the body temperature. The inside walls of the mouth and lips as well as the lungs acting as heat sink.

A basic principle of refrigeration.

Also, the relatively high velocity air stream produced does not allow much time for the expanding air stream to reabsorb heat it lost to the lungs and mouth until it reaches the destination; the cup of soup.

However it is broken down, or whatever technical name(s) anyone wants to apply to it. Pursing the lips reduces the temperature for a variety of secondary reasons. IMO. All of which disappear without such a restriction.

My opinion or intuition doesn't make it fact, of course.

Another thought that occurs to me is that, if blowing a stream of air across the surface of hot soup or tea, cools the surface, this might very well drastically reduce evaporation, as evaporation relies mainly on the escape of hot molecules at the surface.

[sethoflagos]

On 5/8/2021 at 7:39 PM, exchemist said:

It seems to me @Bufofrog has got it pretty well spot-on. Air entrainment will account for the faster stream of air being a bit cooler. There is negligible pressure drop across pursed lips, so adiabatic cooling won't be relevant. Cooling by evaporation - accelerated by rapid removal of evaporated vapour - will be very important in the soup/tea case. A cup of tea drunk outside cools a lot faster than indoors at the same temperature, if there is a bit of a breeze.   

  

 

This.

Some pertinent considerations:

Our lungs are not simply compressors. They are also highly efficient heat exchangers and humidifiers. In short, the process is near isothermal, and exhaled breath emerges from our lips (pursed or otherwise), at core body temperature and saturated with water vapour.

Skin thermoreceptors sense rate of temperature change rather than absolute temperature.

Simultaneous mass and heat transfer can become quite an intense study subject. However, when evaporation or condensation are involved, these processes tend to dominate overall heat transfer rates (compared to 'dry' conductive/convective processes). 

 

[studiot]

26 minutes ago, sethoflagos said:

This.

Some pertinent considerations:

Our lungs are not simply compressors. They are also highly efficient heat exchangers and humidifiers. In short, the process is near isothermal, and exhaled breath emerges from our lips (pursed or otherwise), at core body temperature and saturated with water vapour.

Skin thermoreceptors sense rate of temperature change rather than absolute temperature.

Simultaneous mass and heat transfer can become quite an intense study subject. However, when evaporation or condensation are involved, these processes tend to dominate overall heat transfer rates (compared to 'dry' conductive/convective processes). 

 

All good points pertinent to a much more in depth analysis than my offering.  +1

However it should be noticed that the effect of blowing over soup is always cooling on the soup, however you blow.
But the effect of blowing over the hands is cooling or warming depending upon how you hold you hands and how you blow.

Edited May 10, 2021 by studiot

[sethoflagos]

1 hour ago, studiot said:

All good points pertinent to a much more in depth analysis than my offering.  +1

However it should be noticed that the effect of blowing over soup is always cooling on the soup, however you blow.
But the effect of blowing over the hands is cooling or warming depending upon how you hold you hands and how you blow.

Not wishing to nit-pick, but what happens when you blow on a really cold gaspacho?

Somebody previously mentioned that faster air velocity would entrain more ambient (cooler) air and up the heat transfer coefficient quite considerably. I wouldn't want to steal their thunder!