Cooling with bottles

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I came across this claim that blowing air through a cut bottle somehow cools it down. I don't believe it should work and the explanation given is rather weak.
Does anyone have any ideas as to whether this should work, and if so, based on what principle?

Originally posted by @twhitehead
http://www.cleverly.me/diy-ac/?ref=fb
https://www.youtube.com/watch?v=oSbZWNk84F4
https://www.youtube.com/watch?v=Oh9LhrLGUc4

I came across this claim that blowing air through a cut bottle somehow cools it down. I don't believe it should work and the explanation given is rather weak.
Does anyone have any ideas as to whether this should work, and if so, based on what principle?

I feel like it violates the Second Law of Thermodynamics, but its hard to tell without some serious thought. Also, I'm no expert in the fields of Fluid Mechanics/Thermodynamics.

Working "outside" the second law (complete idealization) I can see one way it works, using three principles.
1) The Ideal gas law
2) Bernoulli Prinicple
3) Continuity

Number "3" first: Continuity

The volumetric flow rate entering the bottle is equal to the volumetric flow rate exiting the cap. This means the flow is incompressible ( this is not exactly true, but probably close enough for this case...the pressures are not changing much). Hence the average velocity exiting the cap is greater than what is entering.

Number "2", Bernoulli principle is applicable to idealized invicid flows.

It states the pressure head + the kinetic head+ elevation head is constant along a streamline in a flow field. It means energy is conserved or the gain in kinetic head at the cap was provided by a drop in pressure at the cap. The elevation remained unchanged.

For your reference in Fluid Mechanics Head is a specific Energy (Energy per unit mass). Calculations are typically done with this metric.

Finally, #1 the Ideal gas law for adiabatic process ( without heat transfer) states the Pressure to Temperature ratio remains constant for an Ideal gas such that the temperature at the cap is equal to the ratio of the pressures times the temperature of the entering air.

T_cap = P_cap/P_entr.*T_entr.

P_cap/P_ent < 1

T_cap < T_entr.

So under completely idealized conditions the models I used do not explicitly forbid it.



However, the second law states that in any real process entropy is generated.

Conservation of energy and the second law invalidate Bernouliis principle for "real" viscous flows with a head loss term ( that is heat is generated from friction through the exchange)

The Energy Equation for this case states: pressure head + kinetic head + elevation head at the entrance is equal to the pressure head + kinetic head + elevation head + head loss

The head loss term is really a group of terms that represent the thermal energy of the flow ( sum of internal and transfered heat) across the boundries. By the second law it is always positive. This means that the change in internal energy of the flow is always greater than the heat that could could be transfered out between the two reference points. As a consequence, the internal temperature of any "real" flow necessarily increases in the direction of the flow.

So I'm going to say it violates the second law, and is actually not a real phenomenon.

Originally posted by @joe-shmo
I feel like it violates the Second Law of Thermodynamics, but its hard to tell without some serious thought. Also, I'm no expert in the fields of Fluid Mechanics/Thermodynamics.

Working "outside" the second law (complete idealization) I can see one way it works, using three principles.
1) The Ideal gas law
2) Bernoulli Prinicple
3) Continuity

Numb ...[text shortened]... flow.

So I'm going to say it violates the second law, and is actually not a real phenomenon.

My best guess is heat is released (due to slight compression) as air is traveling through the part of the bottle that narrows down to a small hole, and much of that released heat would be sent back outside (into the open air) in the backwash. So there is no loss of energy, because much of that released heat is simply redirected back out of the larger opening.

A fan running in a closed room is a different story. All you would be doing there is cooling the air in one little portion of the room. You wouldn't be cooling down the entire room, because it would only temporarily separate warmer from cooler air.

Originally posted by @lemon-lime
My best guess is heat is released (due to slight compression) as air is traveling through the part of the bottle that narrows down to a small hole, and much of that released heat would be sent back outside (into the open air) in the backwash. So there is no loss of energy, because much of that released heat is simply redirected back out of the larger ope ...[text shortened]... cooling down the entire room, because it would only temporarily separate warmer from cooler air.

Are you saying you believe the effect is real or not real? I'm having trouble deciphering your position from you post.

Originally posted by @joe-shmo
Are you saying you believe the effect is real or not real? I'm having trouble deciphering your position from you post.

I don't know if it's real or not. All I'm saying here is I believe the effect could be real.
This wasn't my first impression, but after thinking about it I believe there is a rational explanation for how it might work.

I can start off by making one of two assumptions:
1. I can assume it's not real
2. I can assume it is real
I prefer approaching most subjects with an affirmative ssumption, no matter how goofy something might first appear. I do this simply because it's easier for me to look for ideas, and possibly find (or not find) proofs with this approach. If I start off with the assumption that something is not real, then I'm not likely to give it any real thought after that.

It obviously wouldn't be the most effective way to cool a room. But if I lived where there was no electricity, and I could count on a consistent breeze blowing in through one of my windows, I would definitely want to give it a try.

Originally posted by @lemon-lime
I don't know if it's real or not. All I'm saying here is I believe the effect could be real.
This wasn't my first impression, but after thinking about it I believe there is a rational explanation for how it might work.

I can start off by making one of two assumptions:
1. I can assume it's not real
2. I can assume it is real
I ...[text shortened]... onsistent breeze blowing in through one of my windows, I would definitely want to give it a try.

I agree the "feeling" is real. If their is a breeze it will amplify the velocity, thus increasing the convection coefficient. Which will help cool the human, provided there is a temperature differential.

However, the temp of the air goes up (very slightly), not down as it passes through the bottle.

Originally posted by @joe-shmo
I agree the "feeling" is real. If their is a breeze it will amplify the velocity, thus increasing the convection coefficient.

My feeling is that it may increase the velocity for a few cm beyond the neck of the bottles, but further than that it will spread back out again and have no effect on anyone standing say a metre away from it.

Originally posted by @twhitehead
My feeling is that it may increase the velocity for a few cm beyond the neck of the bottles, but further than that it will spread back out again and have no effect on anyone standing say a metre away from it.

Yeah, I agree.

I see this as an example of placebo.
If you believe in it, then you can also feel the effect.

Originally posted by @joe-shmo
I agree the "feeling" is real. If their is a breeze it will amplify the velocity, thus increasing the convection coefficient. Which will help cool the human, provided there is a temperature differential.

However, the temp of the air goes up (very slightly), not down as it passes through the bottle.

This is interesting because we don't have to stop at speculating about it... it can easily be tested under lab conditions. Air blowing on a person can make them feel cooler because of moisture evaporating from the skin, but we're not limited to 'feeling' the effect to know if it actually works. This can be tested. And if we discover it's real before knowing why it works, it would be like knowing bumble bees can fly before discovering how they do it.

By the way, scientists now know how bumble bees can haul their big *ss bodies through the air, so it has (officially) become aerodynamically possible. 😀

Originally posted by @joe-shmo
I agree the "feeling" is real. If their is a breeze it will amplify the velocity, thus increasing the convection coefficient. Which will help cool the human, provided there is a temperature differential.

However, the temp of the air goes up (very slightly), not down as it passes through the bottle.

However, the temp of the air goes up (very slightly), not down as it passes through the bottle.

Yes, but only if all of the air entering the large opening is passing out through the much smaller end. There is (forgive me for saying this) a 'bottle neck' effect to take into account.

This is what I meant by 'backwash'. Air is not being forced through the bottle in such a way that none of it is allowed to flow back out of the larger opening. Not all of the air blowing into the large end will go through the smaller hole... in order for that to happen the large end would have to be blocked to prevent backwash, and air would need to be forcibly pumped into the bottle. But this isn't what the videos are showing, so there's no reason to believe that all of the air going into the bottle is passing out through the other end.

Originally posted by @lemon-lime
[b]However, the temp of the air goes up (very slightly), not down as it passes through the bottle.

Yes, but only if all of the air entering the large opening is passing out through the much smaller end. There is (forgive me for saying this) a 'bottle neck' effect to take into account.

This is what I meant by 'backwash'. Air is not being ...[text shortened]... eason to believe that all of the air going into the bottle is passing out through the other end.[/b]

No, I don't agree. All the air, whether it moves through, back, left, right, up,down... is slightly heated to different, but some extent. It takes energy to change its direction, and in every one of those exchanges (without exception) heat is generated.

Originally posted by @joe-shmo
No, I don't agree. All the air, whether it moves through, back, left, right, up,down... is slightly heated to different, but some extent. It takes energy to change its direction, and in every one of those exchanges (without exception) heat is generated.

Okay. So if a little heat is generated because the air changes direction, in what direction would most of that now slightly warmer air be moving? Would most of it be moving toward or away from the smaller hole?

There would be less resistence from the interior surface of the bottle traveling back toward the larger hole. So I suspect the lions share of that newly created heat would be moving in that direction. But an increase of heat generated by interior resistance (or changing air direction) would only add an imperceptible fraction of a degree to the already warm air. It would (in my opinion) be negligible compared to a possible one (or two?) degree difference in heat already (in theory) being naturally extracted and redirected back out of the larger hole.

Originally posted by @lemon-lime
Okay. So if a little heat is generated because the air changes direction, in what direction would most of that now slightly warmer air be moving? Would most of it be moving toward or away from the smaller hole?

There would be less resistence from the interior surface of the bottle traveling back toward the larger hole. So I suspect the lions share of ...[text shortened]... n heat already (in theory) being naturally extracted and redirected back out of the larger hole.

I think you are confusing some concepts here, so bear with me ( I'm not a Physicist, but this should basically be correct).

Heat is not being extracted from the flow, it is generated from the flow.

The flow entering has all its energy tied up in its pressure, velocity, elevation, and temperature (as far a classical mechaincs is concerned). Now lets say a group of particles/molecules under a certain pressure, velocity, elevation, and temperature hit an obstuction. The molecules experience a change in momentum and scatter about with different velocities than previously held. From conservation of energy, where they aquired the energy to change their velocity is from the stored potential of pressure energy in this situation. In an ideal world the energy given up from pressure is equivalent the the change in kinetic energy and/or elevation (gravitational potential) gained by the system.

However, the Second Law of Thermodynamics dictates that entropy is generated in the exhange of every real process. Which means that some of the energy given up as pressure does not make it into changing the particles kinetic or gravitational potentials. Some of it, however small in this case, is converted to heat. The only place left for the heat energy to go is into the internal ( temperature ) energy of the flow and the obstruction. The flow of heat to the obstruction and the flow itself manfests as a positive change in temperature for both bodies. That is the flow and the obstruction necessarily get hotter. After that occurs heat can leave the original system of particles and obstruction and propogate to its surroundings.

The eventuality of these processes and the implications of the Second Law is the universe itself will eventually suffer a heat death, where all its usefull energy will be converted to heat ( assuming it cannot be trasfered or gained from beyond the universe - which is just a theory at this point).

We know that the plastic bottles being used were not made for this purpose. So if it is discovered and confirmed that any heat is redirected, causing any difference in temperature (1/2°, 1/4°, 1/8°, etc) the design of soda bottles would be purely a happy coincidence. Someone would then have a reason for creating different variations of that design, and run tests to find one that extracts and throws off the most heat.

But if I'm going to start off not believing something, with little to no evidence of it not being true, then it would have to be that the shape of a soda bottle just happens to provide optimal heat extraction.