KL7AJ,

While we're all rumiinating on the original post, here's another teaser in a similar vein. This is a pure thought experiment.

Let's say there's only one object in space. There are no other stars or other reference points. Can you tell if that object is spinning around its own axis? Does spin even have any meaning without outside references? How would you measure it? Or is spin only a RELATIVE parameter?

There are a lot of things you are not defining here. Is the size of the object in relation to the observer? Can the observer move around the object? Is the object close? If so, why can't the observer look at the object and see if it is rotating. And finally, you should post this in another new thread.

studiot,

I proposed an answer to your original question. Why don't you answer whether it is right or wrong so we can move on? And if wrong, why is it wrong?

Ratch

 

I proposed an answer to your original question. Why don't you answer whether it is right or wrong so we can move on? And if wrong, why is it wrong?

I presume you mean this statement?

Newton's Law of Cooling if for convection, and Fourier's Law of Cooling is for conduction. So neither of those methods work in a isolated vacuum, such as outer space or a vacuum chamber.
Gotta go with radiation if you want to dissipate heat energy in those places.

I already said there are probably lots of exceptions one could choose, but I thought everybody was having fun responding to this thread.

There have also been several incomplete or inaccurate statements posted during the discussion.

And no air flow can still support conduction.

This is just plain wrong. Conduction can and does occur through any material substance, moving or not.

Then there is the question of Newton's Law. Newton's original experiments which lead to the law were conducted with forced convection. Strictly the law applies to forced convection only, hence Bill's question about free convection.

However all three modes of heat transfer thus far mentioned reduce to proportionality to the difference between the temperatures. I will address this in a later post.

 

And no air flow can still support conduction.

I took that to mean that the absence of air flow can still support conduction, which would mean that there is no disagreement on this issue.

 

I was thinking about my cup of java actually.

mmm, i like coffee too. Were you watching the steam leaving your cup? Loss of mass in the system would seem to violate the assumptions implicit in Fourier's Heat Law. Or, perhaps it is the phase change, water to vapor, which carries internal energy away.

 

Well yes Steve, we are getting there.

Phase change and latent heat alter things as does physical shape.

A tall thin cup of coffee cools more quickly than a short squat one.

Notice there is no Newton's law which covers for example china in general, but only one that applies to a specific object.

Think also what happens if I take a cloth, dip it into cold water, and wrap it around the cup, and then perhaps remove it after a short interval.

The relevance of all this was to the working of a heatpipe, which the laptop technician I mentioned did not understand.

 

studiot,

There have also been several incomplete or inaccurate statements posted during the discussion.


Quote:
And no air flow can still support conduction.

This is just plain wrong. Conduction can and does occur through any material substance, moving or not.

Then there is the question of Newton's Law. Newton's original experiments which lead to the law were conducted with forced convection. Strictly the law applies to forced convection only, hence Bill's question about free convection.

How is that wrong? Lack of air flow means that air does not move. It does not mean that air is absent. If air does not move, then no convection. Conduction still occurs if air is in contact with a heated object and does not move. Newton's Law of Cooling applies to both natural and forced air movement, but the coefficient will be different between the two methods.

You still have not said anything about whether a hard vacuum will stop both convection and conduction.

Ratch

 

Ratch,
You are quite right, I read the squabbling-over-definitions posts too quickly and though you meant air was absent.

You did in fact say words to the effect that conduction occurs so long as air is present.

So we are all agreed on that one.

As regards to conduction and convection and vacuums, I did not introduce them and so see no need to comment. But yes I agree that there is neither convection nor conduction in a vacuum.

So what?

That does not alter the fact that two otherwise identical cups of coffee will cool at different rates if one is wet on the outside, and I can change the rate of cooling by wetting or drying the surfaces.

Nor does it alter the fact that different shaped cups will cool at different rates.

For your information, Newton's law does not apply to free convection, and as I said before, he did not experiment with this.

However Dulong and Petit did and after 'careful and exhaustive experiments' concluded that the rate of cooling is proportional to the temperature difference raised to the power 5/4.
Lorenz later provided the mathematical derivation for this.

Look it up. Here is a link to Molnar's thesis on the subject.

http://www.the-aps.org/publications/tphys/legacy/1969/issue1/9.pdf

 

studiot,

That does not alter the fact that two otherwise identical cups of coffee will cool at different rates if one is wet on the outside, and I can change the rate of cooling by wetting or drying the surfaces.

No doubt. Evaporative cooling comes into play, which is beyond the purview of Newton's Law of Cooling.

Nor does it alter the fact that different shaped cups will cool at different rates.

Different shapes or different cup materials can be explained by a different convection heat transfer coefficient.

For your information, Newton's law does not apply to free convection, and as I said before, he did not experiment with this.

Doesn't matter if he did not experiment with natural convection. It is still moving air, and it just changes the convection heat transfer coefficient for that situation. The law still holds.

However Dulong and Petit did and after 'careful and exhaustive experiments' concluded that the rate of cooling is proportional to the temperature difference raised to the power 5/4.
Lorenz later provided the mathematical derivation for this.

Are you sure that they factored out radiation? Hard to beliee that if convection was exponential, good text books would not know about it by now. Does that pass the sniff test?

Look it up. Here is a link to Molnar's thesis on the subject.

http://www.the-aps.org/publications/...9/issue1/9.pdf

The first paragraph talks about three ways heat is transferred. He only mentions radiation, convection and "etc". Never once is the document is the word "conduction" found. Neither is the 5/4 power. So he grinds on about constructing a rudimentary thermometer, and plots some points which show that deviations occur, which can probably be accounted for by secondary effects. I would like to see the convection of a large flat wall.

You might as well give the answer to your beginning question. I don't think anymore more suggestions are forthcoming.

Ratch

 

However Dulong and Petit did and after 'careful and exhaustive experiments' concluded that the rate of cooling is proportional to the temperature difference raised to the power 5/4.
Lorenz later provided the mathematical derivation for this.

Look it up. Here is a link to Molnar's thesis on the subject.

http://www.the-aps.org/publications/tphys/legacy/1969/issue1/9.pdf

Very educational! Can you explain the 5/4 power law? Why this number? Is this related to fractals/chaos theory? - Perhaps from turbulent effects? It's clear that the radiation effects were factored out. So, it's hard to visualize why this power law should be in effect.

 

Very educational! Can you explain the 5/4 power law? Why this number? Is this related to fractals/chaos theory? - Perhaps from turbulent effects? It's clear that the radiation effects were factored out. So, it's hard to visualize why this power law should be in effect.

This quite a task you ask. I remember this done in my 2nd year calculus course. The professor was keen on fluid dynamics and this was way of showing a 'practical' application for partial non-linear differential equations for us physics students. He first did the derivation for laminar natural convection which yielded the 5/4 power dependence on the temperature difference. He then did it for turbulent flow, which yielded a 4/3 power dependence.

Nasty stuff…

Intuitively, since it is the heat of the body that drives the free convection, as the body cools the convection necessarily slows lengthening the time it takes to cool the body compared to even moderate forced convection.

 

This quite a task you ask. I remember this done in my 2nd year calculus course. The professor was keen on fluid dynamics and this was way of showing a 'practical' application for partial non-linear differential equations for us physics students. He first did the derivation for laminar natural convection which yielded the 5/4 power dependence on the temperature difference. He then did it for turbulent flow, which yielded a 4/3 power dependence.

Nasty stuff…

Intuitively, since it is the heat of the body that drives the free convection, as the body cools the convection necessarily slows lengthening the time it takes to cool the body compared to even moderate forced convection.

Thank you. This answer is enlightening!

As an electrical guy, I can't say I'm up on all the details of fluid dynamics, so forgive if the following question is simple-minded.

Am I interpreting you correctly if I assume that the reason why their experiments yielded the 5/4 power law is that their experiments were under conditions of laminar flow?

 

It is still moving air, and it just changes the convection heat transfer coefficient for that situation. The law still holds.

It doesn't. Natural convection is driven by the external temperature of the body. As the convection starts, it begins to cool the outside of the body which in turn slows down the convection.

The outer temperature of the body, hence the temperature difference on which Newton's law depends, and the convection rate are interdependent. As the body cools over time the convection slows, reducing the temperature difference, slowing the cooling of the body...etc...

 

Am I interpreting you correctly if I assume that the reason why their experiments yielded the 5/4 power law is that their experiments were under conditions of laminar flow?

I think that would be a safe assumption. I'm not up on fluid dynamics that much myself, nor the experimental apparatus used in these experiments. My feeling on this though would be that turbulent flow would be difficult to maintain around reasonably smooth bodies.

My reasoning here is, that if the flow turned turbulent, the cooling effect would diminish. This would result in a corresponding rise in the outer temperature of the body, which would tend to drive the convection more.

Sounds like a hypothesis on which to conduct an experiment. Any students out there want to take this on?

 

BillO,

It doesn't. Natural convection is driven by the external temperature of the body. As the convection starts, it begins to cool the outside of the body which in turn slows down the convection.

The outer temperature of the body, hence the temperature difference on which Newton's law depends, and the convection rate are interdependent. As the body cools over time the convection slows, reducing the temperature difference, slowing to cooling...etc...

Yes, no one denies that cooling rate slows down as the temp differential becomes less. Sort of like the energy rate discharge of a capacitor slowing down when its voltage decreases. After all, that is what Newton's Law of Cooling states. You are engaged in figuring out the heat loss of a cooling object. I was thinking of an object where the heat is constantly replenished. Like a transistor heat sink. Once equilibrium is reached between the temperature of the sink and the ambient air, there should be no exponentials involved.

Ratch

 

You might as well give the answer to your beginning question. I don't think anymore more suggestions are forthcoming.

I think we got that already....

The relevance of all this was to the working of a heatpipe, which the laptop technician I mentioned did not understand.

 

BillO,



Yes, no one denies that cooling rate slows down as the temp differential becomes less. Sort of like the energy rate discharge of a capacitor slowing down when its voltage decreases. After all, that is what Newton's Law of Cooling states. You are engaged in figuring out the heat loss of a cooling object. I was thinking of an object where the heat is constantly replenished. Like a transistor heat sink. Once equilibrium is reached between the temperature of the sink and the ambient air, there should be no exponentials involved.

Ratch

Well, I don't think you can apply Newton's law to this situation at all.

 

BillO,

Yes, no one denies that cooling rate slows down as the temp differential becomes less......

Ratch

That's not the entire gist of what I am saying. What I am saying is that, in natural convection, the temperature difference is dependant on the convection rate and that the convection rate is dependant on the external temperature. This relationship does not apply to forced convection which occurs at a constant rate.

 

BillO,

That's not the entire gist of what I am saying. What I am saying is that, in natural convection, the temperature difference is dependant on the convection rate and that the convection rate is dependant on the external temperature. This relationship does not apply to forced convection which occurs at a constant rate.

Isn't that backwards? Isn't the convection rate (the rate at which heat is lost) dependent on the temperature difference, and of course, the convection rate is dependent on the external temperature. Why would it not apply to forced convection. Unless the heat is replaced like a transistor heat sink, would not the convection rate slow down as the object temperature lowered, even in forced cooling?

Ratch

 

Ratch

Different shapes or different cup materials can be explained by a different convection heat transfer coefficient.

I don't recall introducing such a coefficient, what is this when it's at home?

Are you sure that they factored out radiation? Hard to beliee that if convection was exponential, good text books would not know about it by now. Does that pass the sniff test?

Sneering is the last resort of someone who considers themselves in danger of learning something from others.

This is a great pity as you have introduced some good and valid observations into the thread.

In particular you have noticed the thermodynamic difference between Newton's cooling experiment and Fourier's.

The cup of coffee in Newton's experiment had a lid on, to prevent evaporation. In thermodynamic terms it is a closed system. That is there is no energy input from the outside and no mass exchange with it.

Since you did not read the paper in my reference properly you missed the very clear statement that for free convection the cooling deviates significantly from direct proportionality.

For the benefit of others reading this thread I will state categorically that it has been proved experimentally by many others (including some of the names I mentioned earlier) that direct proportionality only applies to forced cooling.

Since you are interested in flat walls look up the work of Ezer, Griffiths and Davis. They experimented with exactly these.

 

Steve,
you don't need to get into solving second order partial differential equations to get some useful insight into matters thermal.

Interestingly all three modes of heat transfer (conduction, convection, radiation) can be reduced to sensible proportionality to the absolute temperature difference over suitably restriced ranges. Newton's law (forced convection) has been experimentally proved over very large ranges.

I'll post some stuff later as I have things on at the moment.

This bolsters the view that temperature difference is the driver behind the action, but remember that we are talking about heat (energy) transfer, not actual rate of cooling. And as Ratch observed it matters whether there is energy input ( replenishment) to the system or not. With replenishment we have to take an energy flow balance to calculate a rate of cooling.