studiot,

Quote:
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?

Home or work makes no difference. The coefficient comes from the equation expressing Newton's Law of Cooling.

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.

That is the trouble with only written communication. It is sometimes hard to tell what attitude a person has.

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.

The word "free" is not mentioned in the paper at all. The word "natural" as in natural convection is only mentioned twice in the same paragraph on page 17. The first mention is about "vertical current of convection" and the second mention says "Much more important is the fact that as cooling proceeds in still air and the surface to air temperature difference diminishes, the natural convection generated by this temperature difference diminishes. The heat transfer coefficient therefore progressively diminishes and cooling is retarded more and more." You see, I have a problem with that. The very definition of convection is moving fluid, so if the fluid is still, we don't have convection, only conduction.

Another problem area is the first paragraph which states. "By cooling we mean only the fall in temperature, not the transfer of heat." I ask, how can the temperature of a body fall if no heat energy is removed?

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.

And were there other non-Newtonian factors present? Remember Newton's law is for the rate of cooling at a temperature point, not the temperature of an object while cooling. If anything interferes with that rate, the temperature at a particular time will not be the expected value.

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

If their results are on the web, I will try to find time to do so.

Ratch

 

BillO,

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?

It has already been done. When I built a power supply many years ago, I put in a couple of heat sinks with staggered fingers for the pass transistors. They were said to have superior cooling characteristics because they caused the air to burble, thereby picking up more heat to carry away.

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....

And here I thought we were going to find out where Newton's Law and Fourier's law did not apply, as stated in the first post of this thread. Did I miss something?

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

Where was heatpipe mentioned in this thread before?

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

My money says that is exactly where Newton's Law applies. And perhaps other laws as well.

Ratch

 

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.

Here is a simple physical justification for basing rate of cooling on temp difference for all three modes. Sorry about the handwriting, but I didn't feel up to fighting the TEX.

 

That scenario kind of begs the question of how the object achieved its spinning state in the first place..