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Re: [Phys-l] Heat transfer lab



I seem to recall that the heat pipes included some sort of wick to return
the liquid to the hot end.




On Wed, Nov 17, 2010 at 8:54 AM, Michael Edmiston <edmiston@bluffton.edu>wrote:

I don't differ greatly with what Brian wrote except for some points that
need clarification in my view.

(1) He used slightly different words from what I usually hear. He used
"fluid displacement" and "fan aided convection" and said "convection
depends
on buoyancy effects." I think most people use the word "convection" to
cover all three processes of diffusion, natural convection, and forced
convection. So I would point out that... (1b) The wordings
"displacement"
and "fan-aided convection" are not needed (nor is "pump-aided convection").
We can just used the wording "forced convection." (1a) Buoyancy can be
involved in natural convection, but not always, and forced convection does
not depend on buoyancy. Incidentally, the Wiki article found under
"convection" strikes me as reasonably good.

(2) Heat pipes are more than simple gas-phase displacement. The
evaporation
at the hot end absorbs the "heat of vaporization," and this energy is
released at the cold end as the "heat of condensation." That transfers a
lot more energy than simply moving a gas that heats at one end and cools at
the other end (always remaining in the gas phase). Also, the expansion
from
liquid to gas at the hot end, and the contraction from gas back to liquid
at
the cold end acts as the "pump" that moves the vapor from the hot to cold
end. I think this would be classified as "natural convection" but it is
much more than buoyancy and diffusion dependent.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817

419.358.3270 (office)
edmiston@bluffton.edu


--------------------------------------------------
From: "brian whatcott" <betwys1@sbcglobal.net>
Sent: Wednesday, November 17, 2010 7:54 AM
To: <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] Heat transfer lab

On 11/16/2010 10:00 PM, Bernard Cleyet wrote:
Sci.Am. long time ago discussed these w/ the comment, carrying a bucket
of hot water was much faster than any conduction.

Reviewing heat transfer, conduction, convection, and radiation come
immediately to mind,
then recalling the topic of current interest, heat pipes, one adds
displacement.

As convection depends on buoyancy effects fan aided convection, which
amounts to fluid displacement, is faster.
Heat pipes use gas phase transport. Radiation runs at the speed of c.

Brian W
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l




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Oakland University
Dept. of Physics, (retired)
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