Re: [Phys-l] Heat transfer lab

[Michael Edmiston <edmiston@bluffton.edu>]

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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