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Questions on Heat/Energy Transfer Mechanism(s)

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Posted by Morris Dovey on July 20, 2007, 9:05 am
How is heat energy transfered from one material to another?

This even feels like a dumb question, so please feel free to re-phrase
the question to make it "smarter"...

Is the mechanism identical no matter what the materials are?

Is the mechanism symmetrical? IOW, for any two materials are the
characteristics of energy flow independent of direction?

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Anthony Matonak on July 20, 2007, 11:03 am
Morris Dovey wrote:


Conduction, Convection, Radiation.

The mechanisms are identical no matter the materials.

They don't have to be symmetrical.

Solar collectors use this to their advantage. Glass is transparent
to most of the spectrum of solar radiation but not the infrared
which is radiated by objects that are just hot and now glowing.

This means sunlight (radiation) passes through the glass and
is absorbed by something which then gets hot. This absorber
will then re-radiate infrared radiation which is mostly blocked
by the glass. The heat transfer from this absorber is limited
to conduction and convection and these can be minimized.

Another example is a selective surface. This is a surface that
absorbs radiation better than it re-radiates it (or the other
way around). Selective surfaces are very popular for making solar
absorbers for obvious reasons.

Another example using convection is the use of automatic dampers.
Since convection requires flowing fluid, a damper can be setup
so that it opens when the flow is going in one direction and
closes when the flow reverses.


Posted by Morris Dovey on July 20, 2007, 11:43 am
 Anthony Matonak wrote:
| Morris Dovey wrote:
|| How is heat energy transfered from one material to another?
| http://en.wikipedia.org/wiki/Heat_transfer

Good references - thanks. If I read these articles correctly, heat
transfer involves some mix of molecular collision, atomic collision,
and/or radiation-induced electron path change. Did I misinterpret or I
miss any?

At the molecule and atom level, heat energy appears to equate to
kinetic energy. Is that a correct view, and should I look at the
electron path change the same way?

I'd never really thought about it before, but there's a lot of
difference in the masses of these participants...

| Conduction, Convection, Radiation.

These are old friends, but what I was after was how they work.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by nicksanspam on July 20, 2007, 2:56 pm
In conduction, atoms excite their neighbors. In convection, stuff moves
on a macroscopic level. Radiation can act over large distances, with
a 4x0.1741x10-8(T(F)+460)^3 Btu/h-F-ft^2 linearized conductance, where
T is the mean temp. For instance, a non-shiny floor and ceiling with
a 70 F mean temp would have a U1.04 radiation conductance.

Rich Komp would add phase change, which engineers tend to forget.
A fridge can gain lots of heat quickly when the door is open as
water vapor from room air condenses on cold surfaces.


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