> Kevin & Kim wrote:
>
> Not sure if I asked this already,,,, This is a PC question
with
airplane application.
> Does anodizing aluminum change the heat conduction property
of
aluminum? Specifically, does an AL heat sink with an anodized
surface
retain heat
more than
> the same with out hard surface annodization?
> Or, do different alloys of AL have different thermal
conductivity properties?
> Thanks.
> Kevin in Denver
Good question. A black surface radiates heat better than a
silver
colored surface.
Paul Lamar
Paul,
Black anodize does radiate heat slightly better. Only when a thin
layer is used though. The use of more than about .0003-.0005 in.
insulates the transfer to a
draw. Lots of study was done for aircooled auto engines. Black
body
radiation is a well studied fact. The problem is always with making
a
thin enough coating
that still has a heat transfer in the same range as the base
material.
Bill Jepson
-Dave Nieman wrote:
I just finished a infrared camera training coarse. Anything that is
shiny is a poor conductor of heat. All the show auto builders know
this... They make them shine but they can't get rid of the heat
worth a
flip. All show and no go. Black will radiate heat better than a
shinny
silver object. However it will do it even better if it is dull and
or
not very smooth.
All objects have 3 characteristics. They reflect, transmit and emit
energy. A good reflector is a poor emitter or transmitter of heat
energy. A smooth shinny piece of aluminum for example would radiate
heat
better if it was lightly sand blasted. A shiny surface works better
as
a mirror (reflector) not a emitter or transmitter which is what you
are
looking for when dissipating heat. You could paint it any dull color
and
increase its ability to transfer heat. Or is you blast it you have
no
worry of paint insulating the surface.
Dave Nieman
Al Gietzen wrote:
All this discussion of radiation heat transfer is interesting, and I
guess
everything said is correct, but - for a part exposed to air,
especially
moving air; the radiant heat flux at anything under a couple hundred
degrees
F is very small compared to conduction and convection heat flux. So
whether
it is black, white, silver, shiny or rough isn't going to make much
difference. Do what you think looks nice.
At room temp, radiation heat transfer is essentially zero. It
increases as
the absolute temperature (degrees R or K) to the fourth power, so it
goes up
quite rapidly with temp; but still small at temperatures seen on the
surface
of the engine. Now the exhaust pipe is a different story. At those
temps
the radiation may well be the dominant heat transfer mode.
At 100C (212F) the most effective of those surfaces will radiate about
1/2
watt per square inch. Very little. Convection/conduction to the air
could
be about than 10 times that. You can calculate radiant heat flux as:
H = e x s x T^4
where e is emissivity (less than about .8 for any surface, .1 for
shiny),
s is the Stefan-Bolztman constant (.0000000567) and
T is in degrees Kelvin. It's that S-B constant that will give you a
very
small number until T^4 gets very large.
Worry about radiation for any surface that gets hot enough that it
might
glow in the dark, like the exhaust manifold and pipe.
FWIW,
P.S.
Sorry; I meant to add to my pervious post that a 1500F exhaust pipe
might
radiate 40 Watts/sq in. It can add up.
Al
-Dave Nieman wrote:
Actually most energy that is expended through Convection and or
Conduction takes place outside of the visible light spectrum. Hence the
reason for infrared technology. Radiation takes place in the normal or
visible light spectrum. So I would agree with you. Emissivity is the
key. And shinny surfaces to do not emit well regardless of the color
Dave Nieman
Wright Runstad and Company
206-328-7798
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