We recently had some discussion on this subject. On another list a
knowledgeable
person posted the following(posted here with his permission) -
>>There seems to be a lot of misinformation on the thermodynamics of heat
dissipaters. I hope I can clear some of it up. I have many years in designing
infrared missile sensors in which the seeker housing is emitting radiation at
the same wavelength you are trying to see. It's like having your camera body
emitting white light and trying to take pictures without fogging your film.
This is the exact same issue or problem in trying to take heat out of a system.
Heat dissipation has nothing to do with mass. It has to do with the
materials emissivity. In a nut shell, it has to do with the surface
reflectivity at
the wavelength you want to emit at.
Thermal radiation is mostly between 8 and 14 microns (visible is 0.4 to 0.7
microns). Just because a material looks black does not mean it will be a good
thermal emitter. You MUST look at the surface in the LWIR (long wave
infrared, 8-14 microns). There is emissivity data on the internet for
different
surface treatments. Cadillac Black works well (flat black paint), but black
anodized aluminum is not great.-
In addition, thermal emission is NOT normal to the surface. It is basically
lambertian, with the zero angle normal to the surface. The amount of thermal
energy being emitted peaks at normal incidence and drops to maybe 30-40% at
90 degrees. Fin separation is not that important if you have air moving over
the surface. Surface area and surface emissivity are the two most important
factors in determining the efficiency of thermal emittance. Just make sure you
blow air over the surface, especially in between closely spaced fins.
Circulating coolant also works great, like anti freeze. That will carry away
the
energy better than air.
Fin geometry is not important provided you get enough air or coolant flow
between them. The only reason some designs use thin fins is to reduce overall
mass and for the ability to space the fins closer together (like a car
radiator). Thickness is not a driver in terms of thermal emittance, but there
ARE
reasons for going to thin fins.
--
Paul <<
Thought this might still be of interest.
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