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Old 26th August 2012, 11:19 PM   #1
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Question thermal emissivity question

First, let me say that I'm not a physics or engineering student looking for an easy answer. I'm an idiot looking for an easy answer.

If I have an aluminum tube with cool air flowing through it, what is the best scenario to mitigate heat transfer from the outside air?

Inside/outside polished?
Inside/outside black?
Inside black, outside polished?
Inside polished, outside black?

Thanks
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Old 26th August 2012, 11:36 PM   #2
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If you're trying to isolate from around the outside of the tube, polish the outside.

The interior finish wont make much difference since any radiation will mostly be picked up by other portions of the tube. Just leave it natural.
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Old 26th August 2012, 11:40 PM   #3
jcx is offline jcx  United States
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wrap it in insulation? - “super insulation” combines convection blocking with layers of low emissivity reflective foils

the radiative coupling to air only occurs where the air is opaque to the radiation wavelengths

you are more often radiatively coupled to other opaque materials in the field of view of the radiating surface – can be clouds, cloudy nights are warmer than clear – you can freeze water by radiative cooling in clear dry high desert summer nights
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Old 26th August 2012, 11:56 PM   #4
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Thems big words there! If you meen heat trasnsfer though:
An unpolished surface has more surface area, therefore more heat transfer, negligible though. If the heatsink is in the dark, it does not care what colour it is. I have not done any tests, but I would assume that anodized aluminium would have a lower rate of heat transfer than natural alu. E
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Old 27th August 2012, 12:08 AM   #5
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Thanks guys. The tube is already semi-polished, unfinished aluminum, so I guess I'll just give it a good shine.

I'll look into insulation too.
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Old 27th August 2012, 09:30 AM   #6
DF96 is offline DF96  England
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If your concern is heat transfer from the outside air, as you say, then insulation is the answer; polishing will make little difference because air is not a good radiator or absorber of heat. If it is heat transfer from nearby (radiating) objects then polishing is the answer, although appropriate insulation will work too.
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