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Originally Posted by Xakepa
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-The surface could be twice as big, but also could be much more - all you have to do is to scratch the penny with a Dremel or a rough file. Surface could easily go up tenfold, so the thermal conductance. I love to scrub the paint and scratch the bottoms of the heatsinks in my computer. ... snip ...
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I doubt this is the case. The Reynolds # confirms that the size of air molecules is fixed. For instance, increasing surface area by thinning fins has no improvement unless you make certain there is also a corresponding space for the air. Your scratches will still be seen as a single surface by the air molecules. In fact, heat transfer may be inhibited, because the optimum surface area
exposed to the air is actually a smooth surface.
This is analogous to the use of thermal grease - the greatest contact is if the mating surfaces are smooth. Any roughenss will inhibit the thermal contact, and thus, the heat transfer. In the case of contact with air, air is simply another contact surface (although a moving one, hence -
convection). Increasing the area of that contact surface has benefit, but not through simple surface roughness - those are not "fins."