"For a 10 cm diameter device, an air gap resistance of 0.02 C/W is certainly feasible. I read one comment a couple of days ago where someone asserted that the laws of thermodynamics state that air is an insulator. The key thing to keep in mind that it’s a matter of degree. Air isn’t incapable of conducting heat. Moreover, the violent shearing of the air residing in the gap region between the stationary and rotating surfaces generates substantial convection. In fact, in later version we’re experimenting with the introduction of surface textures that do a better job of disturbing the air to promote convective heat transfer than when you simply have two smooth surfaces.
Some commenters suggested filling the air gap region with some kind of thermally conductive fluid. This seems logical, but the problem is that frictional shearing losses become prohibitively large even at low rotation speeds. It’s not that it wouldn’t work at all, but that it wouldn’t work very well; thermal conductivity is important, but so is viscosity. If you can’t spin the heat-sink-impeller reasonably fast, you don’t get much of a boundary layer thinning effect."
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