I mean there's no shortage of them from suppliers,
https://www.digikey.com/short/h3btm0p9you're welcome to use them any way you like of course. But I also have plenty in my junk collection, if you stipulate things in finished products (many of which were indeed passive convection).
You can try it out by wrapping a laptop power brick in some aluminium foil and see how much hotter it gets. Since the aluminum is thin and well conductive it shouldn't hamper any conductive cooling, but it will stop thermal radiation dead in this tracks.
Not quite, though for a couple of maybe tricky reasons?
The air space between enclosure and foil, will drop quite some temperature. So it will take time for the brick to heat up to its new equilibrium temperature. That's fine. At that point, the
foil surface will be at the equilibrium outer surface temperature, and will be slightly hotter as you suggest.
However, if you go to measure it, by putting your hand on it, or touching it with a thermocouple say -- you will feel the
internal temperature, because the foil has almost no heat capacity and you're smooshing it into the inner surface and feeling that. For the thermocouple, it will hardly make thermal contact anyway, or if greased, its heat capacity again dominates over the thin layer of foil (even if you aren't pressing it against the enclosure in the process -- which because of the point contact, is probably okay in this case), or its conductivity just as well.
And it's no good trying to measure it with a thermal cam, for obvious reasons.
So, while that's technically true, it's a devil to try and actually measure, and I wouldn't suggest doing it this way (besides the small risk of cooking the poor brick like a hot potato
).
So, if you don't mind getting a bunch of sticky goo on your power brick -- you can do the same test but with say adhesive foil tape, or do it with plain foil but grease it the fuck up with heatsink compound so it makes good thermal contact with the enclosure. (Tape adhesive shouldn't be much worse than the enclosure already is, and it's pretty thin; I wouldn't expect it to drop much temp here.) Then you can get a good reading through either method (touch or instrument). You should find the temp is a couple degrees higher.
And this is of course keeping everything constant, so, same load, same supply voltage, etc.
Once there is a fan or some ducting that actively moves air trough a heatsink, those will indeed usually be bare aluminium. This can sometimes raise the conductive cooling potential of a heatsink up to 10x or even more, so at that point the radiated cooling is so low that could be pretty much ignored.
I've seen plenty of forced-convection heatsinks in colors, too! Anodize really doesn't amount to anything (it's modestly conductive, porous aluminum hydroxide) so it's mainly just used for visual appeal (look at all the nice shiny colors in CPU heatsinks etc.) and corrosion resistance.
For the cooling potential to differ by "10x or more", it would have to be compared to some
really thick paint, or very high flow rates (or fluid cooling)... not saying that's impossible, but it seems unlikely for most product designs. A much less alarming "50%" might be more in the ballpark, but really, without any qualification at all it's an utterly arbitrary number, so I guess my real point is, why bother giving any figure at all?... Anyway, I digress.
Tim