Apparently, the black anodized aluminum outperformed the plain aluminum in a significant way...on the order of 5C reduction in semiconductor case temperature just by anodization.
But, yes, you understand my original intention for starting this thread. Basically, forgetting about "whether it's worth it or not", what would be the theoretical best coating/surface finish for a copper heatsink to maximize radiation performance?
Yay! A data point!
Ahhhh! A half a data point!
Critial information is: out of what HS and ambient temperatures?
I appreciate the calculations and visuals. Thanks. I understand you are showing the drop in thermal resistance, presumably from heatsink to ambient, for radiation. You noted that emissivity = 1, so you are assuming the heatsink material is a perfect black body radiator? Are the calculations you show independent of actual heatsink size, dimensions, and other factors?
Right. The way radiation works is, everything is radiating, all the time. Maybe not 100% in all frequency bands, but always something, somewhere. An ideal dielectric is the only thing that doesn't (there are ideal conductors but not at AC, so superconductors also radiate; granted, it's not much at their low operating temperatures). And vacuum is really the only thing of that sort. Which means the temperature of a vacuum is defined by the temperature of its bounding surfaces, if that's any help. (The universe itself is bounded by the cosmic horizon, which comes to us as cosmic background radiation ~2.7K.)
Radiation is a power density, so we can compare types of heat transfer in these terms, to the extent they are applicable to the other types anyway. It makes intuitive sense that bigger things can dissipate more heat. How much more, is the question; convection is very sensitive to geometry and orientation.
Conductivity, is linear or very nearly so, so we can approach it in the same way. It does depend on thickness being conducted through, but if we define that -- say looking at a particular thermal pad -- we find an area thermal resistance (r_th = thickness / bulk conductivity).
Can you explain what a concave and convex object is, in the context of this discussion? Obviously I am familiar with the terms in general. By concave, do you mean a radiating object which radiates back on itself due to its shape? Whereas, for an enclosure, the radiating surfaces (i.e hot surfaces facing "cool" ambient) are all pointing away from each other?
Basically this,
https://en.wikipedia.org/wiki/Convex_hullI'm not going to try and explain the whole thing in detail (as I don't understand radiation rigorously enough myself to be able to prove something like that..), but that's the sort of idea.
More specifically I think, if you look at any point on a surface, take however much solid angle of space is visible. Radiation is Lambertian, so integrate solid angle over the object. This should be better than the convex hull, as long spindly projections have excellent visibility, like a, think of whatever the star-shaped polyhedra are. Or, consider the point cloud in the article itself: if we suppose a small sphere around every point, the visibility of any given sphere (as in, solid angle not obstructed by other spheres) is very good, even in the middle of the cloud (though certainly worse there, and depending on density). We should expect such a configuration to radiate very effectively.
Though I'm also not sure if such shapes radiate as well as their convex hull, which takes up the same space (if the bounded volume is what's important in your application..) but has the most unobstructed surface area in that shape(?).
In any case, in terms of conventional shapes, heatsink fins, don't count towards radiation, at least very much. Broad surfaces or overall dimensions, yes. Pointy shapes, not so sure, but you rarely see, like, a pin array type where the pins are very far from the surface (and from each other), so it's at least not a question of high commercial interest.
Finally, the numbers you suggest 50% for convex objects, and 10-20% for concave objects...are you saying this is the fraction of the total energy released from the heatsink? (i.e. radiation + convection)
Yes, breakdown between radiation and convection.
Again, as mentioned, the comparison depends wholly on a reasonable figure for convection; the 150 figure I pulled out is probably from the low end of the scale, and obviously not the high end where radiation would handily account for that alone.
Tim