black anodized heatsinks

Black anodizing will help them radiate heat better, but that is not the primary cooling method for heat sinks. Moving air is what cools heat sinks and the color doesn't impact that at all unless you paint them which makes them work worse.

I would only anodize them for esthetic reasons or if it is very inexpensive. Pay attention to keeping the airflow unobstructed and you will do a lot more to increase their efficiency.

Phil
 
If you're a little adventurous, you can anodize yourself without too much trouble. See here: http://www.focuser.com/atm/anodize/anodize.html. What isn't mentioned on this page any more is that you can use common RIT fabric dye (available at Safeway etc.) instead of expensive commercial anodizing dyes. Only black RIT dye will be inferior to the commercial stuff, but other colours should be just fine. RIT dye is cheap enough that you don't have to bother trying to keep and preserve the solution to get max mileage out of it. :cool:
 
black buity

Hi,

what Alberto said (and gave as a calculation tool, thanks!) is totally right: by painting black the efficiency of the cooler will almost double!

At about 60°C in average (depending on the geometry) 30% of the heat are emitted by heat radiation, the rest of 70% by convection. And the emission-factor for this goes up from less than 0.1 to above 0.9. Factor 1 would be the ideal black radiator.

Also the conducted heat transport increases, because a part of this transport also is radiation. Of cause the main transport effect is convection – energy given directly from “metal-atom to air- molecule”.

So, silver coolers are almost nonsense, all effective coolers are black. Black is beautiful :cool: !

Regards

Klaus
 
105mAh/in^2 is close (900A/min/ft^2)... so that's roughly 16.3mAh/cm^2. If you follow the directions on that website, you should leave an additional margin... maybe 20%, and of course the time will vary with the acid temperature - the cooler the better too. I haven't tried this before, so I'm just going on what I've read, but it doesn't look very hard to do if you're careful.
 
Roberto Amato said:
Well, if we trust that Excel sheet, actually the plain polished aluminium sink roughly has a C/W about twice of the same black
anodized. A great deal of heat goes away through radiation.:devilr:
When happens to the heat energy radiated from a surface? Answer is it travels through space until it is absorbed by either air molecules or some solid object. Most of the heat is going to be radiated out perpendicularly from a flat surface. If you are talking about a flat plate radiator I agree that a black anodized surface makes an improvement, but we are talking about finned heat sinks.

The vast majority of the surface area in a finned heat sink is facing another fin in the same heat sink. That means the most of the radiated energy is just being reabsorbed by the fin right next to it. Net result is almost no improvement in heat transfer due to increased radiation. FYI. Black is also the optimal color for absorbing heat as well as radiating it.

By all means anodize your heat sinks if you like how it looks (I do), but don't plan on using a smaller heat sink because it's black.

Phil
 
haldor said:

When happens to the heat energy radiated from a surface? Answer is it travels through space until it is absorbed by either air molecules or some solid object. Most of the heat is going to be radiated out perpendicularly from a flat surface. If you are talking about a flat plate radiator I agree that a black anodized surface makes an improvement, but we are talking about finned heat sinks.

The vast majority of the surface area in a finned heat sink is facing another fin in the same heat sink. That means the most of the radiated energy is just being reabsorbed by the fin right next to it. Net result is almost no improvement in heat transfer due to increased radiation. FYI. Black is also the optimal color for absorbing heat as well as radiating it.


Very interesting indeed. I didn't think about it in these terms.



By all means anodize your heat sinks if you like how it looks (I do), but don't plan on using a smaller heat sink because it's black.
 
Sorry, previous message wasn't over... a very quick finger, that was.

In the same time, black fins radiating and absorbing each other gets much hotter than plain polished ones... taking heat away from devices. In the end this heat has to be transfered to the air, no other place to go. Possibly, the slanted sides of fins helps too, not radiating to each other squarely face to face.
Otherwise I can't understand why the C/W difference is so great, assuming that Excel sheet is trustworthy.

To say that (aesthetics aside) if you have to choose, choose black. :darkside:
 
Hello Roberto,

That was a very honorable reply. I hope I can do the same when it's my turn (probably in about 15 minutes based on my track record). :D

My readings on heat sinks indicate that turbulance (which mixes the air up as it flows over the fins) is the real key to improving the effectiveness of a heat sink. The biggest benefit from adding a fan is not so much the increase in airflow itself (which does help some), but rather in breaking up the laminar airflow over the fins that occurs with convection cooling. This is why heat sink manufactures recommend the fans blow onto the heat sink instead of pulling air through it (the second scenario is less turbulent therefore less effective).

I have also seen heat sinks designed with staggered rows of pins rather than fins to increase turbulance without adding a local fan.

Phil
 
haldor said:
Hello Roberto,

That was a very honorable reply. I hope I can do the same when it's my turn (probably in about 15 minutes based on my track record). :D


:D :D :D Phil, exchanging ideas peacefully is a real pleasure.


My readings on heat sinks indicate that turbulance (which mixes the air up as it flows over the fins) is the real key to improving the effectiveness of a heat sink. The biggest benefit from adding a fan is not so much the increase in airflow itself (which does help some), but rather in breaking up the laminar airflow over the fins that occurs with convection cooling. This is why heat sink manufactures recommend the fans blow onto the heat sink instead of pulling air through it (the second scenario is less turbulent therefore less effective).

I have also seen heat sinks designed with staggered rows of pins rather than fins to increase turbulance without adding a local fan.

Phil
This is interesting too, it could also explain why some fins surface is purposedly built very rough. My idea is that blowing air from under VERTICAL fins, so helping the natural tendency of hot air to rise, is the best way to take heat away using in the same time the least amount of RPM (and so the noise) on the fan. I'm building two monoblock SOZ using (only under the mosfets area) two small ball bearing PC fan, wide exactly as the fins are (plain luck). This way fresh air is where is most needed and the fans are almost invisible.

ciao,
Roberto
 
off topic...

Aahh! Brings back memories from my college days...

During my time studying Metals and Materials Engineering, I took several courses on Thermodynamics and "Transport Phenomena" (aka, thermo combined with fluid dynamics)... For a given fin dimension (height, mainly) and fluid viscosity, specific heat, etc.. there will be an optimum spacing which balances frictional flow losses against the convective contribution of the fluid heating in a small layer at it's surface, and will give the highest fluid velocity, and therefore the best heat transfer rate... all of the equations of course, only applied if you had laminar flow, and so you'd have to do a second calculation to ensure that the fluid flow remained laminar. If not, then calculation #3 would give you numbers for turbulent flows, and therefore a different optimal spacing etc. Each calculation was over a page of cramped handwritten equations.

<hr>
Check this out:

:up: My favorite engineering problem of all time went something like this:

You have a case of warm beer at temperature X. Assuming the cans are perfect cylinders with dimensions (_h, _d), how do you stack them in the refrigerator (temp Y) to chill them to temp Z fastest?

Is that a classic 'geer problem or what! :rolleyes:

It turns out to be a <i>lot</i> more complicated than you may think... First, you calculate black body radiation losses based on surface area and emissivity (the prof allowed us to ignore re-absorption by adjacent cans... you'll see why later!). This gives you some differential equation describing an exponential temperature decay, but you don't find the numerical solution, because this equation needs to be combined later with the fluid convective cooling equations to produce some other, much more complex differential that describes the temperature curve...

You then had to work out the solution for optimum inter-can distance. But there were several cases for the orientation of the cans: standing vertically, laying parallel and laying end-to-end. IIRC, the vertical case was tricky because you didn't have parallel flat plates, you had parallel vertical cylinders, so the optimal distance would be affected by the surface curvature - part of each can would be closer than optimal, and part would be further away than optimal. Some simple calculation reduces this to a prallel flat-plate problem, I think, with a linear factor to get the actual inter-can distance afterwards. With the cans vertically, you could ignore convection from the ends of the can - a surface which had to be included in the convection calculations for the other two orientations. The parallel plate simplification is not needed for the cans laying parallel, since there's a nice set of equations for flow perpendicular to cylinders.

But, there was another factor which you had to consider: you had to find the fastest <i>overall</i> cooling time to temp Z. This means that the initial temperature differential couldn't be used in the calculation of optimal distance. Instead you had to do an additional proof that the optimal distance should be calculated for a lower temp somewhere between X and Z. We'll call that temp W. W is a trade-off for the lower fluid convective efficiency due to non-optimal distance at temps above and below W, accounting, of course, for the integral of the cooling rate curve above and below W... This calculation was truly huge, and had to include the radiative heat losses from the first calculation. With both the fluid equations and the radiative cooling equations in there, this differential balooned in a big hurry! In many cases, you'd just replace big chunks of the equation which appear more than once with a single symbol. Then, you could write the whole thing on maybe two lines, if you wrote really small. Assuming laminar conditions at all times, and after a bunch of carefully executed manipulations, everything neatly cancels and W drops out of this big messy differential. Now you had the temp you should use in your steady-state fluid flow equation to calculate optimum distance. Don't forget to multiply by your factor to get back to cylinder distance instead of the parallel plate simplification! The final step was to back-check that your flow was still laminar <i>for the closest inter-can distance on the curved surface</i>, and at the maximum temp differential Y-Z.

Repeat above for each of the other can orientations :att'n: :bawling:

Finish problem at 6am. Hand in assignment at 8am. Drag tired a** through rest of friday lectures... :drink: ... :drunk: ... :faint:

Ahh, good ol' university. I have to say, despite the difficulty, I actually enjoyed my thermo courses more than any of my electrical engineering courses. Maybe it was the great prof(s), maybe it was the small class/department, or maybe I really am a just freak for big equations that do cool stuff! I really miss that stuff. I still have my all-time favorite textbook: "An Introduction to Transport Phenomena in Materials Engineering" by David R. Gaskell. Or, as we used to refer to him "Gas-kill"! Death by gaseous flows! Maybe it's time to take a flip through that book again, just to put to rest these nostalgic thoughts!