Electronics > Projects, Designs, and Technical Stuff
Thermal Management : How to go fanless in a small space?
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ebastler:
@3roomlab, I think I understand your calculations of heat conductivity. But how did you estimate the radiation??


--- Quote from: 3roomlab on April 27, 2019, 08:23:46 pm ---now then, say we have the 3cm radii PGS radiating to air. this becomes a tricky estimate. a thickness of approx 2mm seems to give plausible numbers.
C/W = thickness/(w/mk x area) = 0.002/(0.024*0.00283) = 29.4 C/W
add the disc radii resistance + 2.498 = 31.9C/W

--- End quote ---

Are you saying that you just modelled conductivity (through a layer of what material?), and postulated some arbitrary layer thickness to make the numbers "fit"? If so, fit to what?

And what about convective transport, which happens in additon to radiation?

I am truly curious about good (i.e. quantitative and simple) ways to estimate radiation and convection. If you have experience with such methods, could you  please eleborate?
daslolo:

--- Quote from: ebastler on April 27, 2019, 06:16:36 pm ---No, I am afraid that will not work:

* First, the heat needs to be transported to the surface. A plastic case will have bad heat conductivity, and you will not change that in any way if you treat its outer surface.
* Heat transfer away from the surface, whether by convection or radiation, is proportional to the surface area. You would still need large fins, with at least comparable area to the aluminium heatsinks.
* Radiation of energy works best for a perfectly black surface. A suitable surface layer (PGS, paint...) can probably be slightly better than black-anodized aluminuim, but not a whole lot. I dont see how you could even come close to offsetting the limited conductivity of the plastic.
* Convection cooling does not depend much at all on the surface finish, to my knowledge. Surface area and thermal conductivity (to transport heat to the surface) rule here. So the only way your "plastic heatsink" could compensate for its bad conductivity vs. aluminium is by an even larger surface area.
--- End quote ---
transport is via a heatpipe made of a slice of PGS. It seems to work so far.
surface area would be the surface of the goggles, like the idea someone floated earlier about making the case aluminium.
it's not a plastic heatsink but a heatsink layer taped on the plastic body


--- Quote from: 3roomlab on April 27, 2019, 08:23:46 pm ---this is my approximations or rant

the PGS mechanism, from the spec we have 1000W/mK. assume PGS70 is stuck on a heat source 1cm x 1cm
on this end we can approximately calculate the heater to PGS thermal resistance
C/W = thickness/(w/mk x area) = 70e-6/(1000*0.01^2) = 0.0007 C/W (this did not include the glue, which they said requires some pressure)
...
but you can now try and guess things like sun heating, and it no longer dissipates heat properly.

--- End quote ---

Power draw is 8W. What heat W is produced you think?
Could you run your math with:
1- a layer of PGS70 glued to the surface of the goggles?
2- 14x14x10mm pin copper heatsink pointed 45 degrees down mounted at the bottom where the blower was https://www.amazon.com/dp/B002BWXW6E/ref=psdc_2998409011_t1_B077VM8FJW
3- a 30x30 fan blowing air out of the case
4- a 20x20x6 fan blowing air out of the case
5- arctic silver adhesive instead of the Panasonic stuff
6- use of SSM mazipan looking thing vs PGS A-M adhesive PGS

Thanks
coppercone2:
Yea no one is gonna like this suggestion: I recommend thin walled heat exchanger with liquid or forced air (in plumbing) cooling. for the electronics and the face contact area. Possibly even two seperate loops so you can keep your face comfortable. And forehead band.

If its possible. Check the design weight when loaded with fluid to make sure its good. See if you can cool the face a bit too.

Complicated and expensive but its on your face and its gonna be fucking annoying. I can imagine now you wanna fly outdoors on a nice summer day and its 30C+ and your standing in the sun on a air field. Fuck that. I would literary go to stand by a damn picnic table with a big ass power pack that powers a peltier pump that connects to the goggles with coolant. And it might keep it lighter.

Why inefficient peltiers? so it can tune the loop to a temperature that you find comfortable on your face. Dual heat exchanger (one is peltier-air/coolant other is goggles/face/transistor-coolant. It would require a powerpack like a fanny pack at the very least and possibly a small backpack but IMO worth it. Gonna pull that thing off at 6-7PM then the fucking gnats are gonna land on your sweaty face and its gonna attract mosquitos and all that nasty shit.

Fucking hate head-lamps too for that reason, when I do out-doors stuff I like being by water (lake) in the evening and any kind of heat, sweat, light... swarms your ass. Unless its something tactical.. go for luxury here. Man I got itchy just thinking about wearing some black plastic goggles in the sun At least a welder helmet is loose on the eyes!  |O
Marco:

--- Quote from: daslolo on April 27, 2019, 05:49:06 pm ---You're saying that thermal resistance goes up the thinner the PGS is?

--- End quote ---

Yes, the thermal conductivity goes up slower than the area of the cross section goes down, so the thermal resistance indeed goes up.

--- Quote ---Why is the thermal conductivity higher on 40 than on 70?

--- End quote ---

Dunno, structure is more regular I guess. But thermal resistance is Length/(Area * Conductivity), 1000*70 is larger than 1350*40 so thermal resistance for the 70 um sheet is smaller.

If you do the math that also shows why you really don't just want to make a long sheet into a wavy heatsink ... it has relatively low thermal resistance, but not that low. It's nothing like a real heatpipe.
ebastler:

--- Quote from: daslolo on April 28, 2019, 01:52:14 am ---Power draw is 8W. What heat W is produced you think?

--- End quote ---

I'm afraid you are way out of your depth here.  :-\

That whole goggle design looks complex enough, not like a one-man effort. Aren't there others on the team who can get involved with the heat management questions? I don't think this can be designed between just you and a couple of forum members who have seen a few photos.
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