[Beowulf] immersion cooling for ASIC Bitcoin miners

Prentice Bisbal prentice.bisbal at rutgers.edu
Wed Nov 27 11:36:27 PST 2013


On 11/26/2013 10:22 AM, Ellis H. Wilson III wrote:
> On 11/26/2013 07:22 AM, Bogdan Costescu wrote:
>> One of the pics shows a bubbling liquid surface, we've seen recently
>> similar pics. From what I remember from past discussions on this very
>> list, the bubbles are bad for heat transfer - it's OK to have them at
>> or very near to the surface, but not down below. Has the physics
>> changed behind my back ?
> <queue non-physicist comment, 99% chance of being wrong>
>
> I have heard (a long time ago, admittedly) from little overclocking
> birdies that in poorly-designed or tightly-packed arrangements, putting
> an aquarium bubbler or some such hacky bubbling mechanism allows you to
> avoid the hotspots in the oil you would otherwise suffer from around
> your main heating components (passively cooled bridges in home setups
> are known issues here).  Overall bubbles don't help heat dissipation
> (and may hurt it as suggested), but if your problem isn't the overall
> temp of the tank but instead making sure the temp is fairly average
> throughout, bubbles may help.  I've kept my mouth shut on the matter so
> far because I have no proper training in the area to ground this
> suggestion on.
>
> <dequeue>
>
> Going back to my storage hole now.
>

I already addressed Bogdan's question in a separate e-mail, but I want 
to address why this might work:

I suspect the bubbles are acting as stirrers that pull some of the 
liquid with them as they move upward, helping natural convection, or 
"stir" the liquid.  Even though liquids have higher thermal capacities 
and conductivies than gases, you still want the liquid to circulate so 
that the liquid that has absorbed the energy and heated up moves away 
from the heat source, allowing colder liquid to take it's place.

The reason for this is that heat transfer is proportional to the 
temperature difference between the heat since and heat source, so the 
best way to maximize heat transfer is to maximize the temperature 
difference between the since and source. This kind of movement helps 
this. If the liquid moves, you get convective heat transfer, which is 
much more effective than just conductive heat transfer.

--
Prentice


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