[Beowulf] oil immersion cooled blades

Prentice Bisbal prentice at ias.edu
Fri Mar 16 07:51:49 PDT 2012


On 03/15/2012 11:47 PM, Mark Hahn wrote:
>>> I guess I'm a bit skeptical about the utility of this approach -
>>> would be nice if they had some technical literature.  something about
>>> thermal resistance.  define how the oil bath dumps the heat (water
>>> hookups in the back?)  comparison to modern heatpipe-based solutions, etc.
>> No need for water hookups. You can circulate the oil through an oil to
>> air heat exchanger. That's what those green cooing guys at the SC
> is that really better than going to air directly?
> I guess I'd like to see the the numbers - to my way of thinking,
> it's almost all about the thermal resistance.  transferring heat to oil, 
> then to air, means two stages of resistance.  using oil would permit
> a bigger air interface, though I suppose.
Oh boy... I'm having flashbacks of college. The short answer is "yes"
that is much better than going to air directly. I'm tying to think of a
simple way to explain why, and I'm starting to have flashbacks of my
Chem Eng. classes: Thermodynamics, Transport Phenomena... ugh.

Here's an attempt at a longer explanation. Sorry if I'm rehashing what
you already know.

Yes, going from oil, or any liquid, to air is better than using air
directly. That is why car engines are liquid-cooled instead of air
cooled. It comes down to two physical properties: Thermal conductivity,
and thermal capacity.

Thermal conductivity is the ability for a material to move energy in the
form of heat from one place to another. Thermal capacity is how much
energy in the form of heat can be stored in a given quantity (mass or
volume) of a substance.

In general, liquids and solids have better thermal conductivity and
capacity than gases. We use liquid for cooling because the liquid's
superior conductivity draws the heat away from the object to be cooled
quicker, requiring less surface area, and less "residence time" for the
coolant. The superior capacity allows us to use less volume of coolant
to store the same amount of "heat" (energy). And liquids are better than
solids because they flow, and the bulk movement allows us to easily
transport the heat stored in the coolant away easier than if were trying
to move solids.

Using liquids allows to do draw heat out of tiny confined spaces, like a
1U server, or a cars engine block, that we can't do effectively with
air.  Once we draw that heat out, it needs to go somewhere, which, in
most cases, is atmospheric are, so we pump the liquids to a cooling
tower outside, the radiator in the front of  our engine bay, or
somewhere where is a large amount of air and room for all the surface
area the heat exchanger will need (since air has a low thermal
conductivity, it needs a lot more surface area to transfer the heat into
it than a liquid does). This last point you hit on in the last sentence
of your post. That is why  car radiators are large and have lots of
think aluminium fins in them  (more fins = more surface area for the air
to contact), and we have massive cooling towers or heat exchangers
outside our office buildings.

You are correct in that there are inefficiencies when you transfer
energy from one material to another, but those are usually trivial and
offset by the greater effectiveness of liquids in pulling the heat away
from the heat source in the first place.


I hope that helps.

Prentice




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