[Beowulf] Innovative liquid cooling

Prentice Bisbal prentice.bisbal at rutgers.edu
Thu Feb 28 13:29:10 PST 2013

On 02/28/2013 02:58 PM, Mark Hahn wrote:
>> consumption from 41 kW (electricity, probably same amount again for cooling)
> I really doubt it.  there is something profoundly wrong if an
>    HPC-type datacenter, with completely conventional servers, air/DX cooling
> runs at a PUE of more than about 1.3-1.4.
> immersion cooling sounds appealingly unconventional, but if you think
> about the heatflow, you've still got to move it around.  you still need
> a heatsink on the CPU with fins, some way to move the fluid past these
> fins and get them to the secondary heat exchanger.  as with airflow
> management in a conventional DC, surely one has to ensure that cool
> fluid gets to the CPU fins and heated fluid finds its way to the
> rejection exchanger.  surely convection wouldn't be good enough without
> some serious re-engineering.  or do these systems rely on boiling?

Actually, since the thermal conductivity of the liquid is so much better 
than air, you probably don't need finned heat sinks. Heat transfer rates 
is dependent on 3 factors: temperature difference, area the heat is 
flowing across, than the thermal conductivity of the conductor. The main 
reason for heatsinks is to increase the surface area to increase heat 

Since a liquid is about 1000 times better than air at conducting heat, 
you would only need about 1/1000 the surface area to have the same heat 
transfer rate as a gas, essentially eliminating the need for heatsinks.

Another angle is that when working with a liquid, the closely-spaced 
fins of a heat sink would retard heat transfer in a liquid. Why? Because 
the viscosity of a liquid is so much higher than a gas. The liquid can't 
flow through those narrow gaps between the fins, causing stagnant liquid 
which would collect the heat, but not carry it away through convection.

For a real-world example of this, think of a car's radiator, or 
hot-water baseboard heating. In both of the cases, the liquid flows 
through a smooth-walled pipe with metal fins on the outside. The fins 
are on the air side, not the liquid side where the increased surface 
area is needed. Putting fins on the inside would hind the flow of the 
liquid and create a thicker-boundary layer.

For the reasons stated above, I think air-cooling requires more 
engineering to do right than liquid-cooling.

Boiling at these temperatures would require a relatively volatile 
liquid, which would probably flammable (explosive), or bad for you in 
other ways (carcinogenic, etc.)  Change of state is very efficient for 
heat transfer, though. That's why many still consider steam heat to be 
the best form of heating.

> so I wonder what the effective thermal resistance is for such a
> fluid system (assuming some fishtank-like pumping).  risk-wise,
> I'm pretty sure I'd be more comfortable with a heatpipe inside
> the chassis mating to a coldplate/exchanger built into the rack
> running the water loop.
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