[Beowulf] More about those underwater data centers
Prentice Bisbal
pbisbal at pppl.gov
Thu Nov 8 07:40:04 PST 2018
Heat fins are used to increase the surface area used for heat transfer,
since the rate of energy transfer by conduction is directly proportional
the surface area. Heat fins are needed when air is involved because air
has such a low thermal conductivity.
Thermal conductivity of liquids are much high, so heat fins aren't as
necessary. For example, I've read that water can transfer heat orders of
magnitude better than air, so using water to remove hear from a
processor would need orders of magnitude less surface area for the same
energy transfer rate.
Also, liquids have higher viscosities than gases, so we have to worry
about 'boundary layers'. A boundary layer is area where the edge flowing
fluid is in contact with a solid. The friction between the liquid and
the solid slows down the fluid near the solid. This affects both gases
and liquids, but since liquids have higher viscosities, the effect is
more noticeable.
Think about a car's radiator - the air side has all the fins on it, and
the liquid side has smooth pipe walls.
https://en.wikipedia.org/wiki/Boundary_layer
Convection heat transfer is an equally important mode of heat transfer
in fluids, and in the boundary layer, where the liquids aren't moving as
fast, heat transfer isn't as good, so you need to keep your boundary
layer from becoming too thick.
Since fluids have much higher thermal conductivities, and boundary layer
effects are more of a concern, I actually think a smooth heat transfer
surface would be better in these immersion cooling cases. I'm sure
smaller, more spaced out fins would probably help heat transfer
without creating too much of a boundary layer, but making those heat
sinks adds cost for increased performance in a situation where it
probably isn't needed.
Now direct-contact cooling systems like Asetek products do have fins on
the liquid side, if I remember correctly, but that in those systems,
there are pumps to provide forced convection. In immersion cooling, you
are relying on natural convection, so there isn't as much driving force
to overcome viscosity/boundary layer effects to force the liquid through
the heat fins.
That's my thoughts, anyway.
Prentice
On 11/07/2018 04:12 AM, John Hearns via Beowulf wrote:
> Thinking about liquid cooling , and the ebuillient cooling, the main
> sources of heat on our current architecture servers are the CPU
> package and the voltage regulators. Then the DIMMs.
> Concentrating on the CPU die package, it is engineered with a flat
> metal surface which is intended to have a thermal paste to transfer
> heat across to a flat metal heatsink.
> Those heatsinks are finned to have air blown across them to transport
> the heat away.
>
> In liquid immersion should we be looking at having a spiky surface on
> the CPU die packages and the voltage regulators?
> Maybe we should spray the entire board with a 'flocking'' compound and
> give it a matt finish!
> I am being semi-serious. I guess a lot of CFD simulation done
> regarding air cooling with fins.
> How much work has gone into pointy surfaces on the die package, which
> would increase contact area of course and also act as nucleation
> points for bubbles?
>
> One interesting experiment to do - assuming the flat areas of the CPU
> in an immersive system do not have (non thermal paste) heatsinks
> bolted on:
> take two systems and roughen up the die package surfacewith sandpaper
> on one. Compare temperatures.
>
> ps. I can't resist adding this. Sorry Stu .
> https://www.youtube.com/watch?v=kHnifVTSFEo
> I guess Kenneth Williams is a typical vendor Site Engineer.
> pps. the actress in the redress had her career ruined by this film -
> she ver got a serious role again after perfectly being typecast.
>
>
>
>
>
>
>
> On Tue, 6 Nov 2018 at 22:57, Prentice Bisbal via Beowulf
> <beowulf at beowulf.org <mailto:beowulf at beowulf.org>> wrote:
>
> On 11/06/2018 02:03 PM, Lux, Jim (337K) wrote:
>>
>> True enough.
>>
>> Ebullient cooling does have some challenges – you can form vapor
>> films, which are good insulators, but if you get the system
>> working right, nothing beats phase changes for a heat transfer.
>>
> If I recall what I learned in my Transport Phenomena classes in
> engineering school, you need a reasonably high temperature
> difference to get a stable film like that. For that to happen,
> radiant heat transfer needs to be the dominant heat transfer
> mechanism, in the range of operation we are talking about, the
> temperature difference isn't that great, and conduction is still
> the dominant form of heat transfer.
>
> Here's an example of what 3M Novec ebullient cooling looks like.
> It doesn't look like it's anywhere near the film boiling regime:
>
> https://www.youtube.com/watch?v=CIbnl3Pj15w
>
> --
> Prentice
>
>> *From:*Beowulf [mailto:beowulf-bounces at beowulf.org] *On Behalf Of
>> *Prentice Bisbal via Beowulf
>> *Sent:* Tuesday, November 06, 2018 8:17 AM
>> *To:* beowulf at beowulf.org <mailto:beowulf at beowulf.org>
>> *Subject:* Re: [Beowulf] More about those underwater data centers
>>
>> . And serviceability is challenging. You need to pull the
>> "wet" boards out, or you need to connect and disconnect fluid
>> connectors, etc. If you're in an environment where you can
>> manage that (or are forced into it by necessity), then you
>> can do it.
>>
>> I think everyone on this list already knows I'm no fan of mineral
>> oil immersion (It just seems to messy to me. Sorry, Stu), but
>> immersion cooling with other liquids, such as 3M Novec engineered
>> fluid addresses a lot of your concerns. It as a low boiling
>> point, not much above room temperature, and it was originally
>> meant to be an electronic parts cleaner (according to a 3M rep at
>> the 3M booth at SC a few years ago, so if you pull a component
>> out of it, it dries very quickly and should be immaculately clean.
>>
>> The low boiling point is an excellent feature for heat transfer,
>> too, since it boils from the heat of the processor (ebullient
>> cooling). This change of state absorbs a lot of energy, making it
>> very effective at transferring heat away from the processor. The
>> vapor can then rise and condense on a heat exchanger with a
>> chilled water heat exchanger, where it again transfers a lot of
>> heat through a change of state.
>>
>> Prentice
>>
>> On 11/05/2018 06:30 PM, Stu Midgley wrote:
>>
>> I refute both these claims.
>>
>> You DO want to run your boards immersed in coolant. It works
>> wonderfully well, is easy to live with, servicing is easy...
>> and saves you almost 1/2 your power bill.
>>
>> People are scared of immersion cooling, but it isn't that
>> difficult to live with. Some things are harder but other
>> things are way easier. In total, it balances out.
>>
>> Also, given the greater reliability of components you get,
>> you do less servicing.
>>
>> If you haven't lived with it, you really have no idea what
>> you are missing.
>>
>> Serviceability is NOT challenging.
>>
>> You really do NOT want to run boards immersed in coolant
>> - yeah, there's folks doing it at HPC scale
>>
>> Whatever the coolant, it leaks, it oozes, it gets places
>> you don't want it to go. And serviceability is
>> challenging. You need to pull the "wet" boards out, or
>> you need to connect and disconnect fluid connectors, etc.
>> If you're in an environment where you can manage that (or
>> are forced into it by necessity), then you can do it.
>>
>> --
>>
>> Dr Stuart Midgley
>> sdm900 at gmail.com <mailto:sdm900 at gmail.com>
>>
>>
>>
>>
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