OooohâŠ. liquid cooling video from sC18
Post by Lux, Jim (337K) via BeowulfIâll bet the surface is rough enough that there are plenty of nucleation
centers. Consider things like leads on parts.
*Date: *Thursday, November 8, 2018 at 7:47 AM
*Subject: *Re: [Beowulf] More about those underwater data centers
One comment - my dissertation below is specifically about non-ebullient
immersion cooling. As Jim Lux pointed out in a later e-mail, in ebullient
cooling, some kind of surface feature to promote nucleation could be
beneficial. Ebbulient cooling is a whole different beast from normal
(non-ebullient) immersive cooling, since in that case you have changes of
state and gas bubbles flowing through a liquid.
However, in all of the live and video demonstrations I've seen of Novec,
the processors were completely bare, bubbles were forming at a pretty rapid
rate, so again I think creating some sort of heat sink for this would add
cost with no significant benefit.
Prentice Bisbal
Lead Software Engineer
Princeton Plasma Physics Laboratory
http://www.pppl.gov
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
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
take two systems and roughen up the die package surfacewith sandpaper on
one. Compare temperatures.
ps. I can't resist adding this. Sorry Stu .
http://youtu.be/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 <
True enough.
Ebullient cooling does have some challenges â 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
http://youtu.be/CIbnl3Pj15w
--
Prentice
*Sent:* Tuesday, November 06, 2018 8:17 AM
*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
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
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