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-boun...@beowulf.org] *On Behalf Of *Prentice Bisbal via Beowulf
*Sent:* Tuesday, November 06, 2018 8:17 AM
*To:* beowulf@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
    sdm...@gmail.com <mailto:sdm...@gmail.com>




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