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<p class="MsoNormal">The “boilers” for high power tubes have “warts” all over the inside, specifically to provide nucleation sites.<o:p></o:p></p>
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<p class="MsoNormal">But this brings up a whole bootstrapping thing – use a cluster to do CFD for the cooling for the next cluster.<o:p></o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">Beowulf <beowulf-bounces@beowulf.org> on behalf of "beowulf@beowulf.org" <beowulf@beowulf.org><br>
<b>Reply-To: </b>John Hearns <hearnsj@googlemail.com><br>
<b>Date: </b>Wednesday, November 7, 2018 at 12:13 AM<br>
<b>To: </b>"beowulf@beowulf.org" <beowulf@beowulf.org><br>
<b>Subject: </b>Re: [Beowulf] More about those underwater data centers<o:p></o:p></span></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">Those heatsinks are finned to have air blown across them to transport the heat away.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">In liquid immersion should we be looking at having a spiky surface on the CPU die packages and the voltage regulators?<o:p></o:p></p>
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<p class="MsoNormal">Maybe we should spray the entire board with a 'flocking'' compound and give it a matt finish!<o:p></o:p></p>
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<p class="MsoNormal">I am being semi-serious. I guess a lot of CFD simulation done regarding air cooling with fins.<o:p></o:p></p>
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<p class="MsoNormal">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?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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:<o:p></o:p></p>
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<p class="MsoNormal">take two systems and roughen up the die package surfacewith sandpaper on one. Compare temperatures.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">ps. I can't resist adding this. Sorry Stu . <a href="https://www.youtube.com/watch?v=kHnifVTSFEo">https://www.youtube.com/watch?v=kHnifVTSFEo</a><o:p></o:p></p>
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<p class="MsoNormal">I guess Kenneth Williams is a typical vendor Site Engineer.<o:p></o:p></p>
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<p class="MsoNormal">pps. the actress in the redress had her career ruined by this film - she ver got a serious role again after perfectly being typecast.<o:p></o:p></p>
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<p class="MsoNormal">On Tue, 6 Nov 2018 at 22:57, Prentice Bisbal via Beowulf <<a href="mailto:beowulf@beowulf.org">beowulf@beowulf.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On 11/06/2018 02:03 PM, Lux, Jim (337K) wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">True enough.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">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. </span><o:p></o:p></p>
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<p class="MsoNormal">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.
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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:
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<a href="https://www.youtube.com/watch?v=CIbnl3Pj15w" target="_blank">https://www.youtube.com/watch?v=CIbnl3Pj15w</a><br>
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--<br>
Prentice<br>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b>From:</b> Beowulf [<a href="mailto:beowulf-bounces@beowulf.org" target="_blank">mailto:beowulf-bounces@beowulf.org</a>]
<b>On Behalf Of </b>Prentice Bisbal via Beowulf<br>
<b>Sent:</b> Tuesday, November 06, 2018 8:17 AM<br>
<b>To:</b> <a href="mailto:beowulf@beowulf.org" target="_blank">beowulf@beowulf.org</a><br>
<b>Subject:</b> Re: [Beowulf] More about those underwater data centers<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">. 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.<o:p></o:p></p>
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<p>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.
<o:p></o:p></p>
<p>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.
<o:p></o:p></p>
<pre>Prentice <o:p></o:p></pre>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On 11/05/2018 06:30 PM, Stu Midgley wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I refute both these claims.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Also, given the greater reliability of components you get, you do less servicing.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">If you haven't lived with it, you really have no idea what you are missing.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Serviceability is NOT challenging.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">You really do NOT want to run boards immersed in coolant - yeah, there's folks doing it at HPC scale<br>
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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.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">--
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Dr Stuart Midgley<br>
<a href="mailto:sdm900@gmail.com" target="_blank">sdm900@gmail.com</a><o:p></o:p></p>
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<pre>_______________________________________________<o:p></o:p></pre>
<pre>Beowulf mailing list, <a href="mailto:Beowulf@beowulf.org" target="_blank">Beowulf@beowulf.org</a> sponsored by Penguin Computing<o:p></o:p></pre>
<pre>To change your subscription (digest mode or unsubscribe) visit <a href="http://www.beowulf.org/mailman/listinfo/beowulf" target="_blank">http://www.beowulf.org/mailman/listinfo/beowulf</a><o:p></o:p></pre>
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<p class="MsoNormal">_______________________________________________<br>
Beowulf mailing list, <a href="mailto:Beowulf@beowulf.org" target="_blank">Beowulf@beowulf.org</a> sponsored by Penguin Computing<br>
To change your subscription (digest mode or unsubscribe) visit <a href="http://www.beowulf.org/mailman/listinfo/beowulf" target="_blank">
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