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Thermodynamics question you only see once.

  1. Mar 25, 2005 #1
    hi, i've got an unusual q. that i don't think computer forums
    can handle. they know their watercooling very well and that's
    what my question relates to. but more specifically it has to do with
    system effects of ducted DC fans(thermodynamics). you see, i want to cool my
    pc with a watercooling circuit, consisting of a pump, heater
    core, CPU waterblock and one or both of my 6.75",24V, 283CFM,
    .75"H2o Papst fans. the purpose is low noise output/cost.
    here's the deal, i'm trying to design a wood enclosure, that
    contains a 6" by 11" by 2"(louvered fin portion) heater core.
    all the watercooler boxes i've seen, the builders, just mount
    the fan(s) and radiator/core to the exterior walls of their
    external boxes. this allows the noise from the fan(s) a more
    direct path back to your ears. same goes for the fan sound
    coming through the core as well. this will not do. OK, so i
    spent a few weeks learning a little about what you guys/girls
    know. keeping the airflow velocity low, helps to keep the
    sound level down. no turns in the ducting, something like
    2.0-2.5x the fan dia., by inlet or outlet. air in a duct
    doesn't like being squeezed or expanded suddenly. i could go
    on, but you know this stuff a lot better than i do. don't
    leave this post yet, please. what i had in mind before my
    research enlightened me some, was to have a core installed in
    a ducted inlet/outlet, where the air pathway went back and
    forth at least once, on both sides of the core after
    entering/exiting the enclosure. i hope that made sense, for i
    know not how to import and post a pic. because of what i
    learned, i violated many of the rules of system airflow and
    noise. and the part i just don't know, but i'm hoping someone
    can tell me if i'm right or wrong is this; i have two of these
    fans that can really move a lot of air each(in parallel, a
    whole lot). also, in series they can provide greater pressures
    and i understand these facts only apply to a properly,
    respectively matched system, impedance wise. but these fans
    won't need to run anywhere near 24volts, even at my 5000ft.
    elevation in order to properly cool the water passing through
    my core, which is why i've already built a variable DC supply.
    there i finally asked my "question", almost. i mean from what
    i learned, the ducting should be straight before and after any
    fans and be long enough to allow the airflow to develop a
    smooth(laminar?) flow, also, without any obstructions near the
    duct's inlet/outlet? i know there's more to it but that's the
    basics of what i learned. i'm just a landscaper. i do hope you
    can help me, not many people ever respond to my questions in
    the computer forums.
    thank you! slicey
  2. jcsd
  3. Mar 26, 2005 #2
    Unfortunately, the more turbulent the airflow over a cooling surface, the better the heat transfer rates from the surface. There are several methods of precipitating turbulent flow; but the most popular for cooling systems is just a more powerful fan, because it kills two birds with one stone by generating turbulent flow as well as higher flow rates to convect the heat away.
  4. Mar 28, 2005 #3


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    Science Advisor

    You may also want to consider rubber-isolating the fans to make sure they don't transmit sound directly through the walls of the wood box as well, and maybe rubber feet or chunk of carpet underneath the wood box to keep it from vibrating the table it sits on.

    Adding sharp turns with sound absorbent material lining the walls is a method that can help reduce sound levels. You can use egg-crate foam like used to pack equipment and its a lot cheaper and easier to find than acoustic specific foam (used to be able to get it at musician stores too, couldn't hurt to check). There will be little reduction in flow from a few bends when the airflow rate is slow but the sound reduction could be far more.

    Just because the sound doesn't have a direct path doesn't mean everything - if you change the sound dispersion you could increase the level you hear! Add some lined walls with couple turns to really knock the energy down and then point the inlet and outlet away from you or any sound reflective surface and you should have a very quiet setup.
    Last edited: Mar 28, 2005
  5. Mar 28, 2005 #4


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    Science Advisor

    I answered you in the Technology section.
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