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An unusual but not too technical thermodynamic question for you guy/gals

  1. Mar 25, 2005 #1
    an unusual but not too technical, thermodynamic question for you guy/gals

    hi, i've got an unusual q. that i don't think computer forums members
    can handle too well. they know their watercooling very well and that's
    what my q. 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
    i am sorry, i posted this in the wrong forum. i now realize this should be in Mechanical and Aerospace Engineering forum.
    Last edited: Mar 25, 2005
  2. jcsd
  3. Mar 25, 2005 #2


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    Science Advisor

    Without looking at any diagram or getting into specifics, I would relay these things to think about:

    1) In any application with a fan, especially those that move a lot of air, the ducting noise will be far less than the noise created by the fan tip/air interaction. Especially when the fan tips are moving at a considerable speed.

    2) If I have a good mental picture of what your configuration is like, I would stick to your guns in terms of the ducting arrangement and work more on isolating the fans from the enclosure. You will get a nice dampening by using wood for the box, but you may want to look into further dampening by elastomer mountings for the fans. Possibly also placing foam along the inside of the entire box.

    3) In your case in which the flow is along a relatively short path, I would think less about the noise created by adding bends in your flow circuit and consider them more along the lines of effecting the total flow, i.e. frictional effects. The larger you can make the ducting, you will decrease friction and reduce the airstream velocity, thus less noise.

    4) The straight runs of ducting prior to and after the fan will help to get the highest efficiency from the fan by having a fully developed and symmetrical velocity profile entering the fan face. This will mean that, in theory, every spot on the fan face is receiving the same amount of air as it's counterpart on the exact opposite side of the face.

    If you can figure out how to post a schematic that would really be a help.
  4. Mar 25, 2005 #3
    thank you very very much, Fred. i have to head into the valley to get some supplies. (payson,az-scottsdale[80miles one way]) therefore, it'll be late before i get back and post pic. please don't forget me, i WILL return.
  5. Mar 26, 2005 #4
    Fred, couldn't get you a pic, but everythin you said, except for a little bit about paragraph 3, i am acutely aware of. so, back to what you said in paragraph 3. i considered making the ducting larger but how much larger? but, before that q. can be answered, i must 1st know if the fans would better serve me in a series or parallel configuration for my enclosure of limited size. the gross volume of the enclosure is whatever could be made from a 4 by 8 sheet of 3/4" MDF. some of that material would have to be used to make the interior partitions that would form the ducting(passage ways), much like transmission-line speakers, chosen by some loudspeaker enclosure designers.
    i was originaly planning on some kind of lining in the duct walls, like the padding found under automotive carpets. but this too adds friction. thus reducing airflow. i'm so confused! mainly w/ the fan configuration.
    Last edited: Mar 26, 2005
  6. Mar 28, 2005 #5


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    Science Advisor

    Sorry I didn't get back to you in so long.

    If your box has good venthillation, I would run the fans parallel. That will give you a greater increase in overall flow. If you were to run the two in series, you would, in theory, get more head increase which you really don't need in your case.

    In terms of thinking about the velocity of the air, it's definitely a tradeoff. The heat transfer you want will improve as a function of Reynolds number, which is a function of the free stream velocity. However, with that velocity, you will gain noise. It's always a trade off.

    I wouldn't worry too much about lining the inside of the ducts, unless you were to line them with something to reduce the frictional effects. If you were to somehow wrap the outside of the ducts that would help deaden the sounds resonating from the ducts.
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