Thermodynamics question you only see once.

[slicey]

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.
CAN'T I STILL JUST BUILD MY ORIGINAL DUCTED ENCLOSURE, THE WAY
I DESCRIBED, WHICH I THINK WILL STILL DO A BETTER JOB OF
ACOUSTICALY ISOLATING THE SYSTEM NOISE FROM REACHING MY EARS?
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

 

[Speed]

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.

 

[Cliff_J]

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.

 

[FredGarvin]

I answered you in the Technology section.