[Beowulf] Desktop fan reccommendation

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Wed Jun 6 06:24:50 PDT 2012


I used to work for a company that made fans...

-> a very, very tiny fraction of the power into the fan goes into acoustic
noise, so it's not a big driver of efficiency
-> acoustics of fans are a black art.   There are things that you know
make it worse.. But once you avoid those, there's a lot of empiricism:
Very tough to model accurately, even with a big cluster, and the price
premium for a quieter fan isn't worth it.

-> Do not have the number of blades be the same as the number of support
struts or things in the way.  In general, odd number of blades is better.
Taken to an extreme, this is how you build a siren (two plates with holes,
one spinning)

-> big fan turning slower makes less noise than small fan turning fast

-> noise is strongly dependent on the air speed.  In HVAC design, the
usual rule of thumb is to keep the airspeed below 1000 linear feet per
minute  (yeah, non SI unit.. But it's duct cfm/duct cross sectional area)

-> having fan blade tips close to surrounding shroud makes it more
efficient AND quieter, but requires tighter mechanical tolerances in
manufacturing

-> the spacing between blades is important, and a real challenge in any
rotating fan.  Near the hub, the trailing edge of one blade is closer to
the leading edge of the next.  AND, the tangential velocity of the blade
through the air is different at the hub(root) than at the tip.  Fans with
large hubs are easier to optimize (smaller variation), BUT, you give up
airflow area for a given outside dimensions.

-> funky notches and swoops in the blades sometimes help, sometimes don't.
 I think mostly they're for patent protection.  If I sell a fan with 3
asymmetric notches in each blade, and a container load of Chinese copies
shows up at the port, it's easier to say that they infringe my patent.

-> blade balance is important. Not only in terms of rotating mass, but in
terms of aerodynamic balance.  If the blade pitch is slightly different on
each blade, then it will be noisier.

-> well designed input and output vanes (particularly the latter) seem to
make it quieter, but I don't know why.



On 6/6/12 2:38 AM, "Daniel Pfenniger" <Daniel.Pfenniger at unige.ch> wrote:

>Nathan Moore wrote:
>> All,
>>
>> This is barely beowuf related...
>>
>> New desktop machine is a Shuttle SX79R5,
>> http://us.shuttle.com/barebone/Models/SX79R5.html
>>
>> In the past, shuttles have been very quiet, but this one has a fairly
>>loud
>> variable speed fan on the CPU heat exchanger.  I normally buy
>>replacement parts
>> from vendors like newegg, but their selection of 90mm case fans mainly
>>seems to
>> be described by CFM and whether the fan has LED lights mounted in it
>>(FYI, that
>> is not a selling point).
>>
>> So, is there an engineer's version of newegg that ya'll know about?
>>There must
>> be a super quiet 90mm fan out there that I can pick up for $10...
>
>I remind ads for quiet and more efficient rotor-less fans for PC's but
>cannot find such products anymore.
>
>The idea was to maximize the air flow area by displacing the central motor
>to the blade edges.  Not only the larger central area would allow a lower,
>quieter blade speed, but the blades being accelerated at their extremities
>by the circular motor would be mechanically more stable, less subject to
>vibrations.  My guess is that such fans, although technically better, were
>too expensive in regard of the advantages.

Yes.. Fans are a very cost sensitive product.  For a lot of applications,
nobody cares how noisy the fan is.

>
>The Dyson bladeless and silent fans are based om a different principle,
>a cylindrical thin air layer carries along the inner air column, the
>air flow is then laminar (http://www.dyson.com/store/fans.asp).

But you still need a fan to generate the pressurized air for the slit.
However, that fan can be hidden inside the base and can be baffled for
noise reduction.
>
>	Dan
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