Cone for fans to reduce turbulence?

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gavinm
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Cone for fans to reduce turbulence?

Post by gavinm » Mon Jun 21, 2004 2:18 am

I'm a newbie looking at building my first quiet PC and I couldn't help but wonder why cooling fans don't have a cone in front of the central dead-spot (like on jet or propeller engines). Surely this would reduce turbulence (and therefore increase air-flow / reduce noise)? Similarly, a cone behind the central dead-spot would seem like a good idea (but would require cutting away of any obstructions such as grills behind the fan).

Has anyone experimented with this idea?

Does anyone know of somewhere that might sell cones suitable for this application?

Gav.

gavinm
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Post by gavinm » Mon Jun 21, 2004 8:15 am

Anyone out there???!

greeef
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Post by greeef » Mon Jun 21, 2004 8:38 am

i must say i've thought of this before, but never too hard hehe...

Try making some out of paper/card and doing some subjective testing, see if someone you know with good hearing can tell the difference. Blindfolded of course.

griff

hyperslug
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Post by hyperslug » Mon Jun 21, 2004 11:00 am

greeef wrote:i must say i've thought of this before, but never too hard hehe...

Try making some out of paper/card and doing some subjective testing, see if someone you know with good hearing can tell the difference. Blindfolded of course.

griff
I've only seen it done by one person who posts at arstechnica, mnpctech: http://www.mnpctech.com/

gavinm
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Post by gavinm » Tue Jun 22, 2004 6:03 am

Thanks hyperslug - the nose cone from a remote-control prop plane sounds ideal.

I'll have to hunt around to see if I can find a UK supplier.

mnpctech_guy

Post by mnpctech_guy » Thu Jun 24, 2004 1:26 pm

Theyre called Prop Spinners and I get them from http://www.carlgoldbergproducts.com :D :wink:

My latest creation, the Deuce

albatros_la
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Post by albatros_la » Wed Nov 15, 2006 6:23 am

I think there are two main reasons why producers do not concentrate their engineering effort on this way. The first: the air flow is relatively slow and so the noise coming from there is not so high (the fan blades are noisier and so reducing the hub noise does not make the fan more silent). The second: a "long" hub, like the ones used in airplane fans and turbofans, it's necessary for inlet high speed air flows in order to deflect the air towards the fan blades and avoid the air hitting the shaft: it produces a big drag but in these cases you must use it. In little fans, like the ones used in computer electronics, the air flow at the hub is naturally deflected towards the blades by the low pressure region generated by the blades.
Putting a hub like those at the exhaust is also not usefull: the flow is turbulent and it's better if the surface it hits is as little as possible. Here the flow speed is bigger than at the inlet and so a hub will produce a big increase in air drag.
The better way in order to reduce the noise of a fan is to put it in a tube. This increases the fan efficiency and so the fan moves more air at the same sound level (or produces less noise at the same flow rate).

jaganath
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Post by jaganath » Wed Nov 15, 2006 7:24 am

The better way in order to reduce the noise of a fan is to put it in a tube. This increases the fan efficiency and so the fan moves more air at the same sound level
But surely the tube introduces some impedance (friction/adhesion between walls of tube and airflow) thus reducing CFM? Are you sure the reduction in sound level isn't simply because of the blocking effect of the tube, rather than any increase in fan efficiency?

danwright
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Post by danwright » Wed Nov 15, 2006 7:39 pm

I think of the main problems is that alot of the time fans are in a fairly enclosed space (eg in a psu) so you're looking for a small footprint.

albatros_la
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Post by albatros_la » Fri Nov 17, 2006 5:48 am

jaganath wrote:
The better way in order to reduce the noise of a fan is to put it in a tube. This increases the fan efficiency and so the fan moves more air at the same sound level
But surely the tube introduces some impedance (friction/adhesion between walls of tube and airflow) thus reducing CFM? Are you sure the reduction in sound level isn't simply because of the blocking effect of the tube, rather than any increase in fan efficiency?
The increased fan efficiency lets you run the fan at lower speeds, so it produces less noise at the same air flow (cfm).

EndoSteel
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Re: Cone for fans to reduce turbulence?

Post by EndoSteel » Fri Nov 17, 2006 7:36 am

gavinm wrote: Has anyone experimented with this idea?
I have. No measurable effect, except ruined impeller balance.

NeilBlanchard
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Post by NeilBlanchard » Fri Nov 17, 2006 10:47 am

Hello,

On the range of items that make a different in the noise a fan makes, I have a feeling that this is swamped by the 3-4 items that are more important. Blade shape & tip clearance, frame & strut shape, bearing quality, motor quality, balance...all are probably more important than the shape of the surface of the hub.

jaganath
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Post by jaganath » Fri Nov 17, 2006 11:39 am

There are a couple of interesting magnetic drive fans around which drastically reduce the size of the hub, for example:

http://www.dansdata.com/tmdfan.htm

qviri
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Post by qviri » Fri Nov 17, 2006 12:16 pm

Interesting idea, although the fact that the review is from 2002 doesn't exactly inspire me with optimism...

I'd like to see a 120 mm fan like this, though. Even if just for the entertainment value.

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