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Author Topic: Wind Tunnel  (Read 2065 times)

brutal

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Wind Tunnel
« Reply #10 on: June 12, 2002, 01:03:00 PM »

Quote

On 2002-06-11 17:11, GrendelPrime wrote:
If you really must have the screaming wail of rushing air, you might want to look into http://www.arizonavortex.com/vortextubes.htm">Vortex Tubes, which split compressed air into two streams, one hot, one cold.



sorry for the 2 posts but it needs to be :)


grendel, I've seen those before, but I dont understand how they work and how they can be applied to computer cooling.  It seems like a good concept but i don't know.  maybe ill request info and a test setup to see what it is
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Wind Tunnel
« Reply #11 on: June 12, 2002, 01:10:00 PM »

Thanks brutal.  I guess blowers and axial fans cannot be really compared to each other since one performs very differently than the other in terms of how the push air.
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brutal

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Wind Tunnel
« Reply #12 on: June 12, 2002, 02:01:00 PM »

its too bad too, because axial fans dont provide concentrated air. a blower fan is perfect for video card cooling and some other things too
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strongShock

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Wind Tunnel
« Reply #13 on: June 13, 2002, 09:55:00 AM »

more heat is conducted when the airflow moves faster because the boundary layer, which acts as a stale layer of insulating air between the heatsink & fast moving cool air, decreases in thickness with higher velocity. Faster moving air cools better generally. BTW, the boundary layer also grows with flow distance.

The exception to this is the aerodynamic heating that occurs at hypersonic speeds. This heating is a result of sharply angled oblique shockwaves boming very close to an aircraft's (or spacecraft's) skin. In fact, aerospace designs must be thermally validated in those mach7 wind tunnels.

Regardless, any time you reduce the cross-sectional area of a subsonic flow it will speed up (constant mass flow at incompressible mach #s). This holds for all PC fans, since they will never reach 0.33*(gamma*R*T)^1/2 velocity for compressibility to set in.

The area reduction is easily done using a duct. keep in mind that using a duct will always rob energy from the flow (frictional effects). The energy loss results in reduced flow velocity, increased fan backpressure, & slight heating of the air. the energy loss gets worse when a duct is used to "pinch" the flow, since the streamlines will tend to "scrape" against the walls more, resulting in more friction.

Also, note that higher velocities over a heatsink will also tend to increase the level of turbulence. Turbulence helps cooling by breaking up the boundary layer & mixing the cool air along the surface more.

sorry about my "non-technical" jargon.
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strongShock

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Wind Tunnel
« Reply #14 on: June 13, 2002, 10:08:00 AM »

sorry about double posting, but there was another good question I didn't get to. The one regarding static pressure & CFM of certain fans.

Few fans use "stator outlets" to increase pressure potential. Delta screamer fans make use of these despinning blades to recover the wasted rotational energy of the air & put it back into axial energy (the 80mm 80CFM & 90mm 100CFM models come to mind). the end result is that the flow is compressed against the stators, which makes higher pressure, which directly results in more CFM. Why? because air is accelerated when it passes through what is called a positive pressure gradient. Relax, this only means that the air goes from high to low pressure. Incidentally, I learned in my propulsion classes that all axial turbine engines (jets) use the same design. they have multiple rotating compressor stages that are each separated by stator blades. Now, even though you may not understand aerodynamics at a professional level, you know stators wouldn't be used on aircraft if they was anything better.

Delta fans are only louder because they also use very high output motors (18W low turn "racing tuned motors"). If you slow down a Delta (or similar well designed fan), it can operate as quiet as the dominated fans, yet be more efficient & capable of delivering higher static pressure, simply because of the stators.

note: you may be saying, "well, why don't propellers use stators?". because they have high aspect ratios. by this I mean the blades are much longer than they are wide, so they act like traditional wings. In their design, minimizing induced drag is more important that recovering rotational energy, because the energy losses are much greater.
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brutal

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Wind Tunnel
« Reply #15 on: June 13, 2002, 11:42:00 AM »

Quote

On 2002-06-13 10:55, strongShock wrote:
more heat is conducted when the airflow moves faster because the boundary layer, which acts as a stale layer of insulating air between the heatsink & fast moving cool air, decreases in thickness with higher velocity. Faster moving air cools better generally. BTW, the boundary layer also grows with flow distance.


Ive been thinking this and have said it to a few people in recent weeks becuase I feel heatsinks are incorrectly made.  they blow air onto a perpendicular surface where the turbulence has to kill the heat transfer and significantly reduces velocity.

thx for the info on the other items. just for your reference and others almost all case fan (axial) makers have high rpm, high db/a fans so its just not a delta thing, they just take it to the extreme with 32watt fans that move 200cfm.

Strong,  going back to the funnel concept, i have a 80mm fan on a 80mm to 60mm adapter to provide my cpu the same cfm as the stock delta at 32cfm instead of 45.  your saying there are some minor heating and frictional effects.  since fans produce most of their airflow from the wingtips and very little from the center, the funnel is basically just moving the air to a "vacant" area.  Is there a  way to calculate how much cfm can be "redirected" in a funnel before it becomes counter-productive?  Since I feel that heatsinks are so incorrectly designed that adding the max cfm that you could would only help to accelerate those vortexes that happen on 90 degree angles



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« Reply #16 on: June 13, 2002, 12:09:00 PM »

How about this kind of setup?

http://www.geocities.com/_x24/hs.txt">

This might give the intake air a bit difficulty flowing into the fan so maybe raise the position of the fan a bit.  Anyway, Since the driving mechanism of the TMD fan is on the edges of the fan, we can leave the middle as a through-hole for the heatsink.

Heatsink manufacturers: Just in case this tickles your fancy, just remember.  Stealing is a crime.  Give credit where credit is due :D
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GrendelPrime

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Wind Tunnel
« Reply #17 on: June 13, 2002, 01:42:00 PM »

X24, I hadn't thought about it, but if the TMD fan doesn't have a fixed central axis, you should be able to remove the center, provided it is kept balanced.  Downside would be a loss in CFM from the center gap, but your design would overcome that.  Only design flaw I see is the 4 corners would get little to no involvement in the cooling.  Maybe a thicker, X-shaped structure to bind all the fins together and move the heat?
I'll have to order another TMD to try removing the hub, could be an awesome looking mod, thanks for the idea, I'll let you know how it turns out.
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brutal

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« Reply #18 on: June 13, 2002, 02:01:00 PM »

the hotest part of the heatsink is the center where the hub is, your design wouldnt work

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« Reply #19 on: June 13, 2002, 02:24:00 PM »

Hmm.  Maybe have the center lug more thinner so as to conduct the heat to the thin fins much faster and dissipate much quicker.  Anyway, it's just a setup idea.  I think a round HS shape would eliminate the corners.  How about the same idea but have the fan in between the heatsink.  meaning same setup but more fins below the fan, like a sandwich.

Hey, I just noticed.  That looks like an AX-7 :D
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