Basic aerodynamic application for HSF's

[m0ti]

hello,

I think anyone seeing this message is an OC'er of some sort, and so, like the rest of us, is after maximum CFM, and some of us, may just be interesting in the noise. I, myself, try to find a good balance between the two, which is what gave me the idea, which comes from... airplanes! yup, those great lumbering things that go soaring through (and crashing down from) our polluted skies.

So what's an airplane got to do with my HSF, you ask, and then smack yourself since it's so obvious. Air flow! How does a plane stay up? As most of us know, the air moving over the top of the wing moves faster than the air moving below the wing. This means that there is less air pressure above the wing than below the wing and the air there pushes the wing up and we get lift. How can this be used in a comp?

Normally, with a 60 mm fan what happens is that the air outside the heatsink is moving slower than the air exitting the heatsink (since the slower air just left the heatsink and lost even more speed to the air already there), and this SLOWS down the air exitting the heatsink! Not what we want!

So let's fix this. Imagine putting an oversized fan in your HSF. To be really outrageous, lets say its 120 mm. What happens here? Well, the air going through the heat sink is going through a torturous obstacle course when compared to the air not going through it. Thus, it's speed should be significantly less than that of the air outside the heatsink. Which means, that it's under more pressure than the air outside, and it gets pushed out/sucked out (if this confuses you think of blowing over a sheet of paper while holding one end. you blow out, pushing air out of the way, creating a short-lived vacuum which pulls the paper up).

Now, I don't think a mobo exists which'll let you put a 120 mm fan on a heatsink. But an 80 mm might fit, as might a 92 mm. You could also try using a quieter 60 mm, and stacking a larger sized fan on top of it. The air coming out of the heatsink should still be much slower than the air being pumped out by the larger fan. I haven't figured out anyway of getting an approximation of the air flow for a stacked setup (could answer questions like how much space between the fans would be ideal?), and my fluid dynamics knowledgeable roommate isn't in right now. Any ideas?

it should also work for fans set to suck and to not to blow, as air'll get sucked in from further on the mobo (more surface area = lower average temp).

 

[IcemanJer]

I guess this question may fall under this catagory... Why do some fans suck air into the heatsink, and some fans blow air out from the heatsink? What are the advantages and disadvantages of either method? Thanks.

 

[m0ti]

Actually, no. Sucking vs. blowing is a totally seperate issue. What I'm saying is that fitting a heatsink with a fan larger than it should improve performance, as more air can get through the heatsink quicker. Might be useful for running a quiet rig (a quiet 60 mm, with a quiet 92 mm stacked on top). I'll have to try this sometime. The question is how much of an improvement can it make?

 

[IcemanJer]

Well, by stacking the 92 on a 60, can you expect the 60 to increase the air velocity after the air goes through the 92?

 

[m0ti]

Technically, yes. The air arriving at the 60 mm would arrive at speed, meaning that there should be some further gain across the 60 mm fan. It would also be helped by the minor sucking effect going on at the edges of the heatsink which would propogate in to the center. Of course, when stacking, some space has to be left between the two fans, so that the 92 mm can push the air "cleanly". I was thinking of getting some sort of a slotted plate (preferably plastic, due to the weight), to use to attach the 92 mm onto the 60 mm. I could then use a three nuts (one below the plate, one above, and one below the fan itself) to control the spacing between the two fans, and try to find what works better.

 

[Evadman]

odds are going to be that the 60mm is going to act as a "brake" on the 92mm, probably going below the total CFM of the 92mm fan. The 60 would literally be in the way.

 

[m0ti]

The 60 probably would act as a brake on the higher CFM of the 92, but the 60's output would be higher as a result (relative to it's CFM not the 92's). In any case, there've been many attempts to use a larger fan at higher CFM, with appropriate spacing, and they've always achieved worse results than a fan directly on the heatsink (probably because the air loses some speed and under goes some turbulence on the way, not really sure. The air distribution SHOULDNT be even though). What should happen though is that the air pushed by the 90 will fall outside the heatsink, giving the 60 a little more boost, as well as its air arriving at speed to the 60 which should give it another little boost. The question is how much of boost that'll be.

 

[jaydee]

<< odds are going to be that the 60mm is going to act as a &quot;brake&quot; on the 92mm, probably going below the total CFM of the 92mm fan. The 60 would literally be in the way. >>


That's assuming both fans are going at the same speed.

 

[RedBeard0531]

I read that for stacking fans to be sucksesfull(oh, thats just bad), they need to be going in reverse directions. That is because air leaves a fan at a 45deg angle.

PS: This is my 100th post

 

[m0ti]

That could be true. It might also depend on spacing.

There is a question though, of whether one would want the fans to be in two different directions. This might cause the same air to circulate about a path.

Do you have a link to anything about stacking fans?

 

[m0ti]

Did a search on google. Found out the 45 degree thing (maybe): some guy suggested placing the two fans at 45 degs one to the other, and then blocking out the air that comes out the side. That'd only work for two fans of the same size.

 

[RedBeard0531]

It was on Overclockers.com. If I can find the exact link, I'll post it.

Linkage!

 

[m0ti]

That's ok, I found the site. Yeah, the clockwise/counter clockwise thing is good too. The noise can probably be taken care of by using quieter fans, and also the two different sized fans should have very different speeds, which should help cancel out vibrations.

 

[MadRat]

So do a two-stage ducted heatsink. The first stage is meant to pull heat from the CPU. The second-stage is meant to pull heat from the first stage.

1. The first-stage could be ducted so that air can only come from the bottom of the heatsink, like in the ducted Alphas that suck.

2. The second-stage could be ducted to cool the shround around the first stage. (The shroud obviously being integrated into the pad of the first-stage in order to directly pull heat from the CPU.)

3. The heatsink could be held by the four heatsink-holes in the motherboard that nobody seems to currently use.

This way the first-stage could have a smaller (60mm?) fan while the second-stage can have a monster 120mm fan. The four-hole attachment would stabilize it from tipping over when you set the computer case on its side.

 

[MadRat]

btw - the fan pitch determines the potential velocity of air flow, not the size of the fan. A turbofan uses multiple stages with an increasing steeper angle to accellerate airflow from the intake to the exhaust. Likewise, marine props follow the same principle.

I'd advise a slow moving large fan as the second stage simply for its silence.