Stacking case fans

[blueonblack]

From what I've read, stacking case fans on topof one another to try to get more air flow is generally a bad idea. My question is, does it make a difference if the fans are counter-rotating? In other words, if I stacked a 120mm case fan that turned clockwise to pull air on top of another that turned counterclockwise to do the same thing, would this have any appreciable effect on air flow?

[Spawn-Inc]

i'm pretty sure this fan works off that principle, but not 100% sure.


any reason you want to do such a thing?

[blueonblack]

any reason you want to do such a thing?

The same reason many of us do anything we do here: because it's cool. I'm looking at a new CPU cooler that draws air straight into the end of it, right by the side panel and I'm looking at ways to inscrease airflow into the cooler. It has its own intake fan, or course, so by adding two others I'd basically have a three-stage intake system.

The best advice I've found (other than "don't do it, you idiot") is to use a larger, lower-RPM fan on the outside, then a smaller, counter-rotating higher-RPM fan on the inside, with space between them, then a little more space and into the intake hole on the heat sink. With the outer edges of this ridiculous assembly sealed, of course, so air comes in one end and out the other.

[dr.walrus]

had it, done it, bought a stock cooler for one of me early athlons which had two stacked fans (a 1400+ i think) and I woudln't recommend it. The amount of turbulence can make a terrible noise, and can strain the motors (not designed to be used under much load and are damaged by backward i think?).

With a big enough space between them, it'd be fine (half the diameter of the fan), but I don't think counter-rotating would have a massive benefit.

The real answer - just use one, better fan. Here are some options for 65-110 (LOTS) cfm:

http://www.overclockers.co.uk/showpr...id=57&subcat=4

http://www.overclockers.co.uk/showpr...id=57&subcat=4

http://www.overclockers.co.uk/showpr...id=57&subcat=4

[mofo]

I think it would work and looking at the delta posted, the biggest performance/ noise benefit would come from using a variable fan controller. I think the problem with the high noise could be remedied or decreased by finding the perfect frequency of both fans. Notice the deltas don't have the same rpms, the second fan has to spin slower.
I think this was actually similar to a lab in one of my classes, I forget which.

[Airbozo]

I have seen this done in high end SUN and Intel server cases. The fans are exactly the same, but I am not sure if there is something in the fan controller to run the at different RPM's. I do know those cases push a lot of air and would wake the dead.

[Oneslowz28]

I know some aircraft engines use counter rotating propellers on the same engine to produce more thrust or airflow. Hell I say try it on a tower style cpu cooler and run an OOCT with the single fan and the dual fan config and see if it makes a difference.

[artoodeeto]

It should work fine. Oneslowz28 is right - there have been aircraft engines designed with counter-rotating props (the Russian Bear-class bomber made by Tupolev, for instance). If you think about the physics of it, there's a cetain volume of air going through the first set of blades at a certain speed, and the second set slices through that air the other way, making it move even faster. It's not going to double your airflow, nor will it increase the volume of air passing through at any given instant, but it will make air go through a bit faster, thus increasing the CFM of the fan set. All while keeping the RPMs of each individual fan lower, so they're not as noisy. (if anyone sees a flaw in this line of reasoning, speak up! I've never done stacked fans, but it makes sense to me...)

[x88x]

In my (granted rather simple) tests, as long as you don't stack them directly on top of each other it should work fine. The only problem I can think of is if they were placed in such a way that either the sound waves or vibrations generated by the second fans hit the respective resonance frequency and location for the first fan...at which point bad things would happen. I don't think that counter-rotating the second fan will be necessary, in fact it might damage the fans since they're not designed to rotate the other direction.

[dr.walrus]

either the sound waves or vibrations generated by the second fans hit the respective resonance frequency and location for the first fan...at which point bad things would happen.

As the fans vibrations travel in and out of phase you can get a sort of whoomp whoomp noise but overall they're just loud becase you have the second fan is chopping up the first fan's turbulence.