Has anyone tried counter-rotating fans on their heatercore?
 

An interesting idea was proposed over on the OCForums involving counter rotating fans improving static pressure significantly with minimal additional noise. Posted along with this was an article by a Sanyo Denki employee from the Cooling Systems Division that explained the mechanics of it.

http://sanyodb.colle.co.jp/pdf/16e/b.pdf (Skip to page 3)

Pretty interesting stuff. This got me thinking that it might give better results to use two counter rotating 120mm fans instead of sandwiching the heatercore or radiator with a push/pull setup. Of course, there remains the difficulty of finding counter rotating fans, unless someone knows how to disassemble a fan and reverse the blade direction.

Or just buy some Delta EHE 120 X 38mm fans, they have counter rotating sets of blades.

Cool idea. (yuk, yuk). Cooler master implemented this principle in their Dual storm fan. This is from their site.

The basic process of CPU heat dissipation using an axial fan involves the air pressure and volume produced by the fan to exchange heat from the heat sink. However, only the furthest side of the blade from the axle produces the most air pressure and volume, creating an undesired "blind spot" because it is unable to blow towards the center of the heat sink, which is the hottest part of the heat sink. Dual Storm, with dual-layer fan blades, blows with opposite rotating direction, reducing the blind spot at the center and thus improving cooling performance. Similar to the Jet engine's compressed air principal, dual-layer fan blades increase air pressure, an excellent choice for heat sink with high density fins catching up with the latest trend in the market on high density fin heat sinks.

Sounds good in theory. This is the kind of post that makes reading forums fun. Good job.

TMD fans are also supposed to reduce the "blind spot". Of course the biggest TMD fan is only 70mm.

well if you really want to reduce the blind spot why not just get a blower. more air movement and alot less noise. but for small spaces i think the concept would work good

And horrendously expensive. And the point is not the dead spot underneath the motor, but to increase air pressure. The top fan reduces back pressure created by the bottom fan. By negating the back pressure, it effectively makes the bottom fan more powerful. Relatively low static pressure is the main problem with 120mm fans. Same CFM as smaller fans, but it doesn't have the same static pressure because the air is being moved over a larger surface area.

Too loud even undervolted. I was thinking of using a couple low rpm 120's thereby getting a decent amount of air pressure at a relatively low dba level.

It looks like I'm going to have to start up a list of fan specs, cfm, dba, mmH20 and fan rotation direction.

Though counter-rotating fans may have certain applications they excel in... I would expect a centrifugal blower to punish them for their incompetence. If you are looking for performance against backpressure - a blower with respectable diameter will far outperform anything some form of axial fan can muster.

Of course, space and looks is often a governing factor for you kids who stick all those cathodes and toys in your box, so a counter-rotating fan may be a good compromise for some.

I thought SSS or Brian did an article on counter-rotating fans with a Swiftech NB heatsink? Might find an article at OC if you bounce google.

Where would one find a blower of respectable diameter that is not very expensive or loud?

there are some blowers here but it definatly looks like it would have to blow through the heatsink, which may not be a problem if it generates higher static pressure.

or maybe you could use the venturi effect (I think I'm refering to the right effect) and blow across the heater core or some axail fans. Woudl that really create a pressure difference.

Hope this makes some sense as i have confused myself.

If you are looking to stack a pair of counter-rotating fans, looking for increased pressure performance at lower noise levels, I think you are going to be disappointed in the noise department.
I tried this (just casually) a year ago or so (so just one data point). The fans interfered with each other acoustically and were considerably louder than just the two fans side-by-side (experiment done in clear air, no resistance - what axial fans are good at). Note that I was aiming to get fan noise under 18dB or so and was thus undervolting by a lot - I'm sure things are different as you move the fans more into the performance envelope they were designed for
The SD PDF mentions that they have gotten the noise down to the single fan plus 3dB (a 3dB increase is what you would expect if you add two identical noise sources together). IMHO this is quite an acomplishment. Their diagrams are fairly schematic, leaving me wondering what they did to get this result (different fan blades? flow straighteners - like in a multi stage turbine?) The spacing between fans is probably critical - and may vary based on something dynalic like airspeed or blade speed - so even the ones SD designs might be poor candidates for undervolting.
It would be nice if they spent some development money on radial fans that were drop-in replacements for axial fans. Unfortunately the easiest way to do this leaves you with an impeller, not an "expeller" so suck not blow, which is inappropriate for a lot of PC cooling (except for radiators, but we're in a distinct minority with those).
Oh - I've also done some experimenting with under-120mm blowers and motorized impellers. Yes they certainly generate more pressure tha equivalenmd sized axial fans - but they generate more noise, even when undervolted to the point of equivalent airflow in clear air (still superior for overcoming resistance like rads, but not quieter). I've come to the same conclusion as cathar - use a radiator with large frontal area and little thickness - and couple that with quiet axial fans.

Their diagrams don't show, but the text does mention what they did. The interference problem is with vortexes coming of the first set of fan blades (same deal as with vortexes coming off airplane wings) interfering with the wing action of the second set of blades. The solution was in designing a secondary fan with special blades.
Interesting stuff - but probably not useful for us :(
Bob

So you're saying slapping two fans together that happen to counter rotate won't help? That sucks.

Yup, that's what I'm saying. I might even be right :) - at least as far as the noise part goes.
It'd be interesting to get one of these SD units and dissect it - maybe there'd be an obvious mod we could make to the blades of the "secondary" fan.
I don't claim to understand axial fans other than that they behave like a wing - so angle of attack and top of blade curvature (coanda effect) are relevant - and they behave a bit like propellers and helicopter rotors in that the outer circumference is going faster than the inner, so the wing needs to be different between inner and outer sections.
If you've seen the air coming off a wing (like from an airplane flying just over clouds - so it's visible) the air doesn't go straight down, but down and forward (same direction as the wing is going) and loops in a vortex parallel and below the wing trailing edge. I would guess that air flow coming off a rotary wing is a lot more complicated than that. Anyway, I wasn't at all surprised that stacking counter-rotating fans got unpleasantly loud. In retrospect I guess I should have installed them anyway, just to see what sort of airflow I was getting (my airflow gauge is my temp reading - really inaccurate but at least something) but I figured it was a lost cause noise wise right away and didn't go any farther.
Please remember that I tried this once. If you've already got some counter-rotating fan pairs, by all means slap 'em together and see what happens. I'd suggest using a couple of fan controllers and varying speed of the two fans individually - and if you've got a way to separate them with some sort of plenum, that'd probably be worth trying as well.
I would think we've already got data on stacked fans with a "flow straightener" (like in multi-stage compressors). The straightener we've been using is called a "radiator" - and we've been calling those experiments "push-pull" - which got no noise/performance gains for me. I'm now curious about push-pull with counter rotating fans. I expect the same non-improvement, but that's a guess.

What I had in mind was two fans, both pushing since push/pull doesn't seem to do much.

Why push rather than pull?

Does it really make a difference? I was going to go with push since I don't have much room inside my case, I could mount the heatercore inside and the double high fans on the outside.

Pull is a little more effcient since the fan blades impart a swirling motion on the exhausted air. This causes a little less flow if its pushed into a heatercore before being released into the case. The difference isn't huge though, so I wouldn't worry about it if you can't fit it the other way.