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Push/Pull w/ Different Fans

D

Deeky

Gawd
Joined
Feb 7, 2008
Messages
982
I currently have a PWM Slip Stream on my heatsink, but I'm about to grab a Noctua NF-P12 1300 rpm fan to replace it. I wonder, could I then use the Slip Steam and the Noctua in a push/pull configuration? If so, which fan would be better pushing? I assume the Noctua as it's designed for high impedance. Would the Slip Stream do more harm than good pulling, or would I be better off simply having the Noctua push? Would two different fans sporting two different rpm/cfm values somehow be detrimental in push/pull?

EDIT: Or would I be better off running a single Delta 38mm 2900rpm fan at 5v? Naturally, I'm looking for the best possible noise/performance ratio.
 
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Doing the fluid dynamics that would take a Cray a week to crunch in my head, I would agree with you and put the more/most powerful fan in push and the lesser fan in pull.

as far as the rest of it goes, it should be fairly simple to swap around fans. Try it and see. Not sure the Delta would start up at 5V, might have to use 7.
 
Thanks! I'm still flip flopping on which sink to purchase. The Venomous looks great, but then there are Noctua offerings that come standard with two of their fans. Including the cost of even a single high impedance fan, the Vex starts to look a lot less attractive price/performance wise.
 
I think the oppisite is true-
1. Its easier to draw air thru a slower fan than push it thru fins and a slower fan.
2. The faster fan can be slowed down via speedfan if running to fast in the pull mode external header.
 
gwarren007 said:
I think the oppisite is true-
1. Its easier to draw air thru a slower fan than push it thru fins and a slower fan.
2. The faster fan can be slowed down via speedfan if running to fast in the pull mode external header.
Click to expand...

I agree with this.

Since the object that is producing all the heat is in the middle of both fans, you will probably get better performance with the faster fan pulling the hot air off the HSF and pushing it out the back of the case.
 
I'll likely end up pointing the assembly toward the roof of the case, to exhaust through the twin 120mm ... but same difference. I guess the best way to find the answer is to bench it myself. Still not convinced of what cooler to buy. I'll have an HR-05 on my NB, so it needs to be something that will fit in the space between that and the ceiling exhaust fans. It's still a pretty big space to fill, but certainly not Noctua NH-Behemoth size.
 
The Coolermaster 212plus is duing duties on my Xeon 3440 overclocked from 2.53 to 3.55, I run folding at home at 100% load (24/7) and I'm getting 59c @75* ambient.
 
It will most likely not make a difference what order the fans are in, honestly speaking.
 
I would not expct any difference in performance. Each fan will produce a certain amount of dynamic pressure, the sum of those pressures and the pressure drop across the sink will determine the flow rate. If you have a fan controller you can play with the speeds on the two fans and see what gives the best performance/noise. The order of push/pull will make none or negligible difference.
 
Sorry to bring up an old thread but I had a similar question and was wondering if you'd tested it to see what gave the best results?

In my mind putting the faster fan on the pull side would be better, as placing an obstruction on the low pressure side of a fan would have a larger effect on the flow compared to placing an obstruction on the high pressure side. I know from experience simply placing fans in series does not create an additive pressure unless the fans are actually designed to be run like that. More likely you'll get the exact same flow rate as a single fan (maybe a few percent higher) and a pressure slightly but not much higher.
 
From what I learned in fluid dynamics, pulling air creates laminar flow and pushing creates turbulent flow.

In laminar air, different streams don't mix so heat stays localized more. Opposite with turbulent air. also turbulent air is more likely to "hug" the fins of the heatsink, so they tend to have better conduction of heat properties.

Base on this, you should always try push first and then pull. However if you are doing both, then I guess it doesn't matter

edit: I don't know exactly how much putting fans in series increase air flow, but from what I experimented a long time ago I say its about 50%.
 
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Good question. Did somebody ever test this out and see what is the best configuration?
 
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