Fan overlapping bare CPU heatsink?

Blorgon

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Jan 15, 2016
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Been a while since I stopped in. Glad the forum has picked up!

I've been sitting on a design for a while now that I really want to have manufactured (far from that point; all I have is a mock-up at the moment). The whole design depends on how well a section of a large fan placed over a CPU heatsink would be able to cool the CPU.

This would be a C7 heatsink (with no fan of its own) on an i7 6700 (non-K), so relatively low TDP (~65W, IIRC—vs the C7's 100W capacity) What do you guys think? I'm also working with the presumption that this would be a Noctua fan—specifically the 200mm that is coming out soon.

The heatsink and fan are scaled properly, but the heatsink placement isn't exact.



Here's an actual shot with component models:



I'm mostly worried about how much the bearing housing and the frame would affect air flow. From what I understand, Noctuas push a lot of air. I don't know what the static pressure of the 200mm model will be, but their fans usually have decent to high static pressure. I'm wondering if this much coverage would be sufficient to push air through the heatsink fins under those spots.
 
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confusis

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From my experience with hacked together setups it is feasible. However air will try and avoid the restrictive area of the heat sink, so maybe a small plastic shroud pr blocking sheet like inside a PSU may assist in guiding some air into the cooler.
 

Blorgon

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Jan 15, 2016
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From my experience with hacked together setups it is feasible. However air will try and avoid the restrictive area of the heat sink, so maybe a small plastic shroud pr blocking sheet like inside a PSU may assist in guiding some air into the cooler.
I'm not sure I understand. Do you mean like ducting? The fan would be right up against the heatsink, so there wouldn't be any space for that.
 

Josh | NFC

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Jun 12, 2015
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It helps to think of air as a gelatinous fluid. It will act like a semi-solid if you get it fast enough and will bounce off objects you want it to pass through. My jeep for instance displaces so much air at highway speed that it stops moving through my radiator.

Not to get off topic, but the bottom line is I have tried these setups myself with Dynatron server heatsinks, and without dams to force the pressure through the fins it usually doesn't do nearly as good of a job as a smaller, noiser fan right over the blades.

It's a tradeoff with small form factor as you want to cool as many components as possible with one fan. You need to find the balance between a low restrictive heatsink and enough pressure from the fan.

Peace!
 
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XeaLouS

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Dec 29, 2015
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Noctuas push a lot of air. I don't know what the static pressure of the 200mm model will be
Is the noctua 200mm out already? :)

From my experience a large heat sink (cm 212 evo) with a large fan little distance away (180mm fan, 3-4cm away) performed very well
 

K888D

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Feb 23, 2016
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It helps to think of air as a gelatinous fluid. It will act like a semi-solid if you get it fast enough and will bounce off objects you want it to pass through. My jeep for instance displaces so much air at highway speed that it stops moving through my radiator.

Not to get off topic, but the bottom line is I have tried these setups myself with Dynatron server heatsinks, and without dams to force the pressure through the fins it usually doesn't do nearly as good of a job as a smaller, noiser fan right over the blades.

It's a tradeoff with small form factor as you want to cool as many components as possible with one fan. You need to find the balance between a low restrictive heatsink and enough pressure from the fan.

Peace!
I completely agree with this, I also tried out this exact setup of using a larger fan over the entire motherboard with the CPU heatsink fan removed. I used a Noctua NH-L9i heatsink and a silverstone 180mm x 25mm fan about 1mm above the heatsink positioned centrally over the motherboard, the CPU was an A10-7800 (65W TDP).

The system temps were amazing, just a few degrees above ambient. But the CPU temp was just too high, even when the computer was idle it the CPU was sitting in the region of 50 - 60C, turning the fan up to maximum RPM didn't hardly effect the CPU temperature, which tells me that no airflow was getting in between the heat sink fins.
 

Blorgon

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Jan 15, 2016
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ddejohn.bandcamp.com
I completely agree with this, I also tried out this exact setup of using a larger fan over the entire motherboard with the CPU heatsink fan removed. I used a Noctua NH-L9i heatsink and a silverstone 180mm x 25mm fan about 1mm above the heatsink positioned centrally over the motherboard, the CPU was an A10-7800 (65W TDP).

The system temps were amazing, just a few degrees above ambient. But the CPU temp was just too high, even when the computer was idle it the CPU was sitting in the region of 50 - 60C, turning the fan up to maximum RPM didn't hardly effect the CPU temperature, which tells me that no airflow was getting in between the heat sink fins.
I'm really curious about how much of an effect the dam @confusis was describing would have.

It'd be a damn shame if I have to put this design away. It's using a single 200mm fan to cool the CPU, HDPLEX, and a back-of-mobo M.2 drive using surfaces to redirect the air.

I had a feeling my design was only going to work on paper. I'd love to see some numbers on a "dammed" fan. Anybody have the ability to do a test like this? I don't have any large fans to play around with right now.
 

K888D

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Feb 23, 2016
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It will work, just don't expect great CPU temps unless you can find a way to force the airflow through the heat sink. The airflow from a fan will not penetrate between thin heat sink fins unless it has no where else to go.

I don't see how you could duct all airflow from a 200mm fan down to an 80mm heat sink without adding a tonne of volume to the case with the required funnel/duct.

But you could probably create an exit airflow path that only allowed air to exit the case by fist passing through the heat sink, @confusis may of been referring to some kind of dam system like this:


Not sure how feasible it would be to add walls like this on top of the motherboard, but so long as the case was sealed up and the only exit route for air was through the heatsink first then it would force air through the fins and work very efficiently.​
 

Josh | NFC

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Jun 12, 2015
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If you want to keep the design, bundle it with a low profile Noctua, OR switch to a less restrictive cooler.

IMO the Zalman CNPS2X might be low profile enough for this to work. It's almost 1U.
 

XeaLouS

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Dec 29, 2015
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I completely agree with this, I also tried out this exact setup of using a larger fan over the entire motherboard with the CPU heatsink fan removed.
There's a pretty big deadspot where the centre of the 180mm fan is. was the cpu heatsink centred with the fan or offset?


Its very hard to "force/push" air through the heatsink if the fan isnt mounted onto it. If you are pulling air however, it'd imagine that it should probably work, even without a dam, since you are creating a whole bunch of negative pressure.