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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck, don't blow ! heatfins direction)



 
 
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Old August 21st 12, 08:04 AM posted to alt.comp.hardware.pc-homebuilt,alt.comp.periphs.videocards.nvidia,comp.arch,sci.electronics.design
Martin Brown
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Default (NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck,don't blow ! heatfins direction)

On 20/08/2012 17:48, John Larkin wrote:
On Sun, 19 Aug 2012 10:22:54 +0100, Martin Brown
wrote:

On 19/08/2012 05:18, John Larkin wrote:


If the heat sink doesn't reduce air flow at all, the air is going
around the fins, not through them (as Skybuck suggests) and the air
does no good. And if you block all the air flow, it does no good. So


More to the point if the heatsink fins are not thick enough to conduct
heat away from the thing being cooled it doesn't matter how easily you
can push air through them. Equally it is no good if you get perfect
laminar airflow since then only the air touching the surface warms up
and the core air remains cool. So you have to have some turbulence and
opposition to free flow but the tricky question is how much is enough?

Something like this might be close :

====o ====o ====o
====o ====o
====o ====o ====o

(slightly tighter together than ASCII art will allow)
Airflow from left to right with a blob on the end to mix the air up.

the amount of airflow restriction that results in the lowest theta
must be somewhere between those two extremes. Dead center is a pretty
good guess.


But also very probably wrong. The volume of air going through the heat
sink is proportional to the amount of cooling you get for a given design


It's not. Look at a heatsink theta-versus-air-flow curve. There's also
cooling at zero flow.


Convective and radiative cooling only. And not much of the latter if the
thing is made of shiny metal. Proportional to flow plus a small additive
constant isn't exactly rocket science. In any forced air cooling design
worth its salt airflow cooling totally dominates.

so there is a definite bias towards not blocking off half the free air
flow. I would guess at something more like allowing 2/3 to 3/4 of free
airflow as about the best depending on the exact heatsink geometry. It
could easily be higher - easy enough to do the experiment.


I don't think the experiment is easy. With the same fan, you'd have to
vary the airflow resistance, like the pin or fin density or something,
and keep the remaining geometry the same.


PWM to vary the power provided to the fan so the work done by the fan is
on moving the air through it and to first order the output velocity
field scales fairly well apart from deep inside the heatsink where there
is or should be some turbulence.

It occurs to me in this discussion that the performance of a standard
rectangular vaned heatsink might be improved by putting diagonal wires
through the vanes at say 45 degrees to generate vortex wakes that mix up
the laminar flow after the first or second set of vanes.

This would matter in a case like deciding between two heatsinks that
are the same overall dimensions but have different fin densities,
cooled by the same fan. Heatsink data sheets give you half the
information you need (theta vs flow) but not the other half
(backpressure vs flow).


Measuring rpm of the fan against power supplied will give you a decent
proxy for back pressure and many PC fans are so equipped.

I suspect the perfect shape for an optimum heatsink is rather more
complex than the typical fins we get but the designs used at present are
good enough and much easier to engineer. Heat pipes have helped
enormously with the latest generation of quiet heatsinks.

It is a sobering thought that high performance CPUs often have a heat
output per unit area that exceeds the tip of a soldering iron.


I wonder if CPU chip layouts include hot-spot distribution, like
putting the hottest bits into the corners or something.


Perhaps but I would guess only by coincidence since the main output
drive buffers are probably physically close to the edge of the die.

Regards,
Martin Brown

 




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