On 19/08/2012 05:18, John Larkin wrote:
On Sat, 18 Aug 2012 17:19:48 -0700 (PDT), Robert Macy
wrote:

On Aug 18, 12:59 pm, John Larkin
wrote:
I have this theory that the fins of a heat sink should reduce a fan's
free-flow rate by 50% for optimum heat transfer.


optimum heat transfer? not sure what the criteria would be,

Minimum theta would do.


but think
instead about the air's thermal mass, thermal resistance form metal to
bulk air. and you see you're left with characteristics of the heat
sink, not the characteristics of the fan.

As a mind argument enfisionone hell of a powerful fan. Now block that
to half flow, what do you have? versus an 'underpowered' fan that is
blocked to half flow. .

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
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 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.

Regards,
Martin Brown