On Aug 18, 9:18*pm, 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
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.

--

John Larkin * * * * * * * * *Highland Technology Incwww..highlandtechnology.com* jlarkin at highlandtechnology dot com

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Doesn't lowest theta occur at MAXIMUM airflow? Think of it as a
stiction layer of air. All the heat from the metal must travel through
that stiction layer [which by definition is an insulative sheet of air
on the heat sink.] Now as air moves rapidly over the HS's surface;
that stiction layer becomes thin and voila! lowers the theta. That's
why all the HS curves of temp ris for given wattage always
asymptotically approach some value as the air speed increases. It's
because the stiction layer can be thinned, but not removed.