On Sun, 19 Aug 2012 08:56:05 -0700 (PDT), Robert Macy
wrote:
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
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME *analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
Doesn't lowest theta occur at MAXIMUM airflow?
If you have a veriable speed fan, sure, more air flow cools better.
That's not what I'm talking about.
If you have some fan running full blast, with some flow-backpressure
curve, and you blow it at/through a heat sink, you could vary fin
density, fin shape, ducting, stuff like that. Maximum air flow implies
zero interaction with the heat sink fins, which will make for zero
cooling. 100% blocking by fins is zero air flow, also no cooling
attributable to the fan.
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.
http://tinyurl.com/9xu8rvx
The other thing going on is that the thermal resistance of the
heatsink itself, resistance along the fins or pins, starts to isolate
the extremes of the heatsink at high heat flows. You could attach the
tips of the fins to a zero-theta block and still have the thermal
resistance of the fins themselves. In real life, the baseplate lateral
theta gets to be a problem too; you get a hot spot at the chip or
mosfet or whatever, but the sink is cooler farther away. Heat sink
extrusions are apparently specified with uniform heating of the
baseplate.
For a given chip, there would be a finite theta if it were bolted to
the face of a half-the-universe-sized block of copper.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators