"Jeff Liebermann" wrote in message
...
On Sat, 18 Aug 2012 12:59:41 -0700, 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.

If the input and output temperatures were the same, that might be
true. It might also be true if you consider the reduction in free
flow rate caused by the back pressure due to the head sink obstructing
the air flow.

However, air expands when heated, causing an increase in air flow at
the exhaust end. That's why the exhaust port for a heat removal
system is larger than the intake. My guess(tm) is that the increased
exhaust air flow caused by heating is much larger than the reduction
in intake air flow caused by the fins getting in the way.

On the original assertion, that it's better to suck than to blow,
methinks that's wrong. You can demonstrate this with a dirty
computer. Take a vacuum cleaner and try to remove the dust by
sucking. Most of it will still be in the machine when you're done.
Now, put the hose on the same vacuum cleaner exhaust and blow the dust
out of the machine. Notice that remaining dust is effectively blown
all over the room.

It's dispersion versus concentration. When sucking, one pulls air
from the sides and from all around the heat sink, including air that
does not need cooling. This makes the fan work harder moving excess
air, leaving less air flow for between the heatsink fins. Turn the
fan around and blow air at the heat sink, and the entire air flow is
involved in cooling the fins.

Similarly, you can demonstrate the effect by comparing the CPU
temperatures with the fan in the normal position (blowing air down
towards the heat sink), versus flipping the fan over and sucking air
out. I did this with a Pentium 4 dual core. I forgot the measured
temperatures, but the difference was substantial.

I tried that on an old AMD and got the same results; the CPU was much hotter
with the air being pulled through than it was with it being blown through.
One of the electrical engineers at work explained it this way: If the air is
being pulled through, most of the air is moving through the fin area closest
to the fan, with the lower fins (closest to the CPU) getting the least
amount of flow. Therefore the heat has to transfer through the fins before
it gets to an area where there is enough air flow to actually aid in heat
removal. But, with the air being blown down, through the fins, there is
enough back-pressure to allow the air to travel almost equally across all
fin surfaces before exiting, carrying a larger amount of heat with it. Don't
know if that's exactly how it works, but it made sense to me, and would
explain why most newer heatsinks have the air blown through rather than
pulled through the fins.
--
SC Tom