"Timothy Daniels" wrote in message
...

"Jeff Liebermann" wrote:
[.............] On the original assertion, that it's better to suck than
to blow,
methinks that's wrong. .....

"
Given the same mass/sec flow of air over the fins of a heatsink,
the best heat transfer is by blowing due to the greater turbulence -
which disturbs the boundary layer of air that lies in contact with
the fins and puts more flowing air in direct contact with the surface
of the fins. In the case where the fins rise up away from the source
of the heat, it's best to blow downward from the ends of the fins
toward the source of the heat. IOW, the air should move in a
direction opposite to the heat flow.

This principle is not only used in heat transfer systems, but also in
biological systems in oxygen transfer through membranes - as in
fish gills where the blood moves across the gill membrane in a
direction opposite to the flow of water. The basis of this principle
lies in the finite heat (or gas) capacity of a fluid and that greatest
heat (or gas) flow occurs as a linear function of the difference of
temperature (or gas concentration) between 2 bodies. Apply a little
calculus, and the principle of opposing flows results. This design
principle was recently seen when I opened up the case of a friend's
PC to clean it out: The cooling fins for the CPU rose up from the CPU,
and the cooling fan blew air down along the fins toward the CPU.
Obviously, the designer had paid attention during college freshman
physics.
"

I'd love to see simulation that actually includes dust particles and hair to
see how much effectiveness remains for this theory.

I suspect the simulation software used at the time did not include these
factors, and therefore all designs might be totally wrong for dusty/hairy
environments.

Bye,
Skybuck.