Positive vs Negative airflow

Which is better and why?

On Wed, 29 Oct 2003 11:17:53 -0000, "nardia" wrote:

Which is better and why?


The big advantage of blowing air into a case is you can filter the
dust out.

If you buy a new system every year, this isn't a big problem

Positive, or "pushing air onto or through something" is better at cooling
over a surface. Such as having a fan blow down onto a heatsink. Why?
Disturbed air is better at cooling over a surface than a stream of air.

Look at it this way,

------------------ 3rd level of air has no temperature change
------------------ 2nd level of air gets warm
------------------ 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface

while disturbed air allows the "levels" of air to mix and have a good
conducting of heat.

\/\/\/\/\/\/\/\/\/\/\/\/- 3rd level of air gets warm
/\/\/\/\/\/\/\/\/\/\/\/\- 2nd level of air gets warm
\/\/\/\/\/\/\/\/\/\/\/\/- 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface


At least, this is the way i imagine it. I haven't taken thermodynamics.

-Sean


"nardia" wrote in message
...
Which is better and why?

he means should the pressure inside the case be lower or higher than the
pressure of the surrounding room


"nardia" wrote in message
...
Which is better and why?

Look at it this way,

------------------ 3rd level of air has no temperature change
------------------ 2nd level of air gets warm
------------------ 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface

while disturbed air allows the "levels" of air to mix and have a good
conducting of heat.

\/\/\/\/\/\/\/\/\/\/\/\/- 3rd level of air gets warm
/\/\/\/\/\/\/\/\/\/\/\/\- 2nd level of air gets warm
\/\/\/\/\/\/\/\/\/\/\/\/- 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface

I think you're wrong, air wont sit in levels :-D

nardia wrote:
Which is better and why?

You want an air stream in the shape of a backwards "S" Air should come in
the bottom front, move up and suck out the back. I've heard of people
blowing some type of smoke through the front of the case to watch the path
of the air.

"Josh" wrote in message
...
Look at it this way,

------------------ 3rd level of air has no temperature change
------------------ 2nd level of air gets warm
------------------ 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface

while disturbed air allows the "levels" of air to mix and have a good
conducting of heat.

\/\/\/\/\/\/\/\/\/\/\/\/- 3rd level of air gets warm
/\/\/\/\/\/\/\/\/\/\/\/\- 2nd level of air gets warm
\/\/\/\/\/\/\/\/\/\/\/\/- 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface

I think you're wrong, air wont sit in levels :-D



really!? LOL

"Sean" writes:

Positive, or "pushing air onto or through something" is better at cooling
over a surface. Such as having a fan blow down onto a heatsink. Why?
Disturbed air is better at cooling over a surface than a stream of air.

Look at it this way,

------------------ 3rd level of air has no temperature change
------------------ 2nd level of air gets warm
------------------ 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface

while disturbed air allows the "levels" of air to mix and have a good
conducting of heat.

"Good conducting" is not a good way to look at it. To a practical
approximation, air doesn't conduct heat. More importantly, it gets
heated by direct contact with surfaces, and moving heated air carries
heat away from the surfaces and eventually out of the case. The
movement naturally occurs as the heating air expands and rises,
getting out of the way for cooler air which cools better than warmer
air. That's called cooling by convection. Fans are supposed to move
the air faster than convection, but with smooth airflow, friction (and
viscosity?) tends to cause airspeed to vary between near-zero at the
surface to full speed some distance above causing the so-called
boundary layers shown above. Turbulent airflow tends to break up
these boundary layers, but PC case air is likely to be fairly
turbulent without adding turbulence (which comes at a price) and few
are equiped to know whether adding more turbulence will help or hurt
and few bother thinking about it. (The more important effect of some
in-blowing case fans is to better concentrate a stream of air onto a
HSF than a fanless, ductless suck hole would do at the same (large)
distance.)

Umm sorry, your the one who is wrong. Very close to the surface there will
likely be a boundary layer of air molecules that don't move.....



"Josh" wrote in message
...
Look at it this way,

------------------ 3rd level of air has no temperature change
------------------ 2nd level of air gets warm
------------------ 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface

while disturbed air allows the "levels" of air to mix and have a good
conducting of heat.

\/\/\/\/\/\/\/\/\/\/\/\/- 3rd level of air gets warm
/\/\/\/\/\/\/\/\/\/\/\/\- 2nd level of air gets warm
\/\/\/\/\/\/\/\/\/\/\/\/- 1st level of air gets warm
{}{}{}{}{}{}{}{} - hot surface

I think you're wrong, air wont sit in levels :-D

In an attempt to distance himself from all the other Gary Swearingens,
Gary W. Swearingen wrote:

Fans are supposed to move the air faster than convection, but with
smooth airflow, friction (and
viscosity?) tends to cause airspeed to vary between near-zero at the
surface to full speed some distance above causing the so-called
boundary layers shown above. Turbulent airflow tends to break up
these boundary layers, but PC case air is likely to be fairly
turbulent without adding turbulence (which comes at a price) and few
are equiped to know whether adding more turbulence will help or hurt
and few bother thinking about it.

Yes, and it's good to fill your case half full of golf balls because
it keeps the velocity up and the dimples help the airflow.