Ian White, G3SEK wrote:
Gary Schafer wrote:
Ian White, G3SEK wrote:
Turbulence is vital for effective cooling. If the air flow over a
hot surface is gentle (laminar) there is always a stagnant boundary
layer of hot air which effectively insulates the surface from the
bulk of cool air flowing past it. Turbulence scrubs away the
boundary layer, so it greatly increases the heat transfer.
The key point about the "Blow or suck?" discussion is that
turbulence only exists *downstream* from the blower or fan blades -
so there is a real difference.
I don't entirely agree here. You can have as much turbulence sucking
air as you can by blowing it. It depends on the speed of the air and
the surface that the air is passing over as to how turbulent it is.
Sorry, but that isn't true. Simply hold your hand on the inlet and the
outlet side of a fan - you can feel the turbulence on the outlet side,
but not on the inlet side until you come very close to the blades.
Alternatively, let a thin streamer of smoke into the air going into a
fan. It may show a mild turbulence on the intake side, but on the outlet
side the turbulence is much more violent.
The technical reason is that on the inlet side, all the air comes in
with pretty much the same average velocity; but it comes out at more
like the speed of the fan blades.
Ask an air conditioning company how they deal with turbulence in air
inlet ducts that are noisy.
When you are sucking air you are really lowering the pressure on one
side and allowing the atmospheric pressure to push. You can not really
suck air via a vacuum. It always gets "pushed".
The only difference when "sucking air" is that it is a little less
dense. Lower pressure on it.
Again, that's not true, except in the kind of closely confined ducting
that doesn't apply in most parts of an amplifier.
On the outlet side of a fan, the air goes in the direction the fan
propels it, so even without a duct you can blow a stream of air *at*
something (providing it's not too far away). On the inlet side, if you
don't have a duct the air comes in from whatever direction it pleases.
Next time the air conditioning fails, try turning the desk fan the
opposite way, and see how it feels.
In desktop amps with the Alpha-type through flow cooling system, the
blower is buried deep inside (which helps reduce noise) and it delivers
highly turbulent air direct to the tubes. However, the inward air flow
from the rear of the amp is much more gentle, almost laminar,
Not familiar with the alpha but I would suspect that the air inlet is
a lot larger than the outlet thus lower velocity which would make it
gentler.
Only by a factor of 2 or 3.
You missed the point. Again: "You can have as much turbulence sucking
air as you can by blowing it."
This is the key: "It depends on the speed of the air and
the surface that the air is passing over as to how turbulent it is."
If the air is not going through a duct, and a fan is in free space then
yes there is a difference between inlet and outlet air flow. It is more
concentrated at the output than at the input. There is more velocity
directly in front of the fan than there is on the inlet side of the fan.
Cooling a tube you are forcing or pulling air through a duct. (the tube
chimney)
Blow air through the cooling fins of a tube with a chimney or suck the
air through and you will get the same amount of cooling of the tube.
However blowers / fans are more efficient when pushing than pulling when
there is back pressure involved. But as long as you could move the same
volume of air there would be little difference. Trying to suck air
through a tube system with high back pressure would not work as well as
blowing with a given type of blower.
73
Gary K4FMX
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