Re: CFD - software

Steven M. Schweda wrote:
From: "David J. Dachtera" <djesys.no@xxxxxxxxxxxxxxxx>

Oh, boy. I really should know better...

No argument there.

Question: Why can you not "suck" mercury all the way up a 35 inch column
at sea level under standard conditions? (Vacuum pump at one end, other
end open.)

Mostly because, unlike air pressure, "suck" is not a real physical
concept. Removing air at the top of such a column reduces the air
pressure there. (Remove all the air, and there's no air pressure
there.) What moves the liquid up the column is the outside air pressure
pushing on the liquid at the bottom of the column.

Don't ya just love trying to explain to people that things like 'suck' and 'centrifical force' and such just don't exist?

Before you start pumping air out, the air pressure on the liquid at
the bottom of the tube on the inside is the same as the air pressure on
the liquid outside the tube, so the liquid just sits there. Reduce the
air pressure inside the tube, and the air pressure outside the tube will
force the liquid to rise until the combination of the air pressure inside
the tube and the liquid pressure inside the tube (at the bottom of the
column) matches the air pressure outside the tube.

Even when all the air has been removed from the tube, the air
pressure outside the tube can only raise the column of liquid until the
pressure at the bottom equals atmospheric pressure, because atmospheric
pressure is all there is to do the pushing. "Suck" is not pulling the
liquid up, air pressure is pushing it up, and air pressure is finite. This gives you about 30 inches of mercury or about 34 feet of water. (Quick: what's the ratio of the density of mercury to the density of
water?)

Humm... using your figures, 2.5/34, or close to 1/14. Do you really mean density, vs weight?

It is frequently the case that an "air conditioning system" is a
"closed" system. Typically, this is mostly true in homes (though some do
provide for air replacement using the outside air), especially in the
case of a window ("room") air conditioner which may provide for a little
bit of venting to the outside.

In datacenters, however, it is more common to find that chillers take
ambient air in and discharge it (cooled) into the plenum floor, or the
reverse: take air in from an overhead plenum and simply discharge it
(cooled) back into the room. In this case, the supply of cooled air is
usually approximately the same as the supply of warmed air. Local codes
may require some air replacement if the conditioned space is considered
"occupied".

The fact remains that the primary consideration is the rate at which
the equipment adds heat to the environment, and how fast the
refrigerator can remove the heat. Especially for a closed system. Blowing the air faster doesn't change this.

Well, actually, it does. Not if the heat being introduced is more than or even equal to the heat being removed. But passing more air (of a lower temperature) over the hot spots will remove more heat from the hot spots. A smaller temperature delta could exist. As the cooler air takes on some heat, the faster it's replaced with more cooler air, the more heat can be removed from the hot spots.

--
David Froble Tel: 724-529-0450
Dave Froble Enterprises, Inc. E-Mail: davef@xxxxxxxxxxxxx
DFE Ultralights, Inc.
170 Grimplin Road
Vanderbilt, PA 15486
.

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