Re: Strange cooling methods

Wed, 30 Mar 1994 14:36:00 -0500 (EST)

Bill Smith wrote:

>Since we're veering into physics a bit here: does anyone else
>remember the cooling device that was one of the projects in a fairly
>old collection of _Scientific American_ amateur scientist projects
>that used high pressure air and a strange funnel to create a stream of
>cooled air? I'm afraid I can't remember the details, but at the time
>it wasn't entirely clear how the cool air was produced -- there was
>some speculation about Maxwell's demon.

I believe that the device to which you refer is the vortex tube.
It is a cylinder with an iris inside, at about the midpoint
along the cylinder axis. The iris has a hole centered in it, of
about half the diameter or less of the cylinder. A nozzle is
fixed to the cylinder so that a jet of air may be injected tangentially
to the inner surface. The nozzle is in a plane parallel and adjacent
to the plane of the iris.

Pressurized air is forced into the device and, supposedly, warm
air exits one end of the cylinder and cool air exits the
opposite end. I've forgotten all the details in the explanation,
but I can try to fake it :).

The air molecules which swirl in a vortex inside the cylinder have a
distribution of energy centered about some average value. For some
reason involving angular momentum and other stuff I can't remember,
the molecules containing more energy than the average tend to
collect near the inner wall of the cylinder. (Or it could be
near the cylinder axis, I've forgotten which. Let's assume the
former for the sake of illustration). The molecules containing
energy less than the average tend to collect near the cylinder
axis. The hole in the iris is sized ideally so that its radius
matches the vortex radius of the molecules containing average energy.
So, the molecules become separated along the vortex radius according
to their energies. The cooler ones near the cylinder axis can
escape through the hole in the iris and exit one end of the cylinder.
The warmer molecules are blocked from the same exit by the iris and
so must flow out the other end.

I doubt that a vortex tube would be useful in cooling a terrarium.

By the way, a group at Los Alamos has done a neat job in using
acoustic waves to operate a refrigerator. Fascinating paper.
I've forgotten the frequency they used, but the sound pressure
level was very high, maybe 95 dB. I wonder how well the
Nepenthes in the terrarium would fare under those circumstances :).
You'd probably blow out the glass walls.