Thursday, May 17, 2012

Ranque-Hilsch vortex tube

A tube that separates a neutral temperature stream of compressed air into hot and cold streams, with no moving parts. There are multiple explanations for how it works. Air is fed into the main tube by small tangential inlets, so the gas swirls. After that there are separating-effects at the ends, which have uniquely shaped openings.

This Hack-a-day post shows how to make one, by Otto Belden. Note the efficiency!
"His latest version, which you can see in the video below, takes compressed air at about 78degrees and spits out about 112degrees on the hot side and 8degrees on the cold side."

From Wikipedia:

The vortex tube, also known as the Ranque-Hilsch vortex tube, is a mechanical device that separates a compressed gasinto hot and cold streams. It has no moving parts.

Pressurized gas is injected tangentially into a swirl chamber and accelerated to a high rate of rotation. Due to the conical nozzleat the end of the tube, only the outer shell of the compressed gas is allowed to escape at that end. The remainder of the gas is forced to return in an inner vortex of reduced diameter within the outer vortex.

There are different explanations for the effect and there is debate on which explanation is best or correct.

What is usually agreed upon is that the air in the tube experiences mostly "solid body rotation", which simply means the rotation rate (angular velocity) of the inner gas is the same as that of the outer gas. This is different from what most consider standardvortex behavior — where inner fluid spins at a higher rate than outer fluid. The (mostly) solid body rotation is probably due to the long time which each parcel of air remains in the vortex — allowing friction between the inner parcels and outer parcels to have a notable effect.

It is also usually agreed upon that there is a slight effect of hot air tending to "rise" toward the center, but this effect is negligible — especially if turbulence is kept to a minimum.

One simple explanation is that the outer air is under higher pressure than the inner air (because of centrifugal force). Therefore the temperature of the outer air is higher than that of the inner air.

Another explanation is that as both vortices rotate at the same angular velocity and direction, the inner vortex has lost angular momentum. The decrease of angular momentum is transferred as kinetic energy to the outer vortex, resulting in separated flows of hot and cold gas.[1]

This is somewhat analogous to a Peltier effect device, which uses electrical pressure (voltage) to move heat to one side of a dissimilar metal junction, causing the other side to grow cold.

When used to refrigerate, heat-sinking the whole vortex tube is helpful.

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