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The vortex tube, also known as the Ranque-Hilsch vortex tube, is a mechanical device that separates a compressed gas into hot and cold streams. It has no moving parts.
Pressurized gas is injected tangentially into a swirl chamber and accelerates to a high rate of rotation. Due to the conical nozzle at 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 standard vortex behaviour--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 wanting 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.
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. Vortex tubes can also be cascaded. The cold (or hot) output of one can be used to pre-cool (or pre-heat) the air supply to another vortex tube. Cascaded tubes can be used, for example, to produce cryogenic temperatures.
Additional recommended knowledge
The vortex tube was invented in 1933 by French physicist Georges J. Ranque. German physicist Rudolf Hilsch improved the design and published a widely read paper in 1947 on the device, which he called a Wirbelrohr (literally, whirl pipe). Vortex tubes also seem to work with liquids to some extent.
Vortex tubes have lower efficiency than traditional air conditioning equipment. They are commonly used for inexpensive spot cooling, when compressed air is available. Commercial models are designed for industrial applications to produce a temperature drop of about 45 °C (80 °F).
Categories: Thermodynamics | Heat pumps
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Vortex_tube". A list of authors is available in Wikipedia.|