So, let's demonstrate two simple scientific principles through this tube. That's right, two principles in one!
let's start with temperature. Temperature is an AVERAGE of how fast the particles are bumping into each other. Because it's an average, it means that air is composed of fast AND slow particles. It's hot and cold at the same time! (see right of third picture) How will this be proved? Well, I think that if we just separated the hot and cold, that'd do just fine for proving!
How's it going to work? By utilizing inertia! Which is defined as: "the property of an object to remain at constant velocity unless acted upon by an outside force." Basically, once you get a boulder rolling, it's not going to want to stop. And trust me, it'll resist you changing it's velocity!
There are two things that make up inertia- mass, and velocity. More of either means more inertia.
Now imagine, there's a wall that curves 90 degrees. A boulder and ping pong ball are rolling towards it. (see fourth picture for the explanation) now, as you saw, the pingpong ball was just pushed away with ease. Now, say the pingpong ball was the same size as the boulder (but still very much lighter) and on the right instead of the left. The boulder would still force its way to the wall. If you had a steady stream of boulders and giant pingpong balls, and this was a circle instead of just a curve, it wouldn't be long before there were only boulders rolling along the wall, and all the pingpong balls were all pushed to the inside.
Now, one step further, now because inertia = mass x velocity, say you had lots of molecules of the same weight going around that circle. But some were moving really fast, and others slow, because the faster ones are moving much faster (and have more inertia), they'll push the slower ones out of the way. (yes, just like the boulder and pingpong ball) and before you know it, there's the hot (fast) molecules around the edge, and slow (cold) in the center! (see left side of third picture) and that's exactly what the inside of the vortex tube looks like!
to put it very simply all the vortex tube does is get those molecules moving in one direction (rather then the chaotic right of the third picture) so that the separation will begin!
There ya go, you've learned two scientific principles, and the basic idea of how this machine works. If you want to know the history and a step by step process, go to step one! If not, and just want to build, just move to the step after that.
Step 1: History and step by step explination
Georges J. Ranque, a French physicist noticed temperature differences in vortex separators. He found that the center would get cool, and the outside quite warm. After some due thought, he wrote some theories up and moved on. These ideas, as well as maxwell's thoughts on the subject came to Rudolf Hilsch and he began actually researching and building a refrigerant system to try and beat the standard system for the German military. After building a few prototypes, and getting a very good hold on the dimensions, he left the idea alone, as, the conventional system was more efficient, and less noisy.
Ok, enough history, now how the tube actually works in a step by step process. This is for those science nerds who really want to know how the air separates in this cool machine. Be warned, this is complicated, and I tried to explain it as well as I could. If you've got a good enough idea from the scientific principles above, you may just want to go to the next step.
-First you have the vortex chamber, this is simply where the air starts to spin. the better this is designed, the better your tube works. for the best tube, the faster you can get the air to spin the higher the temperature change.
-Second the air moves down the long hot tube and the hot air separates outward, and the cold air is pushed to the center of the vortex. (effects of inertia)
-Third, the air makes it to the end of the pipe, and, because the ball valve is opened slightly, with a small opening near the WALL of the hot pipe, it siphons off hot air, but, because the pressure is too great to go out that single opening, some of the air must rebound and travel through the center of the vortex, and exit back through the hole in the middle of the vortex chamber.
Why wouldn't it just go through there in the first place? simply because in the vortex chamber, the air is moving so fast, it's being smashed on the walls of the pipe and can't "pull" itself to go through it. If the ball valve is closed, enough pressure builds, and the air just exits there, as, there's nowhere else to go. Because the ball valve is slightly opened near the wall where the air is being smashed, it tries to exit there first. If it can't make it out there, it is forced to go back through the center of the vortex and exit at out the cold tube.
-fourth as the air goes back through the center of the vortex, the faster molecules push back out towards the edge of the tube, and the colder are forced to the inside. Because there is too much air to exit out the hot tube, the air is forced to escape out the cold tube, and your separation is complete. hot air out one end, cold out the other.
Now we're done with theory, thank goodness! Back to the hands on build it part!
Next step: the materials!
EDIT: due to many people asking what "practical" use this might serve besides teaching a principal, once tuned to achieve temperatures below freezing, you can use the tube to freeze all sorts of stuff! Scientists use it for tissue sample freezing, what can you use it for? Just think carefully. Because you're producing a concentrated blast of cold air, you can freeze things REALLY quickly! Honesly, if you've got a large air compressor that's just sitting there, this is worth a build. In my experience, uses come after it's built, not before.