Designing a working Ranque-hilsch Vortex tube to be 3D printable or machinable on a lathe Answered
I have looked far and wide in the attempt to fully understand how these tubes actually work.
Nothing online really satisfies me fully so I made my own theories ;)
I won't bother you with them tough as the foucs is on giving some tips on designing these tubes.
Lets start with the main components as you find them in petents and some of the online videos and tutorials:
1. The tube.
It does not seem to do much but it is a very important part of the design.
The diameter needs to be correct for the pressure and air flow rate so the up and down vortex can form properly on the inside.
The lengh is important as well because you won't the hot end outlet right where the tube is hottest.
Too long and you waste more than you use, too short and you won't get a decent temp difference.
2. The chamber, rotation box, inlet box or whatever you wanna call the thick bit where the air goes in that holds all parts together.
Most designs, even commercial ones, seem to go dirt simple here.
Inlet on the side so the air starts spinning around on the inner wall.
And that's about it as it really is nothing but a simple cylinder.
However, the free space and internal design of the components affect the performance of this chamber.
3. The engine, diffusor or cold end tube.
This little gadget, in commercail tubes, serves a dual purpose.
Firstly I provides small inlets to generate several vortex stream for the tube.
Secondly I provides the cold end outlet nozzle.
Both the nozzle and the inlets are important for the workings.
Depending on their dimensions you need a higher or lower airflow and get a higher or lower temp difference.
With just these three bits you see that a lot of variables are involved.
I simulated most of my parts, but with the lack of proper software this happened in the empty void between my ears ;)
Needless to say I learned a lot of ways to design fance looking tubes with no real function LOL
If you use some aluminium tubing for the actual tube and machined parts you have an advantage over 3D printed plastic parts.
Not just in terms of safety but also in terms of actually measuring the temp of the tube.
Blessed are those who have a heat vision camera :(
Either way, once you start the design there will be a lot of errors you would need address later - and they waste time and material.
Let me share my thoughts on how designed my first working tube:
Compressor delivers a certain pressure and flow rate.
I made something to test how different diameter holes as a substitude for tubes would affect the air pressure and flow.
Once I had a diameter that would not drain the compressor and kept a stable pressure I used this to calculate inlet and orifice sizes.
Goal was to have the same overall cut area as for what worked on the compressor test.
For example if a 10mm hole works fine than a first guesstimate for 5 inlets would be a 2mm diameter.
Usually slightly more as interal friction and such create more pressure in the system.
So far for the inlets and vortex creating parts - what about clearances??
Imagine you have a short cylindrical space where the tube shall be mounted to.
Your inlets need to have enough free space to allow for the air to spin around the center.
Key is to have the clearance tight enough to keep a high pressure in this area.
Imagine the 5 2mm holes from the above example providing the air flow.
A 1mm gap would mean insane pressures that most likely will limit performance.
On the other side a 8mm gap will cause a massive pressure drop and results in the vortext from the inlets to get chaotic.
If you blow compressed air through the housing with the engine inside but no tube mounted and just a backing plate to close it then you should hear a distinct sound.
If it sounds just like air escaping through some holes you got it wrong :(
However, if start screaming like mad when the pressure goes up it means you created a quite powerful vortex ;)
But don't confuse the sound of small holes with something that almost sounds like a siren ;)
A tube lenght of around 15cm seems to work fine for most applications.
However it makes sense to check the tube temp to make sure the lenght is set to the hottest part of it.
If in doubt, try it out ;)
You designed it so well, it makes all the right noises but it won't cool or get hot....
I had this problem a lot in my early days.
An old patent however gave the clue that you won't find that easy.
On the hot end outlet you can use a simple cone to limit what comes out of the tube.
Either with fancy internal outlets or just outer casing with slots.
Not much different to what you find online.
What you might miss though is the diffusor or vortex terminator.
You see, the upgoing vortex is extremely fast in it's rotation.
If it hits the outlet cone like this then most of the air will be forced out and the inner vortex won't form at all.
A star like insert at the end of the tube will cause the outer vortex to become a straight upward stream.
Not all of the air will be able to fllow this route, so the rest is refected and creates the inner vortex.
A tiny detail that often gets lost it seems.