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Vortex Tube

Tom Booth

By Tom Booth
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Tom Booth

Tom Booth

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I'm splitting this off from the Stirling Turbine thread.

 

I made a modification to the vortex tube and it is finally showing some action.

 

I knew that the capillary tubing from the old air-conditioner that I was using for the "injector" or whatever it's called had a slightly smaller orifice than than recommended, by something like 1/2 mm.

 

I thought if I added another, the two combined would be slightly over specs. (too much air input ?) but I figured that it was worth a try anyway:

 

vortex_tube_3_a.jpg

 

I looped another piece of capillary tubing around to the top and opposite side as shown, drilled the holes and soldered it into place.

 

I plugged it into the compressor hose and tested it again. There didn't seem to be any change. Still just blowing luke warm air out both ends. I checked the measurements for the "Hot" tube length and carefully measured it out again and discovered that I had miscalculated slightly somehow. It was about 1 inch too long. I cut this off and tried again. Still nothing.

 

I tried a few things. First I stuck some stainless steel wool into the end of the Hot tube again, as I had done before the modifications.

 

This time the hot side started to feel nice and warm. The cold side felt cold, but not noticeably colder than the copper tubing normally feels, but the hot side most definitely began to warm up.

 

I would guess it was just slightly above my own body temperature as it felt quite warm to the touch. Nothing dramatic but most definitely nice and warm.

 

I should mention that I had not added a "stop cock" to the end of the Hot Tube side as I wanted to be able to experiment with it so I had nothing permanently affixed to the Hot end, so both ends of the Vortex Tube were open unless I was intentionally doing something to close them.

 

So I stuffed some more of the stainless steel scrubbing pad in the tube. As I did so It got, perhaps just a little warmer up to a point but I could get no more out of it by adding more tan about 1/4 to 1/2 inch or so. Just enough to stop up the end.

 

Then I tried various plugs to stop up the hot end with the steel wool inside. It seemed like this created some additional warmth and the cold side really started to feel cold.

 

To see if the steel-wool was helping just because it was blocking the pipe like a rather porous "stop cock" I took it out and just plugged the end of the pipe with a Phillips screw-gun bit that fit snugly in the end of the pipe but which allowed some air to escape around it. This had about the same effect.

 

But when I pushed the screw bit in tighter with my finger against the air pressure until the opening was almost completely closed there was a quite noticeable swing in temperature. The "hot" side got quite warm and the cold side got quite cold. Nothing that would fry an egg or cryogenically freeze anything yet but the thing was definitely showing signs of life. I could vary the temperatures by pushing the bit in tighter or looser.

 

I took the bit out and was trying to see what kind of temperatures were coming out of the tubes without it by blowing the air on the back of my hand when I noticed something that seemed rather strange. I could not feel any air coming out the cold side.

 

I put my finger over the hole and surprisingly, there was SUCTION !

 

My finger got sucked up tight against the hole like it was a vacuum.

 

To test this more objectively I clamped the thing in a vice and tore a strip of paper out of a notebook and gradually brought the paper closer and closer to the tube. When close enough, within about two inches of the tube, the paper strip got sucked towards the tube and stuck to the end. There was most definitely a vacuum on the cold side of the tube, EVEN THOUGH I WAS BLOWING COMPRESSED AIR INTO THE TUBE AT 125 PSI !!!

 

Becoming more and more amazed I noticed that the tube was STILL not making any noise. At least nothing in the audible range. It didn't sound any different than before the modifications, just a slight hiss of compressed air through the capillary tubes.

 

In one You-Tube video I had seen a guy was apparently trying to take a temperature reading and his long thermometer probe got caught in the vortex and caused the thermometer to spin.

 

I had tried doing the same before making the most recent modifications and got no effect. Now I tried it again using a round chain saw file that fit loosely in the tube. It started spinning !!

 

There was definitely a vortex in there.

 

I got carried away dropping various objects into the tube, or rather PUSHING them in against the air pressure. They did not spin until they got down to within about 2 inches of the diaphragm. The further in they went the faster they would spin reaching the greatest RPM when just about AT the air jets from the capillary tubes.

 

At one point I wanted to see if the cold air side would get colder by wrapping some steel wool around the hot side to dissipate the heat.

 

I did this at one point more or less absentmindedly WHILE I HAD A DRILL BIT SPINNING INSIDE THE VORTEX.

 

As soon as I applied the steel wool to the tube, holding a steel wool pad around the tube with my hand, the RPM of the drill-bit inside the tube rose very sharply and dramatically. Apparently the vortex received quite a boost due to the more rapid dissipation of heat.

 

Wow, this was getting to be fun. But the drill bit spun so incredibly fast inside the tube I was afraid it would fly out or do damage to the tube or the diaphragm, but I thought this would definitely be an area for some further experimentation down the road.

 

Out of curiosity, to see what the effect on the temperatures would be while the Vortex Tube was doing "Work" spinning an object, I put the chainsaw file back into the tube and held the tube up on end so the weight of the file would hold it in the tube without me having to push it in with my finger while I checked the temperature of the tubes with the back of my other hand.

 

This seemed almost as strange as anything. The temperature differential was now reversed ! The Hot Tube was now cold and the cold tube was Warm !

 

I took some of the aluminum fins off of the Air conditioners heat exchanger tubes and slipped them over the hot side of the vortex tube.

 

I now found that with this additional heat dissipation, when I put the chain saw file back into the tube, it was possible to get it spinning even though barely inserted into the end of the tube if I had it balanced on end on the table.

 

The tube was 8 inches long - so it seemed that the vortex had grown from about 2 inches from the diaphragm to the full length of the tube with the addition of the heat dissipating aluminum fins from the air conditioner.

 

When I get to fooling around with this some more another day I think I will try making a stainless steel wool sleeve to go around the hot tube.

 

I also noticed that during these experiments whenever the cold end of the vortex tube got cold the handle got cold also. The handle is soldered to the tube just for support. There is no air flow through this tube. It is only used to feed air to the capillary tubes, above that it is pinched off and soldered closed at the end but still attached to the cold tube for support.

 

I think this handle is conducting a lot of heat to the cold end and perhaps the cold end could get much colder if it were removed. I will try both of these additional modifications next time.

 

I might also mention that I made this small unit using the smallest possible tubing I could find (refrigeration capillary tubing) because I wanted to keep compressed air use to a minimum for all this testing. As a result, the compressor did not have to run continually or even very often to maintain a tank of compressed air. Still, the air volume or air flow was above what was specified. One was too little. Two capillary tubes was a little too much I think, but it was close enough to work to some degree.

 

There were moments when the temperature of the cold end really seemed to drop down to what I would call "dangerously cold" temperature. Not really, but I would guess at least "ice cold".

 

One other thing I tried. I cut a Styrofoam sleeve to put around the cold tube to see if insulating it would allow it to produce colder air. This did not seem to make any difference.

 

Getting the cold tube down as cold as I could get it and then adding the insulation didn't seem to make a difference.

 

There were some definite surprises in fooling with this thing. Phenomenon I had not seen mentioned previously. Most notably the vacuum at the cold end when the hot end was left open, the increase in the vortex size, velocity and strength with greater heat dissipation using aluminum fins or steel wool and the apparent temperature reversal when the vortex was doing "work" spinning an object.

 

A very strange device gets even stranger.

 

I'm having a little difficulty wrapping my brain around this things rather oddball behavior. Any ideas ?

 

Strange to say, my acoustic theory seems to have fallen flat. At least this test model did not make hardly a sound. Not within any audible range of my hearing anyway. There was, most definitely a vortex after-all, and a surprisingly powerful one I thought for the size of the thing - able to spin rather heavy screwdrivers and what not that I was putting in there.

 

Also the strength of the vortex, it seems, could be greatly increased by rapid heat dissipation so that it became stronger than what it was as just a "forced vortex" from the compressed air.

 

I did not test this aspect of the thing very thoroughly as the objects were spinning incredibly fast without having them spin any faster. I would first like to make some sort of rod that can be controlled and prevented from lashing around inside the tube and doing damage to the diaphragm or something (like me) I was not very comfortable with the idea of drill bits and such spinning around at incredibly high RPM with nothing but some swirling air to hold them in place, especially when I had to hold them in the tube with my finger. It was real interesting but probably not real smart.

 

I will make some videos of some of this strange phenomenon when I get the chance and the equipment to show temperature readings and such.

Edited by Tom Booth
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Tom Booth

Tom Booth

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I was about to head out to the shop this morning (with a cup of coffee to make some modifications to the small vortex tube as discussed earlier. Probably start by cutting off the top of the "handle" and replacing it with something non-heat conducting, like wood,) when a new theory occurred to me that I thought I would share.

 

I got a clue from the vacuum effect on the cold tube when the stop was off the end of the hot tube.

 

It appears that there is something like a "venturi effect" going on. Air gets thrown to the perimeter of the tube and due to its high speed of rotation cannot exit through the orifice in the diaphragm but goes the other way towards the exit at the other end (Hot or long end) of the tube leaving a vacuum in the middle. The vacuum is sucking air through the orifice from the cold end of the tube (or at least trying to). This is not particularly unusual. Many IC engine carburetors utilize the venturi effect to deliver fuel to the carburetor.

 

But when the end of the Hot tube is blocked the air trying to rush out is bounced back creating a kind of concussion wave that travels back towards the diaphragm. The "Back Pressure" or reverse wave collides with the air being drawn in through the orifice due to the venturi effect. This sets up a kind of back and forth reverberation at the orifice between the two tubes

 

So I'm guessing that there is some sort of "standing wave" situation which accounts for the acoustical-like dimensionality requirements but this is, strictly speaking, I think, more on the order of a concussion wave, shock wave or pressure wave rather than a "sound" wave.

 

By adjusting the valve at the end of the Hot tube it is possible to cause a kind of conflict of interests at the diaphragm orifice where the air is alternately sucked through one way due to the venturi effect and then pushed back due to the rebounding concussion wave from the stop or plug at the other "Hot" end. This sets up a reverberation which is amplified due to the tubes precise acoustical dimensions which creates a kind of pressure or concussive standing wave.

 

I'm thinking that the usual vortex theory is somewhat incomplete, or at least, I'm speculating that the temperature difference is not a result of heat transfer between an internal and external vortex. Maybe it is, I don't know, but my new theory is that the real action is all taking place at the diaphragm orifice.

 

Lets say that there is a standing wave due to the reverberation. The air is traveling through the diaphragm orifice, first one way then the other. If this reversal were in step with the standing wave, say the direction reverses every 1/2 cycle of the wave the pressure/temperature would reverse as well.

 

In other words, each time the air flows one way through the orifice a small pocket of cold air would be delivered through the orifice then there would be a reversal and a small pocket of compressed hot air would be delivered through the orifice in the opposite direction. With every reversal of direction in air flow there would be a corresponding reversal in temperature on one side or the other of the diaphragm opening.

 

In other words there would be a kind of Doppler effect or something where there would be a tendency for the air going left through the orifice to be compressed/heated and to the right expanded/cooled repeatedly in rapid succession but with relatively little actual air flow in either direction so that that the difference in temperature would tend to be cumulative.

 

As left to itself, the air would be suctioned in through the "cold" end and ejected out the "Hot" end, there only need be a valve at the hot end to regulate the flow so that the flow through the diaphragm orifice in either direction can be adjusted more to the left (venturi effect) or more to the right (back flow). - but I still think it is the standing waves set up due to this reverberation that accounts for the heat separation at the diaphragm opening more so, or rather than, the heat separation taking place due to the rapid spinning of the air in the vortex(s) in the hot tube.

 

Of course, this is just another "guess". I'll probably have an entirely new theory by tomorrow, but I think it should be possible to discover something one way or the other through additional experimentation.

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ewmon

ewmon

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Tom, I've read about vortex tubes, and I'm interested in your project.

 

An inch more or less shouldn't affect the tube's operation much/dramatically. A vacuum clearly shows something wrong. I haven't read about plugging either tube with anything. It seems a matter of drawing the outputs from either the perimeter or center of the tube. There's no moving or "spinning" parts that I know, and damaging the inside surfaces may adversely affect its operation. Insulation shouldn't make much difference. There shouldn't be any oscillations or cycling; the operation should be continuous.

 

What's inside your tubes?

 

What does your diaphragm look like?

 

What internally distinguished the cold end from the hot end?

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J.C.MacSwell

J.C.MacSwell

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I must have missed that article back in 1976. Amazing that it would have not surfaced earlier after invention in 1922.

 

I don't expect science to explain it until perhaps 2022, but since it works so well I fully expect it to take off at that time and everyone will want one knowing that it is possible, and doesn't just merely work, as it did 100 years earlier.

 

Ok, more seriously:

 

Moving air causes friction, heat (or thermal energy waste) and expanding (throttling) compressed air at ambient temperature causes cooling, so some inefficient process may allow this to happen.

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ewmon

ewmon

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The first time I heard of vortex tubes was decades ago when someone made them as an aftermarket air conditioner for cars! No power source required except the forward speed of the car.

 

See post #9 here.

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Tom Booth

Tom Booth

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The first time I heard of vortex tubes was decades ago when someone made them as an aftermarket air conditioner for cars! No power source required except the forward speed of the car.

 

See post #9 here.

 

Cool, and very interesting,

 

I'd like to see one of these things and maybe see if it is made in a similar way or what exactly. This sounds kind of like what Airplanes still use for cabin cooling using ram air through an "air-cycle" cooling system while in flight, but those systems are huge. Something you can clip on your car window ? I'd like to see that.

 

Anyway, I came here today to mention that I did the modifications on the tube mentioned previously. Removed and pinched off the top of the handle, added stainless steel padding to the hot tube and whatnot. I also made a "stop-cock" that could be very finely adjusted.

 

None of these changes made much if any real difference.

 

I think I just found out why in reviewing some additional online information last night:

 

----------------clip:

"There is a lot of evidence in the literature that the pipe that you use should be thermally non-conductive or if it is a good conductor (like metal) it should be insulated. I decided to use PVC because I had it laying around and of the three vortex tubes that I have made the metal one didn't work too good. Metal might be the reason for the poor performance."

 

 

http://ottobelden.blogspot.com/2010/12/another-home-made-ranque-hilsch-vortex.html

 

Now you tell me!

 

(The above link is very informative by the way, with some interesting thermal images of the vortex tube.)

 

Of all metals; copper is one of the best heat conductors, aside from silver and gold.

 

By thermometer reading I was only able to get a temperature drop of about 15 degrees Fahrenheit at best.

 

By comparison, I got about 5 degrees or so LOWER than that just blowing air onto the thermometer straight out of the compressor hose nozzle!

 

But, on the other hand, that was a very large VOLUME of air that does not produce heat and cold simultaneously.

 

I have noticed that it seems like all the commercial models have some sort of plastic or Bakelite type inserts for the diaphragm section with the spinner groves and orifice, even if the rest, the tubes themselves and the housing are metal.

 

This again seems to suggest that the critical heat separation is taking place at the central orifice and not in the tubes or these commercial units with metal tubes with a plastic central insert would probably work no better than my metal model. My central orifice is made out of a solid copper washer in a copper housing soldered to copper tubing - no wonder it is barely working.

 

Well, live and learn.

 

I will still make some videos when I get the chance as this tube IS CREATING A VORTEX, though apparently the heat/cold generated is being lost or re-equalized as quickly as it is produced due to the high rate of conduction through the copper.

 

And thanks for the information about the Vortex car air-conditioner. I will be seeing if I can find out any more about it.

 

Tom

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Tom Booth

Tom Booth

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Wow, thanks for that article. Why did I not see your post earlier ?

 

Anyway, I found the part about "There was an ear-piercing scream like that of a jet priming for takeoff".

 

I've read of the same thing before in other articles or accounts about the vortex tube - that it makes a horrific noise.

 

My tube seems to be working a little, just warm and cold rather than very hot and very cold, but it wasn't working at all before I shortened the length of the hot tube some. But it really isn't making any sound at all, just the rather quiet hiss of air.

 

I also appreciated the cut-away view of the inside. I read elsewhere that some tubes are tapered inside.

 

Tom, I've read about vortex tubes, and I'm interested in your project.

 

An inch more or less shouldn't affect the tube's operation much/dramatically.

 

I wouldn't think so, but I didn't seem to be getting any response from the thing at all until I shortened the length of the Hot tube, though, it is hard to say with just one or two trials. The capillary tubing might have been plugged with something and then it blew out later, I can't really be sure without building another one.

 

I'm going to try making some out of PVC pipe as some have said it works better than metal and it is also cheaper and easier to work with, unfortunately it is also bigger in diameter and will require more air.

 

 

A vacuum clearly shows something wrong.

 

There is only a vacuum at the cold end when the plug is taken out of the hot end. I read a forum recently where someone else reported the same thing when the plug was off the hot tube. Well, actually to quote:

 

Regarding the vortex tube, I read that if it is adjusted too far to one side, the cold end will actually induce atmosphere instead of blowing cold air out.

 

http://www.energeticforum.com/renewable-energy/6706-vortex-tube.html#post116251

 

and another:

 

Hi all.

Today I messed around with the vortex tube a bit. I made a simple tube like this:

 

 

(photo missing)

 

 

The tube is 40mm in diameter and 300mm long.

I tried to apply pressurized air to the tube, but all the air just flows out of the hot air side and nothing flows out of the small cold air hole. In fact, the small cold air hole creates suction. I don't have the cone on the hot side, could this be the problem?

 

http://www.energeticforum.com/renewable-energy/3486-hilsch-vortex-tube.html#post44666

Thanks,

 

So, can't really be sure but this doesn't seem to be at all unique to the one I made.

I haven't read about plugging either tube with anything.

 

The hot end is supposed to have a valve of one sort or another to control the air flow. Usually a tapered plug like a cork in a bottle that just lets (Hot) air escape around the edges and is supposed to cause the cold air in the center to return back in the other direction. I'm skeptical that that is the exact mechanism of operation, especially since seeing some thermal images of one in operation. In the thermal imagery I see no signs of any center COLD core inside the Hot tube. The heat seems to be emanating directly from the center orifice, which would seem to lend some support to my theory that this is where the actual hot/cold separation is happening:

 

Scroll down a way on this page for the thermal image:

 

http://ottobelden.blogspot.com/2011/06/homemade-ranque-hilsch-vortex-cooling.html

 

It just looks Hot on the hot side, getting hotter towards the end and cold on the cold side, divided right at the "diaphragm" or orifice in the center. There doesn't seem to be any indication of a reverse cold vortex as far as I can see in the thermal images. (There are more thermal images in his YouTube video but they look just the same.)

 

It seems a matter of drawing the outputs from either the perimeter or center of the tube. There's no moving or "spinning" parts that I know

 

Usually the thing is said to have "no moving parts" though one guy on that other forum reported that some old vortex tubes did have something spinning at the end of the hot tube, though I have never seen this.

 

, and damaging the inside surfaces may adversely affect its operation.

 

I noticed that the tubing I used from the air-conditioner (after I cut some open) has a kind of stamped pattern of some sort on the inner surface that looks like a cloth weave. What the purpose of that might be I'm not sure, perhaps to increase turbulence for heat exchange. Probably not a good thing in this particular application.

Insulation shouldn't make much difference. There shouldn't be any oscillations or cycling; the operation should be continuous.

 

Since this tube isn't making any dangerous levels of heat or cold I was able to test it out by putting one end against my lip. In fact I let the cold end suction my lower lip into it, then I adjusted the valve at the hot end.

 

At the point where the suction began to let go and the cold air began flowing out the cold end I could definitely feel a kind of pulsation or vibration like a vibrating reed. With further adjustment of the valve and a greater flow of cold it was more difficult to detect but I suspect it was still present to some degree, It faded out gradually as I adjusted the valve further.

 

What's inside your tubes?

 

Nothing except for the "diaphragm" or orifice between the hot and cold sides. Otherwise it is just a straight tube

 

What does your diaphragm look like?

 

It is a small copper washer that I ground down around the edges to fit inside the tube and soldered in place.

 

What internally distinguished the cold end from the hot end?

 

Nothing other than the Hot end has the air jets going into it, just to the side of the diaphragm from the capillary tubing and the hot tube is longer, otherwise it is just one long tube divided into two by a "washer" with a hole in the middle. And also the hot end is plugged by a "valve". I just used a bolt with the end ground down to a taper, though I tried a number of other things.

 

One thing I noticed that makes me suspect of the predominant theory is that it seemed to work just as well if the hot end was just plugged up with something porous like a wad of steel wool or even a fish tank aerator (or just my finger) or practically anything. In other words, as far as I could tell it did not seem to be a matter of letting the Hot air escape around the perimeter of the tube but rather just a matter of restricting the air flow out of the hot tube irregardless of how that was accomplished, which seems to contradict the conventional theory of operation. But then, this tube isn't exactly working up to par either.

 

Apparently the size of the hole in the "washer" or "diaphragm" needs to be in proportion with the other dimensions Pipe ID, length, and the ID of the capillary tubes I'm using for "injectors" instead of just drilling holes as is usually the case.

 

I'm thinking though that material is a consideration.

 

Old diagrams seem to illustrate the use of what looks like steel pipe. I'm not sure copper tubing was available when this thing was invented. Copper pipe had different thermal characteristics. It conducts heat much better than steel so there may need to be some compensation for that, though its difficult to guess what modifications might be necessary without really knowing how the thing works. I'm guessing that the copper tube would need to be shortened from the length specified for steel pipe.

 

I'll be doing some additional experimenting and hopefully have more definite information soon.

 

An interesting note about the Popular Science instructions:

 

A plastic, "fiber" or even a wooden diaphragm, washer or "throttle" is specified, though the rest of the thing can be built from brass.

 

This seems to be further confirmation, IMO, that the critical point of heat separation is taking place at the "throttle" or central orifice or "diaphragm" or "washer" as it is variously called.

 

What other reason could there be for this specification other than to prevent heat conduction at that particular point where the heat separation is actually taking place.

 

It doesn't make sense to me that hot and cold air are separated in the Hot tube by the vortex. IMO this would just tend to push ALL the air to the walls of the tube and leave a partial vacuum in the middle which I think is the reason for the suction when the hot tube is not partially blocked with a valve. I think the conflict between the sucking action of the "venturi effect" and the back pressure from the hot end being plugged causes some kind of rapid fluctuation or back and forth impulse through the "throttle" in the middle setting up compression/heat waves that due to the precise "acoustic" dimensions of the tube become "standing waves". But, that is just my pet theory at this point, mostly, though I think the thermal images and some other evidence like the necessity for a plastic or wooden, non-heat conducting washer and general principles. I don't know as there is any evidence to support the idea that a vortex of any sort can separate hot and cold air molecules whereas it is commonly available knowledge that "sound" waves cause compression which causes bands of hot compressed air. Perhaps these bands are separated due to the above described oscillation at the "throttle".

 

This would put this in the same category as a thermal acoustic engine I think.

 

BTW. If you are interested in seeing what's inside the tube, I have some photos I posted earlier in the other thread where I got interested in the Vortex Tube here:

 

http://www.scienceforums.net/topic/46143-stirling-turbine/page__view__findpost__p__623450

 

As can be seen here, there is nothing particularly interesting inside the tube other than light coming in through the other end:

 

vortex_tube_2_c.jpg

 

Oh, another BTW.

 

I'm still not sure what caused the drill bit I dropped into the vortex tube to start spinning faster. (judging by the sound it made) when I held some steel wool around the hot tube attempting to dissipate heat.

 

When I wraped the same steel wool around the tube and held it in place with tape (instead of my hand) I could not get any such effect.

 

I'm thinking that if anything, the steel wool was making the tube warmer or preventing it from getting colder as the shop was rather cool.

 

In other words I was holding the steel wool in my hand which was of course at my body temperature more or less (probably a little less) but this would have been warmer than the air in the shop at the time so in effect, I was not dissipating heat with the steel wool but more than likely adding heat or preventing heat loss by applying the steel wool which was warmed by holding it in my hand. So now I'm thinking that the vortex was strengthened by REDUCING heat dissipation rather than by increasing it.

 

This would make more sense as some say it is easier to get a working vortex tube by making it out of non-heat dissipating material like PVC pipe instead of metal.

 

When I was holding the steel wool to the pipe with my hand it was effectively adding my body heat, but when I just taped it in place this was not the case and it had no effect. Perhaps I should try applying the heat from a candle flame or a torch and see what effect that has.

 

I made a kind of wooden pinwheel I could insert down into the tube to experiment with that shouldn't cause any damage. It is a wooden dowel with a propeller on the end to more easily see it spinning and there is an adjustable stop to keep it from going too far down into the tube.

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Otto

Otto

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Have a look at this post related to some FAQ's about vortex tubes:

 

http://ottobelden.blogspot.com/2011/06/homemade-ranque-hilsch-vortex-cooling.html

 

You do need some back pressure on the hot end of the tube, with a 'plug valve' of some kind. The shape of the valve does have an influence on the temperature difference because of how it will redirect the flow back to toward the cold end. For the tube that I built in the link below I used a bolt and I angled the inlet holes for the air so that the vortex was spinning in the direction of the bolt threads - the air could "unscrew" on the thread and exit. I believe that this facilitated the removal of the hot air around the inside of the tube while deflecting the cold air back because the bold is ground down to a dull point.

 

http://ottobelden.blogspot.com/2010/12/another-home-made-ranque-hilsch-vortex.html

 

The high speed air that is spinning around against the ID of the tube is heated by friction and as it expands it is compressing the air near the middle of the tube. The air near the middle can escape out the cold air plate hole and expend causing further cooling. That is how I think it is working...

 

- Otto Belden

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