Since the last vortex tube that I built worked really well (and the previous one didn't) I decided that I would try to see what was happening inside a vortex tube while it is operating. This isn't an easy thing to do because the air inside a vortex cooling tube is swirling around really fast and you can't see it. I thought that I might be a neat idea to make a large transparent tube and inject smoke into it along with the air and use lasers to see the smoke swirl around. What a great idea!? Well maybe... a large tube would require a very large amount of air to get any thermal separation and at a large air flow rate the smoke patterns disappear fast and be hard to see. With that in mind I built a large transparent vortex tube anyway without any intentions of getting thermal separation and using a very low air flow rate just to see the smoke.
|Vortex Flow With Smoke and Lasers|
In other words this is just a model to get an idea of what is going on inside the tube and how the airflow interacts during the vortex formation, the cold plate orifice and the cone valve. One thing I realized right away is I need a high speed camera to really see what is going on.
As always I had to build this on a budget of $0.00 so it was all made from stuff I already had. The clear plastic tube is 2" in diameter (I.D.) and for half of the tests I used a cold orifice hole of one inch. The other half of the tests I ran with a 0.5" hole. The cone valve was 1.75" in diameter and I could only adjust it's position, not how much air it's cutting off. I have three 1 milliwatt fan lasers that I used to illuminate the tube and 2 small variable speed air pumps to inject air into the tube. One air inlet hole is 0.063" and the other is 0.09" for air and smoke. I clamped the tube to a piece of wood with a big clamp and turned it all on.
I don't have any way to measure the airflow and these tests were more messing around that any designed experiment. What I was looking for is an idea of how the counter flow is started near or around the cone valve and what is happening near the cold orifice hole. I made over 60 videos of the experiments but because of the relatively low light levels it can be hard to see what is happening. A high speed camera would go a long way to seeing things flow but below in this post is a video. Here is a picture of the test setup.
|Smoke Vortex Test Setup|
In the above picture you can see the tube, laser positions and all the rest of the parts. The fan lasers are just regular point lasers with lenses on them to spread the beam out into a wide plane. If you shine one on the wall it will make a thin line rather than a sharp point. I placed 2 of them on adjustable mounts with the beams cutting across the tube perpendicular to the tube axis. The third laser is placed right behind the cold plate orifice hole near the air inlet tubes and is shining right up the center of the tube along the center axis. I arranged the lasers this way so the first two lasers provide a visualization of the swirling around the axis of the tube and the third laser illuminates the flow along the axis of the tube. I made a compilation of some of the 60 odd videos I made here so you can see more or less what I saw. Because of the light level not all of what i saw was captured in the videos.
What conclusions can be made from this experiment you ask? Don't feel alone because I ask myself that too! It's not clear from the videos what was going on inside the tube but I could see a lot more than the camera could image. What I did notice is the vortex formation was much more stable at higher airflow rates. This is not too surprising. It is also clear that there is a great deal of turbulence right near the air inlet and that inhibits quick vortex formation. I believe that the cause of the turbulence was due in part to the low speed of the air pumps I was using. The pumps are single diaphragm and at low speeds were 'puffing' air into the tube is short blasts of air. This puffing and the different sized air inlet tubes prevented a nice swirling flow near the cold plate orifice. I did notice a Secondary Circulation Loop flow pattern very much like the picture in Figure 2.3(b) on page 20 of THIS document. That was very interesting and you can see a little of it in the video I made.
The other interesting thing to note is the latency of air in the center of the vortex. It is clear that the air that gets 'trapped' in the center of the vortex appears to remain there for a much longer time than the air at the outside of the vortex. It is spinning around but it isn't moving as fast. I think because of that latency, the air in the center of the vortex has more time to release kinetic energy into the higher speed outer region. That release of energy cools the air in the center (I think) and heats the air around the outside that is also picking up heat from friction with the tube wall itself.
The next things I would like to try with this is use a higher powered laser to get more light into the tube to photograph what is going on. I would also like to get a high speed camera for a better slow motion look at what is going on. I am in the process of getting both of those and should have another setup soon. The next experiments will also use identical sized inlet tubes that are connected to a manifold to balance the flow and an accumulator tank to smooth out the blasts of air. Lastly I'm going to redesign the cone valve so that I can change the open area annulus and choke off the 'hot flow' in a controlled manner.
If you have any comments or suggestions (critiques!) please email me at email@example.com or leave a comment. Oh yeah! If you have a high speed video camera you want to sell let me know!