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supersonic flow
2

supersonic flow

supersonic flow

(OP)
Hello everyone. I am attempting to build a small vortex tube for spot cooling of milling operations. I relize that vortex tubes are avalible comercialy for under $150 but want to try to build one myself just out of curiosity. A vortex tube employes a high volocity gas jet that creates a vortex which in turn causes denser cold air to separate from less dense hot air.

http://en.wikipedia.org/wiki/Vortex_tube

 I want to use a converging diverging nozzle to create this high volocity gas jet. It would be conveniant to use shop air compressor with max psi of 120psi. I know that for a converging diverging nozzle to work the gas volocity must be Mach 1 at the throat to be further accelerated by the diverging section. So is it possible to create a sonic flow with a converging section and 120 psi compresed air? Does a decrease in orfice diameter cause a increase in velocity if presure is held constant? Does anyone know of any equations that would apply here? Any info will be gladly appreciated.

Thanks everyone for your time.
Jslade

 

RE: supersonic flow

Yes, what you are doing is possible.  For air, the critical pressure is around 60% so for your 120 psig system, as long as your cooling air line is big enough to keep the pressure at the inlet of the vortex tube below about 66 psig (assuming sea level) then you should be able to get to choked flow and at that point it is just a matter of nozzle configuration to go above Mach 1.0.

I'm just not sure what you're gaining by going that fast.  The device heats/cools the air due to heat transfer facilitated by the conservation of angular momentum between the inner (cooling) stream and the outer (heating) stream.  By speeding up the outer stream aren't you decreasing the amount of time that this heat transfer can act?  I've seen Ranke Hilsche Vortex Tubes with back pressure regulators on the outlet of the cold side to increase the heat transfer time, seems like you are going in the opposite direction.

Good luck with this.  I've always been interested in a device that appears to violate the laws of thermodynamics (but doesn't really) and have designed a couple of odd applications to use them that haven't taken off yet (and probably never will).

David

RE: supersonic flow

Commercial tubes use simple nozzles. While maximun velocity would be nice It seems you are getting sidetracked. But since you are doing this for fun - whatever rocks your boat.

There is no generally accepted theory on how vortex tobes work but air density is not one of the factor. My personal conclusion that the votex tube separates molecules where the randon thermal motion is in line with the airflow (high-velocity, hot molecules) from molecules where the random thermal motion is opposite to the air flow (slower-velocity, cool molecules). The spinning can generate tens of thousands of G's as I recall.





  

RE: supersonic flow

no fancy nozzle is required, just tangential entry in the swirl chamber, and a good pair of ear plugs

the valve in the main discharge adjust the temperature

doesn't violate the laws of thermowell nor does it centrifuge the hot cold fractions, but they are neat

 

RE: supersonic flow

(OP)
Thank you all so much for your positive responces. I origanly posted this same post on physics forum.com and did not recieve any responces in over two weeks. Here it looks like there were five very helpfull posts in 24 hours. Eng Tips forum is awsome and I will use it from now on and recomend it to all my friends.

Thanks everyone
Jslade

RE: supersonic flow

jslade:

I do agree with your observation and kind words for Eng-Tips.com.

In my opinion, Eng-Tips.com is a terrific site when one needs to get some information, resources, experience, suggestions etc. on any given engineering topic in no time ...

There are some very knowledgeable people who are willing to go an extra mile when providing requested input ...

Speaking for myself, it is my pleasure to be of some help to those interested in what I can bring to the table ...  

http://www.engineering-4e.com

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