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Sonic Flow

  1. Apr 29, 2009 #1
    I am wondering if I can use a shop air compressor (120psi) to achieve sonic flow? I am building a vortex tube (for spot cooling) and need a high velocity jet to form the vortex. What is the minimum pressure diff. required to achieve sonic flow of compressed air? Does varying the diameter of the orfice change the velocity if pressure is held constant? If someone could help me here or point me towards an equation that will answer my question that would be awsome.

  2. jcsd
  3. May 1, 2009 #2
    So I guess you answer your own questions around here. I posted on Engineering Tips forum and had 5 positive responses in 24 hours. Here is a very helpful responce I recieved.

    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).


    Engineering Tips Forum was very helpful to me and I would recomend it to anyone whos questions go unanswerd here.

    Here is some more usfull info I found by googling choked flow.


    "The Basic Concept
    A greatly misunderstood and misapplied notion is that of “choked flow”, also referred
    to as “critical flow”. In gas flow through an orifice there is an occasion
    where the gas velocity reaches sonic conditions. This occurs for air flow when the
    absolute pressure ratio is .528, i.e. when the downstream absolute pressure (P2) is 52.8% of the upstream absolute pressure (P1)."

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