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Choked Flow!

  1. Jun 6, 2009 #1
    What would be the effect on a choked mass flow rate if there were fluctuations in the downstream pressure?

    Thanks
     
  2. jcsd
  3. Jun 7, 2009 #2

    FredGarvin

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    Downstream pressure has no affect on an upstream choked condition. The only way to increase mass flow in a choked section is to increase pressure and thus density.
     
  4. Jun 7, 2009 #3
    I thought that wit chocked flow upstream pressure did not effect the flow rate?!
     
  5. Jun 7, 2009 #4
    Here is a web-based flow rate calculator for various pipe sizes, lengths, head etc.
    http://www.pipeflowcalculations.com/pressuredrop/index.htm [Broken]
    If this doesn't work out, then try
    http://www.pipeflowcalculations.com/
     
    Last edited by a moderator: May 4, 2017
  6. Jun 7, 2009 #5

    FredGarvin

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    Upstream pressure will change the density and thus change the mass flow through the nozzle. An increase in pressure will not increase the velocity though. The downstream pressure changes will not be able to propagate upstream to the choked area so the downstream pressure change will never be seen by anything upstream.

    Here's a quick read by a manufacturer of precision orifices...orifii...whatever
    http://www.okcc.com/PDF/Choked Flow of Gases pg.48.pdf
     
  7. Oct 16, 2009 #6
    Many years ago my fluid flow professor had this to say when I indicated I couldn't understand how the flow would not increase when the downstream pressure was lowered. He said think of it this way; "It would IF it knew the pressure was lower but the pressure wave that tells it the pressure is lower only travels at the speed of sound so it never gets there to tell it!!"
    Frankly I have the hardest time telling welding enginers about "Choked Flow" as it related to a product I recently patented that saves shielding gas (see www.NetWelding.com[/url]) It is important since some folks sell a product that operates at a low pressure which doesn't use choked flow and the preset flow rate varies all over the placewith minor restriction changes that occur in production. I even wrote a technical paper for our Society's Journal ([PLAIN]http://files.aws.org/wj/2007/04/wj200704/wj0407-22.pdf [Broken]) without the math but it appears to me many folks don't like to think beyond what I cal the CNN 15 second answer to complex problems! Choked flow takes more than that. Electrical analogies don't work well.
    Jerry
     
    Last edited by a moderator: May 4, 2017
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