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C-D Nozzle - Mach No. >1 at Throat

  1. Feb 12, 2015 #1
    Hi,

    As I understand, it is theoretically impossible to have a Mach No. higher than one at the throat of a Convergent Divergent Nozzle.

    I'm currently working on some calculations, and while i've been told the calculations are correct, I'm getting abnormally high Mach Numbers at the throat.

    For example, pressure i'm reading at the throat (4.31 Diameter) is 3.3Bar, temperature is 9 degrees celcius.

    Using Velocity/Speed of Sound, I'm coming away with a Mach Number of over 1.2...

    I'm assuming it's something to do with Shock Waves, but I really have no knowledge of why?
     
  2. jcsd
  3. Feb 12, 2015 #2

    cjl

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    Can we have some more details about your setup and how you're getting the velocity? Is this a simulation, or is this a measured velocity at the throat?
     
  4. Feb 12, 2015 #3
    Sorry, I'd typed this up a bit quick...

    Essentially I'm working on a nozzle apparatus,taking temperature, and pressure readings at the three taps. Then velocity, ambient pressure and temperature at the top.

    upload_2015-2-12_18-46-6.png upload_2015-2-12_18-46-35.png

    The Mach number rises to a value higher than one at the throat, then drops to subsonic speeds after (which is abnormal in itself).
     
  5. Feb 12, 2015 #4

    cjl

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    How are you getting your mach number at the throat?
     
  6. Feb 12, 2015 #5

    Attached Files:

  7. Feb 12, 2015 #6

    cjl

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    Those numbers look odd - you don't have nearly enough temperature variation for your large pressure variation, for example. In addition, your method of obtaining mass flow rate seems prone to errors - specifically, how are you measuring V4? I wouldn't expect the velocity to be uniform across the cross section of region 4, so you'd have to have some way of averaging the velocity at a bunch of points.
     
  8. Feb 12, 2015 #7
    Measured the velocities via pressure sensor, we used the same pressure sensor at the centre of region 4.
    Mass flow-rate's just mdot = pAV, as far as I'm aware? Is it the formula or the figures causing the error?
     
  9. Feb 12, 2015 #8

    cjl

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    Using a pitot-static measurement?

    As for mass flow rate just being rho*A*V, yes, that's true, but I'm saying that I seriously doubt V is constant across the entire cross section, and you'd need some way to integrate/average the velocity profile to get a reasonably accurate value.
     
  10. Feb 13, 2015 #9
    How are you using the pressure measurement to get velocity? Are you just measuring the static pressure?
     
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