1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Convergent Divergent Nozzle

  1. Jan 13, 2015 #1
    I hope that, given I've sourced what I can, you may be able to help? I'm Currently working on a lab report for my Aeroengines unit based on Convergent-Divergent nozzle (http://imgur.com/HQML2Au).

    From the drawing, I have been provided D1, D2, D3, D4, T1, T2, T3, T4, P1, P2, P3, Velocity, Ambient Pressure and Ambient Temperature.

    I need to calculate the following:
    - Mass Air Flow rate through the system at each inlet valve.
    - Velocity at each section (inlet, throat, exit) of the nozzle.
    - Speed of Sound at each section (inlet, throat, exit) of the nozzle.
    - Mach No. at each section (inlet, throat, exit) of the nozzle.

    I don't have a real grasp of this subject, but from some text-books I've figured out the following, and mainly, I just want to ensure what I'm doing is correct.

    •Mass Flow Rate
    Density x Surface Area x Velocity Density = pRT, so I assume I'd take P1, and divide by (T1 x Universal Gas Constant)
    Surface Area = (Pi x D12)/4
    Velocity = Provided as part of the experiment.

    Continuity Equation = p1A1V1=p2A2V2 Now that I have p1 and A1 from the previous part, I should be able to rearrange to get V1? I'm assuming this would be the same for V3, V4?

    Speed of Sound
    I have this for a perfect gas: a = ROOT(yGcRT) So Root(specific heat ratio(?) x Newton's Constant x Gas Constant x Temperature?)
    Not sure how to calculate each part of this

    Mach No. M2 = V2/(specific heat ratio(?) x Newton's Constant x Gas Constant x Temperature?)

    Any help at all would be appreciated, I've worked out what I can, as I don't want to flat-out ask for answers...

    Thank you! :)
  2. jcsd
  3. Jan 13, 2015 #2
    I see a mass balance, but I don't see any momentum or energy balances. Have you heard of Bernoulli's equation?

  4. Jan 13, 2015 #3
    Hi Chet,

    Thank you for responding - I've not been given anything other than what's above, can I calculate momentum or energy from those?

    I've heard Bernoulli's in passing, but its not something we've covered; is this along the right lines? http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html#bcal

    I also have the attached presentation

    Attached Files:

    Last edited: Jan 13, 2015
  5. Jan 13, 2015 #4
    You speak of "inlet valves." Do you really mean that gas is flowing into the nozzle through the side tubes, or are these just pipes at which the pressure and temperature are measured? Let's see your calculations so far.

  6. Jan 13, 2015 #5
    Hi Chet,

    The temperatures are measured at those points.
    I've not fully calculated everything, but this is my initial attempt

    Thank you!

    Attached Files:

  7. Jan 13, 2015 #6
    Well, you know that the gas densities are not supposed to be negative, right? So, what did you do wrong in applying the ideal gas law to get the densities? Hint: Those pressures are not absolute pressures.... they are gage pressures.

    This problem does not look like an exercise in how to apply conservation of momentum and conservation of energy to solve a problem for nozzle operation. It is designed to give you practice in applying the relevant equations to data in order to diagnose the operation of a nozzle.

  8. Jan 13, 2015 #7
    Thought the negatives were a bit suspicious...thanks for that! I'm guessing I'd need to sub the Ambient pressure in instead?

    Do the equations look ok?
  9. Jan 13, 2015 #8
    The absolute pressures equal 1 bar plus the gage pressures. I'm guessing that, when you do this, all the mass flow rates will come out the same.

    The equations look OK because they are taken straight from the presentation. In applying the equations, please be careful to get the units right.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted