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Water, piston-cylinder problem [Thermodynamics]

  1. Jan 25, 2016 #1
    1. The problem statement, all variables and given/known data
    Okay, so these are usually pretty easy for me to understand, but this one doesn't make sense.

    10 kg of water in a piston cylinder arrangement exists as saturated liquid/vapor at 100 kPa, with a quality of 50%. It is now heated so the volume triples. The mass of the piston is such that a cylinder pressure of 200 kPa will float it. Find the final temperature and the heat transfer in the process.

    2. Relevant equations
    v = V/m
    v - specific volume
    V - volume
    m - mass

    y = yf +xyfg

    3. The attempt at a solution

    State 1:

    m1 = m2 = m = 10kg
    x1 = .5 ---> 2-phase mixture
    P1 = 100 kPa = 1 bar
    v1 = .0010432 m3/kg + (.5)(1.694 - .0010432)
    v1 = .8475 m3/kg

    State 2:
    v2 = 3*v1 = 2.5425 m3/kg
    P2 = 200 kPa --> pressure is constant from this point on

    Solutions I have found online say that the final temp is in the 800's. How is that possible? I went into my steam tables (Fundamentals of Engineering Thermodynamics, Moran) and I couldn't figure out the state. I assume it is a superheated vapour. Is this right? If so, I go to the tables and the temp could probably be interpolated between 1.5 bar and 3 bar, but even then it's not close to 800. I believe the answer is 827 or 829 degrees C. Thanks for any help.
  2. jcsd
  3. Jan 25, 2016 #2
    What temperature does the ideal gas law give you? (This is a pretty low pressure).
  4. Jan 25, 2016 #3
    Thank you! I've always been under the assumption to never use ideal gas for non ideal gases though? This is a first for me. Is there a way to know? Thanks again
  5. Jan 25, 2016 #4
    Calculate the reduced pressure and the reduced temperature. Then, see the chart in Moran et al for the Z factor. Or better yet, calculate the pseudo-reduced volume and the reduced pressure and use the Z chart in Moran.

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