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Homework Help: Thermodynamics - find entropy in isovolumetric system

  1. Jul 25, 2014 #1
    1. The problem statement, all variables and given/known data
    A well-insulated rigid tank contains 7 kg of a saturated liquid-vapor mixture of water at 150 kPa. Initially, three-quarters of the mass is in the liquid phase. An electric resistance heater placed in the tank is now turned on and kept on until all the liquid in the tank is vaporized. Determine the entropy change of the system during this process.

    2. Relevant equations

    3. The attempt at a solution
    Using tables I found S1 = 2.88105 kJ/kg K
    by s=Sf + xSfg

    Since it finishes at the saturated vapor line the final entropy should be Sg @ 111.15 °C because this is the saturation temperature which I interpolated to 7.22369 kJ/kg K.

    For some reason the final value is completely wrong. The books gets it to 6.7296 for some weird reason.

    My thought was that if the box has a constant volume, the pressure is increasing so we can't use Tsat to find our final entropy. In other words - this process is not isothermal. Is this assumption correct?
  2. jcsd
  3. Jul 25, 2014 #2
    If you have 7 kg of water, and, initially, 3/4 is liquid, how many kg of liquid water do you have initially, and how many kg of water vapor do you have initially?

    What is the specific volume of the liquid? What is the specific volume of the vapor? What is the average specific volume of the combination? Does this average specific volume change if the volume of the tank is constant?

    What is the specific entropy of the liquid water initially? What is the specific entropy of the water vapor initially? What is the total entropy of the initial combination of liquid water and water vapor?

    If you have pure saturated water vapor in the final state with the same specific volume as the original combination of liquid water and water vapor, what is the final temperature and pressure? What is the final mass of the water vapor? What is the final specific entropy of the water vapor? What is the final entropy?

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