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Thermodynamics Control Volume evaluation 2 inlet

  1. Dec 9, 2015 #1
    Figure P6.95 provides steady-state test data for a control
    volume in which two entering streams of air mix to form a
    single exiting stream. Stray heat transfer and kinetic and
    potential energy effects are negligible. A hard-to-read
    photocopy of the data sheet indicates that the pressure of
    the exiting stream is either 1.0 MPa or 1.8 MPa. Assuming
    the ideal gas model for air with Cp = 1.02 kJ/kg . K, determine
    if either or both of these pressure values can be correct.

    Figure shows that:
    T1 = 800k
    P1 = 1.8Mpa

    T2 = 650K
    P2 = 1.0Mpa
    m(dot)2 = 2 kg/s

    P3 = 1.0Mpa OR 1.8 Mpa (to ues for calculations about wether it is even possible)

    i found h1 = 821.95, h2 = 659.84 using thermo tables

    So far i have used 0 = m1(h1) + m2(h2) - m3(h3) to find that h3 is 713.88

    Im really struggling with the concept of two inlets and finding the change (DELTA)h to use to find the outlet Temp right now.. using (DELTA)h = Cp(DELTA T)...

    Where should i go next? I believe i need to find the Delta h but im really not sure. i've spent hours on this.
  2. jcsd
  3. Dec 9, 2015 #2
    I have some ideas for you to consider. Are you supposed to be using the tables, or are you supposed to be assuming ideal gas behavior? If you are assuming ideal gas behavior, you can calculate the final temperature. Then test each condition to see if the entropy increases or decreases for each of the pressures. If you are using the tables, basically the same thing (particularly if your tables have entropies). Look up in you table h=713.88, and see what the temperatures are at the two pressures (they should be about the same). Then get the entropies from the table, and calculate the change in entropy through the device for the two choices of pressure.
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