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I Use of Entropy for a Control Volume in Energy Balance

  1. Mar 12, 2016 #1
    Hi all,

    I'm having some trouble figuring out why entropy is used instead of enthalpy for an open system.

    From what I understand, an open system uses entropy to calculate internal energy. Since the control volume is constant (i.e. Δv = 0), wouldn't using : h = u + PΔv effectively be h = u? So in this case, wouldn't you use enthalpy instead of entropy since volume isn't changing?

    Just to confirm, in an open system like a turbine or something, with the assumption of no heat transfer and negligible potential energy, the energy balance becomes:

    W = ΔU
    W = m2(h2 + ke2 + pe2) - m1(h1 + ke1 + pe1)

    ... and with steady flow ...

    w = (h2 + ke2) - (h1 + ke1)

    Please let me know if I am misunderstanding something, thanks!
    Last edited: Mar 12, 2016
  2. jcsd
  3. Mar 12, 2016 #2
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