ENGR 260 Section 7'3 7'5

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ENGR 260Section 7.3 7.5
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Entropy change of pure substances

• Entropy is a property the value of the entropy
  of the system is fixed once the state is fixed.
• Specifying two intensive properties (independent
  of mass) fixes the state of a simple compressible
  system, as thus the value of entropy.
• Entropy values in the tables are given relative
  to and arbitrary reference state.

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Entropy Values

• Entropy values are determined like any other
  values from the tables.

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Entropy of a liquid vapor mixture

• Determine the entropy of a saturated liquid-vapor
  mix at 100C with a quality of 0.25.

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Entropy of a compressed liquid

• Determine the entropy of water at 50C and 100
  kPa.

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Entropy of a specified mass in a closed system.
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Entropy T-s diagram
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Example 7-3 and 7-4

• Please work these examples the would make great
  quiz problems for Wednesday!

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Isentropic Processes

• A process during which entropy remains constant.
  ?s0 or s1 s2.
• All reversible adiabatic processes are
  isentropic.
• All isentropic processes are not necessarily
  reversible adiabatic processes.

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Isentropic Expansion of Steam in a Turbine

• Steam enters an adiabatic turbine at 5 MPa and
  450C and leaves at a pressure of 1.4 MPa.
• Determine the work output of the turbine per unit
  mass.

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Property Diagrams involving Entropy
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Isentropic Processes
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Adiabatic steady flow devices

• For adiabatic steady flow devices the vertical is
  the measure of the work and the horizontal is the
  measure of the irreversibilties.

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Carnot Cycle