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Chapter 3: Evaluating Properties

Chapter 3: Evaluating Properties. Property Relations in Engineering Thermodynamics. Lava flowing into the Pacific Ocean in Hawaii. Photo courtesy of Mike Benson. The State Principle.

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Chapter 3: Evaluating Properties

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  1. Chapter 3: Evaluating Properties Property Relations in Engineering Thermodynamics Lava flowing into the Pacific Ocean in Hawaii. Photo courtesy of Mike Benson.

  2. The State Principle Two independent, intensive, thermodynamic properties are required to fix the state of a simple compressible system. For example: P and v T and u x and h Intensive thermodynamic properties:

  3. P-v-T Relation Note the location of the following: - Single phase regions - Two phase regions - Saturation states - Triple line - Critical Point

  4. T-v diagram Diagram courtesy of Jerry M. Seitzman, 2001.

  5. Phase Changes Constant Pressure phase change of water: • Subcooled or Compressed Liquid • Two-phase liquid-vapor mixture (steam) • Superheated vapor

  6. Vapor and Liquid Tables For Water: • Superheated vapor properties in Tables A-4, A-4E • Compressed liquid properties in Tables A-5, A-5E

  7. Linear Interpolation:Between values in the tables Subscripts: L – Value in table at lower end H – Value in table at upper end None – value of interest

  8. For Saturated Mixture (Liquid-Vapor) Region Quality; x; an intensive property x gives fraction that is vapor (gas) (1-x) gives Moisture Content QualityFor use in Tables A-2 and A-3 0 ≤ x ≤ 1; x = 0 → Saturated Liquid (subscript ‘f’) x = 1 → Saturated Vapor (subscript ‘g’) ‘fg’ → ‘g’-’f’

  9. Quality Relations LET b = ANY INTENSIVE PROPERTY • (b = v, u, h, s, etc.)

  10. Enthalpy Enthalpy is a property constructed due to the frequent occurrence of the above combination of properties.

  11. Specific Heats (Heat Capacities)

  12. Using Saturated Liquid Data (‘Compressed Liquid Rule’) Using ‘Incompressible Substance Model’ Approximations for liquids and solids

  13. Properties for Ideal Gases Requirements: The Ideal Gas Model: When specific heats are assumed constant Table A-20(E): Tables A-22(E) and A-23 (E)

  14. Polytropic Process of an Ideal Gas For a closed system: Expansion/Compression (Moving Boundary) Work (Ideal Gas OR liquid): Ideal Gases ONLY:

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