Chapter 3: Evaluating Properties

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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

Property Relations in Engineering Thermodynamics

Lava flowing into the Pacific Ocean in Hawaii. Photo courtesy of Mike Benson.

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:

P-v-T Relation

Note the location of the following:

- Single phase regions

- Two phase regions

- Saturation states

- Triple line

- Critical Point

T-v diagram

Diagram courtesy of Jerry M. Seitzman, 2001.

Phase Changes

Constant Pressure phase change of water:

• Subcooled or Compressed Liquid
• Two-phase liquid-vapor mixture (steam)
• Superheated vapor
Vapor and Liquid Tables

For Water:

• Superheated vapor properties in Tables A-4, A-4E
• Compressed liquid properties in Tables A-5, A-5E
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

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’

Quality Relations

LET b = ANY INTENSIVE PROPERTY

• (b = v, u, h, s, etc.)
Enthalpy

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

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)

Polytropic Process of an Ideal Gas

For a closed system:

Expansion/Compression (Moving Boundary) Work (Ideal Gas OR liquid):

Ideal Gases ONLY: