chapter 2 Properties of Pure Substances

2-1 Pure Substance2-1-1 Definition of Pure Substance

A homogeneous substance is pure substance

2-1-2 Phases of Pure Substance

Solid

Liquid

Gas

Plasma

2-2 Phase Change Process of Pure Substance2-2-1 Constant Pressure Process of Water

p

v

1 2 3 4 5

5’1’ 2’ 3’ 4’

5”1” 2” 3” 4”

c

T

v

1

2 3 4

5

1’

2’ 3’ 4’

5’

1”

2”

3” 4”

5”

c

T=constant

vapor

Liquid +Vapor

Critical point

Liq

uid

Liq

uid

+so

l id

Solid+Vapor

soli

d

p

T

Triple line

2-2-1 Properties of Water

1.One point ： Critical point C

2.Two lines ： Saturated water line, Saturated vapor line

3.Three Regions: Compressed water region, Saturated mixture region, superheated vapor region

4.Five states: Compressed water, Saturated water, Saturated mixture, Saturated vapor, superheat vapor

2-3-1 p-v-T Surface

2-3 P-v-T surface and P-T diagram

2-3-2 p-T diagram

P

T

SolidVapor

liquid

Melting line

Vaporization

Sublimation

Expand on freezing

Extract on freezing Critical

point

2-4 Property Tables

2-4-1 Saturated Liquid and Vapor

One property is enough to Calculate other properties.

t ℃ P, MPa v’ v” h’ h” s’ s”

100 0.1013 0.00104

1.6738 419.06 2676.3 1.3069 7.3564

200 1.5551 0.00116 0.12714 352.4 2791.4 2.3307 6.4289

Enthalpy ---- the combination property:

H=U+PV

Or, per unit mass h=u+Pv

2-4-2 Saturated Mixture

One property to calculate saturated properties, but how to determine the properties of saturated mixture?

One special property to calculate the fraction of liquid and vapor, the quality

total

vapor

m

mx

')1(" vxxvvmix

')1(" hxxhhmix ')1(" sxxssmix

2-4-3 Superheated Vapor

Two independent properties are used to Calculate other properties

T ℃ P=0.1 MPav, m3/kg h, kJ/kg s, kJ/kg.K

100 1.6958 2676.2 7.3628150 1.9364 2776.4 7.5438

2-4-4 Reference state and reference value

Water: Saturated liquid at 0.01 is taken as the referenc℃e state, where u=0 s=0

R-12: Saturated liquid at -40 is taken as the reference s℃tate, where u=0 s=0

Notice: The reference state differ with different tables

2-5-1 The Ideal-Gas

The molecules of ideal-gas have no volume

There are no attraction among molecules of ideal-gas

2-5-2 The Ideal-Gas Equation of State

2-5 The Ideal-Gas Equation of State

(1). pV = mRT pv=RT

R------The gas constant(2) pVm = μRμT pvm=RμT

Rμ-----The universal gas constant

= 8.314kJ/kmol.K

2-6 Real Gases2-6-1 Compressibility Factor

Pv=RT can only be employed for the gas under high temperature or low pressure

For real gas, from pv=RT :

RT

pvz z------- Compressibility Factor:

a measure of deviation from ideal-gas behavior

z=1: Ideal-Gas

z>1 or z<1: Real-Gas

2-6-2 Van der Waals Equation of State

(1)consider the volume occupied by the molecules themselves

from pv=RT :

v

RTp

Since the smaller space for the molecules flying will lead to more chance of hit on its container ,the pressure of the gas will increase then .

Correct this by replacing v as v-b

bv

RTp

(2) consider contraction among the molecules

The contraction will lead to a decrease on pressure, so correct the equation as following:

2v

a

bv

RTp

The contraction should be in proportion to ρ2 which can be write as 1/v2.

‘a’ is proportional coefficient decided by experiment

2-6-3 Other Equations of State

(1) Beattie-Bridgeman Equation

(2) Martin-Hou Equation

(3) Virial Equation

The end of This Chapter

Thank you