Transcript 工程热力学 - China University of Mining and Technology

1 Basic Concepts of Thermodynamics
1-1 Thermodynamics and Energy
1-1-1 Definition of Thermodynamics
Thermodynamics is defined as the science of energy.
Engineering thermodynamic deals with the principles of energy and
energy conversion in engineering and the thermal properties of
substances as well.
It can be divided into: Classical Thermodynamics--------macroscopic approach
Statistical Thermodynamics--------microscopic approach
1-1-2 Contents of Thermodynamics
The Basic Laws of Thermodynamics :
(1) The First Law of Thermodynamics:
an expression of the quantity relationship during energy change
(2) The Second Law of Thermodynamics:
an expression of the quality relationship during energy change
The Properties of Material
(1) Ideal Gas
(2) Vapor: Water Steam, Freon , Ammonia
(3) Mixtures: Gas mixture, Gas-vapor mixture
Practical Application
Gas Power Cycle
Vapor and Combine Power Cycle
Refrigeration
History :
1-1-3 Dimensions and Units 1.1790 French National Assembly charged the
From Le
Systeme
nternational
d’Unites
French Academy of Science
Any physical quantity can be characterized by dimension
The arbitrary magnitudes
2. 1875 The Metric Convection Treaty was signed
by 17 nations.General
Conference ofare
Weights
assigned
to the dimensions
called Units
and Measures ( CGPM ) was established
Primary( foundamental ) dimensions and units:
3. 1960 the CGPM produced the SI
Mass m, Length L, time t, temperature, T, mol
Secondary dimensions:
velocity
V, energy
E , volume V
English system:
Also known as the United States Customary System(USCS)
SI:
International System
Some SI and English units
Refer to our Text book
Dimensional Homogeneity
Every term in an equation must have the the same units.
* the student must keep in mind that a formula which is
not dimensionally homogeneous is definitely wrong, but a
dimensionally homogeneous formula is not necessarily
right
1-2 Thermodynamics System
1-2-1 The definition of Thermodynamics System
System : A quantity of matter or a region in space for study
The real or imaginary surface that separates the system from its
surroundings is called boundary
Anything outside the boundary is called surrounding
The boundary can be real or
imaginary; fixed or
changeable, has no thickness
and volume , and does not
contain any substance.
1-1-3-2 The Classification of Thermodynamics system
Closed system :
A system that only energy can cross the boundary but no
mass can enter or leave ( also known as control mass )
Opened system :
A system that not only energy but also mass can cross the
boundary (also known as control volume )
isolated system :
A system that neither energy nor mass can cross the boundary
(a special closed system)
The Universe
1-3 State and Equilibrium
1-3-1 Definition: All macroscopic characteristics of a system at one time point
1-3-2 Classification:
Equilibrium state:
any part of the system have the same characteristics
any characteristics don’t change with time
Nonequilibrium state:
1-3-3.Conditions of Equilibrium
•Thermal equilibrium
•Mechanical equilibrium
•Chemical equilibrium
•Phase equilibrium
1-4 Properties of System
1-4-1 Definition
Any characteristic of a system is called a property
1-4-2 Characteristic of property
Related with States but have nothing to do with the path from one state
to another
According to mathematics：
∮dx = 0
1-4-3 Classification of property
Intensive property：
Properties independent of the size of system, such as：
temperature, pressure, specific volume
Extensive property：
Properties whose values depend on the size or
extent of system, such as：
volume, entropy, enthalpy
1-5 The state postulate
The state of system is described by its properties,But
obviously we don’t need to specify all the properties to fix
a state,Then how many properties are enough to fix a state?
State postulate
The state of a simple compressible system is completely specified
by two independent, intensive properties
A system is called simple compressible system in the
absence of electrical, gravitational, motion, and surface
tension effects.
1-6 Thermodynamics Property Coordinates
Since two properties can fix a state, it’s very convenient
to draw a state point on a two-dimension coordinates.
p
equilibrium state
nonequilibrium state
v
The usually used coordinates are: p-v coordinates, T-s
coordinates, h-s coordinates
How to
change
under
equilibrium
state?
1-7 Process and Cycle
Process:
any change that a system undergoes from one equilibrium state
to another
Quasi-static process:
a process proceeds in such manner that the system remains
infinitesimally close to an equilibrium state at all time
p
Initial state
Quasi-static process
final state
nonequilibrium process
v
Cycle:
A path-closed process
p
v
1-8 Forms of energy
1-8-1 Total energy
The total energy E of a system is the sum of thermal, mechanical,
kinetic, potential, electric,chemical and nuclear energy.
On a unit mass basis,
e  E/m
1-8-2 Classification of energy
1.Macroscopic form :
m V2
KE 
2
kinetic energy KE and potential energy PE
and
On a unit mass basis
V2
and
ke 
2
PE  m gz
pe  gz
2.Microscopic form:
relate to the molecular structure of a system and the degree of
the molecular activity. The sum of all the microscopic forms of
energy is called the internal energy, denoted by U or u on a unit
mass basis.
1-8-3 Expression of total energy
m V2
E  U  KE  PE  U 
 m gz
2
On a unit mass basis
V2
e  u  ke  pe  u 
 gz
2
1-9 Pressure
1-9-1 definition
The force exerted by a fluid per unit area
Calculation:
F
P
A
Unit:
SI:
Pa--------N/m2
English:
psi------lbf/in2
1bar=105 Pa
= 0.1 Mpa
=100 kPa
1atm=101,325 Pa
= 101.325 kPa
=1.01325 bar
=14.696 psi
The pressure calculated
by formula
P=F/A
1-9-2 Gage pressure and vacuum
Pressure
Absolute pressure: the actual pressure at a given position
is called the absolute pressure
manometer
In SI the unit also is Pa, but in
English psig usually employed
Gage pressure:The pressure over atmospheric pressure
while gas pressure is larger than that of atmosphere，
expressed by Pg
Pg  Pabs  Patm
Vacuum pressure:The pressure below atmospheric
pressure while gas pressure is smaller than that of
atmosphere，expressed by Pv
Pv  Patm  Pabs
Absolute pressure? gage
pressure? vacuum pressure?
woooooooo!
Which one is the state property?
1-10
I think it is the measure
of hotness or coldness
Temperature
Am I right?
What is
temperatur
e?
1-10-1 The zeroth law of thermodynamics
If two bodies are in thermal equilibrium with a
third body , they are also in thermal equilibrium
each other .
1-10-2 thermodynamics definition of temperature
It’s the property to describe if systems are in thermal
equilibrium
1-10-3 Temperature scale
Three factors of Temperature scale ：
•
zero point
•
two states point ( easily reproducible states )
•
Scale
Formerly called the centigrade
scale, in 1948 it was renamed
after the Swedish astronomer
A.Celsuis (1701-1744)
Temperature scale in history named after the German
instrument maker
SI:
Celsius scale
G.Fahrenheit (1686-1736)
English: Fahrenheit scale
Thermodynamics temperature scale:
SI:
Kelvin scale
English: Rankine scale
Relations of scales:
T (K)  T (。C)  273.15
T (R)  T (。F)  459.67
T (。F)  1.8T (。C)  32
T (R )  1.8T (K)