1 basic concepts of thermodynamics 1-1 thermodynamics and energy thermodynamics is defined as the...
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1 Basic Concepts of Thermodynamics
1-1 Thermodynamics and Energy
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.
1-1-1 Definition 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
1-1-2 Contents of Thermodynamics
It can be divided into: Classical Thermodynamics--------macroscopic approach
Statistical Thermodynamics--------microscopic approach
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
Any physical quantity can be characterized by dimension
The arbitrary magnitudes assigned to the dimensions are called Units
Primary( foundamental ) dimensions and units:
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
1-1-3 Dimensions and Units
From Le Systeme International d’Unites
History :
1.1790 French National Assembly charged the French Academy of Science
2. 1875 The Metric Convection Treaty was signed by 17 nations.General Conference of Weights and Measures ( CGPM ) was established
3. 1960 the CGPM produced the SI
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
1-2-1 The definition of Thermodynamics SystemSystem : 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.
1-2 Thermodynamics System
Closed system : 1-1-3-2 The Classification of Thermodynamics 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 Equilibrium1-3-1 Definition: All macroscopic characteristics of a system at one time point
1-3-2 Classification:
1-3-3.Conditions of Equilibrium
Equilibrium state:
any part of the system have the same characteristics
any characteristics don’t change with time
Nonequilibrium state:
•Thermal equilibrium
•Mechanical equilibrium
•Chemical equilibrium
•Phase equilibrium
1-4 Properties of System1-4-1 Definition
Any characteristic of a system is called a property1-4-2 Characteristic of propertyRelated with States but have nothing to do with the path from one state to another
According to mathematics :
∮dx = 01-4-3 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
v
equilibrium state
nonequilibrium state
1-7 Process and Cycle
Process:
any change that a system undergoes from one equilibrium state to another
a process proceeds in such manner that the system remains infinitesimally close to an equilibrium state at all time
How to change under
equilibrium state?
Quasi-static process:
p
v
Quasi-static process
nonequilibrium process
Initial state
final state
1-8 Forms of energy1-8-1 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,
1-8-2 Classification of energy
1.Macroscopic form : kinetic energy KE and potential energy PE
mEe /
2KE
2mV mgzPE
On a unit mass basis
2ke
2V gzpe
and
and
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
mgzmV
UPEKEUE 2
2
gzV
upekeue 2
2
On a unit mass basis
1-9 Pressure1-9-1 definition
The force exerted by a fluid per unit area
Calculation:
A
FP
Unit:
SI: Pa--------N/m2
English: psi------lbf/in2
1-9-2 Gage pressure and vacuum Pressure
Absolute pressure: the actual pressure at a given position is called the absolute pressure
The pressure calculated by formula P=F/A
manometer
Gage pressure:The pressure over atmospheric pressure while gas pressure is larger than that of atmosphere , expressed by Pg
atmabsg PPP
Vacuum pressure:The pressure below atmospheric pressure while gas pressure is smaller than that of atmosphere , expressed by Pv
absatmv PPP
In SI the unit also is Pa, but in English psig usually employed
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 TemperatureI think it is the measure of hotness or coldness
Am I right?
What is temperatur
e?
1-10-2 thermodynamics definition of temperature
It’s the property to describe if systems are in thermal equilibrium
1-10-3 Temperature scaleThree factors of Temperature scale :• zero point • two states point ( easily reproducible states ) • Scale
Temperature scale in history
SI: Celsius scale
English: Fahrenheit scale
Formerly called the centigrade scale, in 1948 it was renamed after the Swedish astronomer A.Celsuis (1701-1744)
named after the German instrument maker G.Fahrenheit (1686-1736)