1 1 basic concepts of thermodynamics objectiveobjectives definition of thermodynamic system concepts...
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1 Basic Concepts of Thermodynamics
ObjectiveObjectives
• Definition of Thermodynamic System
• Concepts of State and its properties
• Pressure, Temperature and Specific Volume
• Quasi-static or Quasi-equilibrium Process
and Reversible Process
• Thermodynamic Cycle
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Thermodynamic System
System—A quantity of matter or
a region in space bounded by an
arbitrary surfaces for study.
Surrounding—The mass or
region outside the system.
Boundary—The surfaces
separate the system from its
surroundings.
The boundary can be real or imaginary, fixed or
variable.
–It is critically important to define your system before
attempting to solve a thermodynamic problem.
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Thermodynamic System
–Closed system (control mass)
– Open system (control volume)
No mass can cross its boundary. It consists of a fixed amount of mass.
A selected region in space. Both mass and energy can cross its boundary.
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Thermodynamic System
–Simple Thermodynamic System
SurroundingsHeat
One Type Work
Adiabatic System —No heat cross its boundary.
Isolated System— Neither mass nor energy can cross its boundary.
( 绝热系统 )
( 孤立系统 )
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Thermodynamic System
Single Composition System —Consists of only one kind
of substance.
Multi-Composition System —Consists of tow or more
kinds of substances.
Single-Phase System —Consists of only one phase.
Multi-Phases System —Consists of tow or more
phases.Homogeneous System —The composition and property are identical all over the system.
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Thermodynamic State
Thermodynamic state of a system –the condition of the system as characterized by the values of its properties.
Property –any characteristic of a system
• Basic Property
–can be directly measured
• Derived Property
–indirectly calculated from basic ones
, ,p T v
, ,u h s
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Pressure
ForcePressure
Area
2
lbf= psi
in
2
N= Pa
m
torr = mmHg
5bar = 10 Pa
Common Use Units( 附表 1)
English
SI
Other
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Pressure Measurement
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Gage and Absolute Pressure Scales
We commonly use three different kinds pressure scales (not
units).
Absolute — is relative to full vacuum (absolute zero).
Gage (Gauge) — is relative to the ambient pressure.
Used for pressure above the ambient pressure.
Vacuum — is relative to the ambient pressure.
Used for pressure below the ambient pressure.
g a b
v b a
p p p
p p p
= -
= -压力计的指示为工质绝对压力与压力计所处环境绝对压力之差
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Gage and Absolute Pressure Scales
0ap
1atm
vp
ap
bp
Absolute
Vacuum
bp
ap
gp
Absolute
Vacuum
1atm 101325Pa
0.1MPa 14.7psi
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Example
用刚性壁将容器分割成两部分，如右图所示。压力表 A 和 C 位于大气环境中，它们分别测量室 2 和室 1 的压力，压力表 B 位于室 2中，测量室 1 的压力。
1. 若 =0.25 MPa, =0.32 MPa, 大气压力为 0.1 MPa, 试计算 压力表 A 的读数及室 1, 室 2 的绝对压力。2. 若 A 为真空计 , 其读数为 0.05 MPa, 压力表 B 的读数为 0.03MP
a ， 则压力表 C 的读数为多少？
CpBp
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1 0 0.32 0.1 0.42MPaCp p p
1 2
2 1 0.42 0.25 0.17MPaB
B
p p p
p p p
2 0 0.17 0.1 0.07MPaAp p p
2 0 0.1 0.05 0.05MPaAp p p
1 2 0.05 0.06 0.11MPaBp p p
(1)
(2)
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Temperature
• Two objects in thermal equilibrium are at the same
temperature.
•Zeroth law — if two bodies are in thermal equilibrium
with a third body, they are also in thermal
equilibrium with each other.Thermometers — measure the temperature
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•Thermometers measure the temperature dependence of some physical property
Temperature Scale
ice point
steam point
0℃ F32O
100℃ F212O
Celsius Fahrenheit
o o( F) 1.8 ( C) 32T T
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Temperature ScaleThermodynamic Temperature Scale
Based on the 2nd Law of Thermodynamics by Kelvin, having nothing to do with physical property of substance.
热力学温度是以水的三相点来定义的，规定水的三相点的温度为 273.16K 。水的三相点是水、水蒸气和冰共存的状态。在标准大气压下，水的冰点实际上是水、冰、空气的一种混合状态，水的冰点的温度是 273.15K ，即冰点比水的三相点的温度低 0.01K 。摄氏温度以冰点为零度。
HART 水三相点瓶
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Temperature Scale
Four temperature scales are
in common use:
–Rankine
–Fahrenheit
–Kelvin
–Celsius
UnitAbsolute Relative
SI Kelvin Celsius
English Rankine Fahrenheitdiffer in:- dependent / independent of property of substance- location of the zero points- size of the degree unit
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Temperature Scale Comparisons
o
o
o o
o
o
o
(K) ( C) 273.15
(R) ( F) 459.67
( F) 1.8 ( C) 32
(R) 1.8 (K)
(K) ( C)
(R) ( F)
(R) 1.8 ( C)
T t
T t
t t
T T
T t
T t
T t
= +
= +
= +
=
D =D
D =D
D = D
Ice point 0
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Specific Volume
Specific Volume 3/ (m )v V m
31/ (kg/m )v
Density ( )
v
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Characteristic of State Property• Mathematical
1
2A
C
B
integral2
2 11d
d 0
f f f f
f
differential
2 2
d ( ) d ( ) dy x
f ff x y
x y
f f
x y y x
p
v
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• intensive and extensive
Characteristic of State Property
–Intensive property: a property that is independent of the extent or mass of the system.
• Example –temperature, pressure, density, …
–Extensive property: a property whose value is dependent upon the mass or extent of the system.
• Example –mass, volume, total enthalpy, …
–Specific property: An extensive property per unit mass. Specific properties are intensive.
• Example –specific volume (= volume/mass)
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Equilibrium
Stable equilibrium state –a state in which the system is not capable of spontaneous change to another state without a finite change in the surroundings. – no driving forces to carry out a change
平衡状态是指在没有外界影响的条件下，
系统的宏观状态不随时间而变化。
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Equilibrium
•Sufficient and Necessary Conditions for Equilibrium: –Thermal: the temperature is the same throughout the system. –Mechanical: the pressure is the same. –Phase: there is no driving force for the total mass in each phase to change. –Chemical: there is no driving force for chemical composition to change.
( 物质交换 )
只有当系统内部以及系统与外界之间都不存在不平衡势差时，系统才处于完全平衡状态。
Equilibrium Homogenousdifference?
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State Equation
We need only specify a certain number of properties to
fix the state of system. The exact number depends on compositions and phases
of system?
2
number of independent properties
number of different compositions of system
number of different phases of system
State postulate( 相律）
State equation ( , , ) 0f p v T =
Example:Water&Vapor
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P-V property diagram
1p
2p
1 Initial state
2 Final state • Any point illustrates the state of system.• The state must be Equilibrium.
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Quasi-Equilibrium Process
驰豫时间系统在无限小势差作用下，
由一个平衡状态过渡到另一个平衡状态所需要的时间。
只要连续两次扰动之间的时间间隔大于驰豫时间，可认为热力过程是由连续的平衡状态构成。即系统有足够的时间回复平衡后再接受下一次不平衡势差。
p
dp
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Quasi-Equilibrium ProcessSystem remains infinitesimally close to an equilibrium state at all times.
Process occurs slow enough to allow system to adjust its properties.
The interval of process change takes much longer time than system spontaneously adjusts to a new state after the previous equilibrium state was destroyed.
1p
2p
1 Initial state
2 Final state
P
V1V2V
Quasi-equilibrium Process
To connect states with line must be QE process.
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萧峰…右手呼的一掌，便向丁春秋 击去，正是降龙十八掌的一招“亢龙有悔”，…
丁春秋素闻“北乔峰，南慕容”的大名，对他决无半点小觑之心，…殊不料萧峰一掌既出，身子已抢到离他三四丈外，又是一招“亢龙有悔”，后掌推前掌，双掌力道并在一起，排山倒海的压将过来。
Quasi-Equilibrium?
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Thermodynamic Process
Thermodynamic Processes–transformations from one equilibrium state to
another. The series of states through which the system passes is called the path.
1p
2p
1 Initial state
2 Final state
path
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Reversible Process
Reversible process: the reverse process could be
performed so that the system and surroundings can be
restored to their initial states with no change in the
system or surroundings.
Sufficient and necessary conditions:•Quasi-equilibrium process• no dissipation in process
-friction, non-elastic deformation, resistance…耗散
Can the system return to initial
state
after irreversible process?
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Thermodynamic Cycle
Cycle –a collection of processes that ultimately lead to the system being returned to the original state.
p
V
•clockwise
heat work
•Anti-clockwise
work heat
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End of Chapter 2
Well done!