Basic Concepts of Thermodynamics

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27 Οκτ 2013 (πριν από 3 χρόνια και 8 μήνες)

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Mechanical Engineering
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Basic Concepts
of Thermodynamics
Chapter 1
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Thermodynamics and Energy
•Thermodynamics is the science that primarily deals with energy.
•Energy can viewed as the capacity to cause change.
•The study of thermodynamics is concerned with the ways energy is
stored within a body and how energy transformations, which involve
heat and work, may take place.
•One of the most fundamental laws of nature is the conservation of
energy principle
.
•The first law of thermodynamics
simply states that during an energy
interaction, energy can change from one form to another but the total
amount of energy remains constant.
•Energy cannot be created or destroyed; it can only change forms.
•It asserts that energy
is a thermodynamic property
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•The macroscopic approach to thermodynamics does not require
acknowledge of the behavior of individual particles and is called
classical thermodynamics
.
–It provides a direct and easy way to obtain the solution of engineering
problems without being overly cumbersome.
•A more elaborate approach, based on the average behavior of large
groups of individual particles, is calledstatistical thermodynamics
.
–This microscopic approach is rather involved and is not reviewedhere and
leads to the definition of the second law of thermodynamics.
•The second law of thermodynamics
asserts that energy has qualityas
well as quantity, and actual processes occur in the direction of
decreasing quality of energy.
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Application Areas of Thermodynamics
Power plants
The human body
Air-conditioning
systems
Airplanes
Car radiators
Refrigeration systems
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Dimensions and Units
•Dimensions
can characterize any physical quantity.
–Primary (or fundamental) dimensions: mass m, length L, time t, and
temperature T.
–Second dimensions, which can be expressed in terms of the primary
dimensions: velocity V (L/t), volume (L3), density ρ(M/L3), ……
•Any magnitudes assigned to the dimensions are called units
.
•Unit Systems:
–English system (United States Customary System, USCS)
–International system (SI)
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Fundamental dimensions and their units
•DimensionSIUnitEnglish Unit
•Lengthmeter (m)foot (ft)1 ft = 0.3048 m
•Masskilogram (kg) pound-mass (lbm) 1 lbm=0.45359 kg
•Time second (s)second (s)
•Temperature Kelvin (K)Rankine(R)T(R) = 1.8T(K)
•Electrical current ampere (A)
•Amount of light candela (c)
•Amount of matter mole (mole)
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Standard prefixes in SI Units
tera, T
giga, G
mega, M
kilo, k
hecto, h
deka, da
deci, d
centi, c
milli, m
micro, µ
nano, n
pico, p
1012
109
106
103
102
101
10-1
10-2
10-3
10-6
10-9
10-12
PrefixMultiples
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Force
•Force is a derived unit from Newton's second law.
•Force = (mass)(acceleration)
or F = ma
–1 N (newton) = 1 kg •m/s2
–l lbf(pound-force) = 32.174 lbm•ft/s2
•The term weight is often misused to express mass. Unlike mass,
weight W
is a force.
–W = mg
•m is the mass of the body and g is the local gravitational acceleration (g is
9.807 m/s2
or 32.174 ft/s2
at sea level and 450
latitude.
•Specific weigh
t
–w = W/

=
ρ
g
•∀is the volume of the body.
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Work
•Work
is defined as force times distance.
•1 J (Joule) = 1 N•m
•1 kJ(kilojoule) = 10
3
J(A more common unit for energy in SI)
•In the English system, the energy unit is the Btu (British thermal unit)
,
which is defined as the energy required to raise the temperatureof 1
lbmof water at 68
0F by 1 0F.
•In the meter system, the amount of energy needed to raise the
temperature of 1 g of water at 150C by 10C is defined as 1 calorie (cal).
•1 cal = 4.1868 J
•1 Btu = 1.055 kJ
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Closed, Open, and Isolated Systems
•Thermodynamic system, or simply system
, is
defined as a quantity of matter or a region in space
chosen for study.
•The region outside the system is called the
surroundings
.
–Surroundings are physical space outside the system
boundary.
•The real or imaginary surface that separates the
system from its surroundings is called the
boundary.
–The boundary of a system may be fixed or movable.
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•Systems
may be considered to be closed
or
open
, depending on whether a fixed mass or a
fixed volume in space is chosen for study.
•A closed system
consists of a fixed amount of
mass and no mass may cross the system
boundary. The closed system boundary may
move.
•Examples of closed systems are sealed tanks
and piston cylinder devices (note the volume
does not have to be fixed). However, energy
in the form of heat and work may cross the
boundaries of a closed system.
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•An open system
, or control volume
, has mass as well as energy
crossing the boundary, called a control surface.
–Examples of open systems are pumps, compressors, turbines, valves, and
heat exchangers.
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•Anisolated system
is a general system of fixed mass where no heat or
work may cross the boundaries.
–An isolated system is a closed system with no energy crossing the
boundaries and is normally a collection of a main system and its
surroundings that are exchanging mass and energy among
themselves and no other system.
Isolated System Boundary
Mass
System
Surr2
Surr3
Surr4
Mass
Heat
Work
Heat = 0
Work = 0
Mass = 0
Across
Isolated
Boundary
Surr1
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Energy
•Consider the system shown below moving with a velocity, at an
elevation, Z, relative to the reference plane.
Z
General
System
CM
Reference Plane, Z=0
V
s
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•The total energy
, E, of a system is the sum of all forms
of energy that can exist within the system such as
thermal, mechanical, kinetic, potential, electric,
magnetic, chemical and nuclear.
•The total energy of the system is normally thought of as
the sum of the internal energy, kinetic energy, and
potential energy.
–The internal energy
, U, is that energy associated with the
molecular structure of a system and the degree of the molecular
activity.
–The kinetic energy
, KE, exists as a result of the system's
motion relative to an external reference frame. When the
system moves with velocity, , the kinetic energy is expressed as
2
2

V
m
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•The energy that a system possesses as a result of its elevation in a
gravitational field relative to the external reference frame is called
potential energy
, PE, and is expressed as
(KJ) mgzPE
=
–g is the gravitational acceleration and z is the elevation of the center of
gravity of a system relative to the reference frame.
•The total energy of the system is expressed as
•or, on a unit mass basis,
•e = E/m is the specific stored energy, and u = U/m is the specific
internal energy
EUKEPEkJ
=
+
+
()
e
E
m
U
m
KE
m
PE
m
kJ
kg
u
V
gZ
==++
=++
()
r
2
2
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•The change in stored energy of a system is given by
•Most closed systems remain stationaryduring a process and, thus,
experience no change in their kinetic and potential energies.
–The change in the stored energy is identical to the change in internal
energy for stationary systems.
•If ∆KE = ∆PE = 0,




EUKEPEkJ
=
+
+
()


EUkJ
=
()
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Property
•Any characteristic of a system in equilibrium is called a property
.
•The property isindependent
of the path used to arrive at the system condition.
•Some thermodynamic properties are pressure P, temperature T, volume V, and
mass m.
•Properties may be intensive
or extensive
.
•Extensive properties are those that vary directly with size---or extent---of the
system.
–Some Extensive Properties
•mass
•volume
•total energy
•mass dependent property
•Intensive properties are those that are independent of size.
–Some Intensive Properties
•temperature
•Pressure
•age
•Color
•any mass independent property
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•Extensive properties per unit mass are intensive properties.
•Specific volume v defined as
•Density ρdefined as
)/(
3
kgm
m
V
mass
volume
==
ν
)(kg/m
3
V
m
volume
mass
==
ρ
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Pressure
•Force per unit area is called pressure, and its unit is the Pascal, N/m2
in the SI
system and psia, lbf/in2
absolute, in the English system.
•The pressure used in allcalculations of state is the absolute pressuremeasured
relative to absolute zero pressure. However, pressures are often measured
relative to atmospheric pressurecalled gageor vacuumpressures. In the
English system the absolute pressure and gage pressures are distinguished by
their units, psia(pounds force per square inch absolute) and psig (pounds force
per square inch gage), respectively; however, the SI system makes no distinction
between absolute and gage pressures.
P
Force
Area
F
A
==
110
11010
3
2
6
2
3
kPa
N
m
MPa
N
m
kPa
=
==
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•Where the +P
gage
is used when Pabs
> Patm
and –Pgage
is used for a
vacuum gage.
PPP
gageabsatm
=

偐µ
癡ca瑭abs
=

偐µ
慢aa瑭条来
=
±
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•The relation among atmospheric, gage, and vacuum pressures
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Example
•A vacuum gage connected to a tank reads 30 kPaat a location where
the atmospheric pressure is 98 kPa. What is the absolute pressure in
the tank?
PPP
kPakPa
kPa
absatmgage
=

=−
=
9830
68
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Example
•A pressure gage connected to a valve stem of a truck tire
reads 240 kPaat a location where the atmospheric pressure is
100 kPa. What is the absolute pressure in the tire, in kPaand
in psia?
•The pressure in psiais
What is the gage pressure of the air in the tire, in psig?
PPP
kPakPa
kPa
absatmgage
=

=+
=
100240
340
PkPa
psia
kPa
psia
abs
==340
147
1013
493
.
.
.
PPP
psiapsia
psig
gageabsatm
=

=−
=
493147
346
..
.
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Manometer
•Small to moderate pressure differences are measured by a manometer
and a differential fluid column of height h corresponds to a pressure
difference of
•ρis the fluid density and g is the local gravitational acceleration
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Example
•Both a gage and a manometer are attached to a gas tank to measure
its pressure. If the pressure gage reads 80 kPa, determine the
distance between the two fluid levels of the manometer if the fluid is
mercury whose density is 13,600 kg/m
3.
h
P
g
=

ρ
h
kPa
kg
m
m
s
Nm
kPa
N
kgms
m
=
=
80
136009807
10
1
06
32
33
2
.
/
/
.
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Bourdon tubes used to measure pressure
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Temperature
•Temperature
is a thermodynamic property that is the measure of the
energy content of a mass.
–When heat energy is transferred to a body, the body's energy content
increases and so does its temperature.
–In fact it is the difference in temperature that causes energy, called heat
transfer
, to flow from a hot body to a cold body.
•Two bodies are in thermal equilibrium
when they have reached the
same temperature.
•Zerothlaw of thermodynamics
–If two bodies are in thermal equilibrium with a third body, theyare also in
thermal equilibrium with each other.
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State
•Consider a system that is not undergoing any change. The properties
can be measured or calculated throughout the entire system. This
gives us a set of properties that completely describe the condition or
state of the system.
•At a given state all of the properties are known; changing one property
changes the state.
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Equilibrium
•A system is said to be in thermodynamic equilibrium
if it maintains
thermal (uniform temperature), mechanical (uniform pressure), phase
(the mass of two phases, e.g. ice and liquid water, in equilibrium) and
chemical equilibrium.
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Process
•Any change from one state to another is called a process
.
•During a quasi-equilibrium
or quasi-static process
the system remains
practically in equilibrium at all times.
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Quasi-Equilibrium, Work-Producing Devices Deliver the Most Work
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•In some processes one thermodynamic property is held constant
.
entropy isentropic
volume isochoric
temperatureisothermal
pressure isobaric
Property held constant Process
Constant Pressure Process
Wate
r
F
System
Boundary
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State Postulate
•The number of properties required to fix the state of a simple,
homogeneous system is given by the state postulate
.
•The thermodynamic state of a simple compressible system is
completely specified by two independent intensive properties.
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Cycle
•A process (or a series of connected processes) with identical end states
is called a cycle
.
•Below is a cycle composed of two processes, A and B. Along process
A, the pressure and volume change from state 1 to state 2. Then to
complete the cycle, the pressure and volume change from state 2 back
to the initial state 1 along process B. Keep in mind that all other
thermodynamic properties must also change so that the pressure is
functions of volume as described by these two processes.
Process
B
Process
A
1
2
P
V
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Many Ways to Supply the Same Energy
Many Ways to Supply the Same Energy
Ways to supply a room with energy equaling a 300-W electric resistance heater
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Bomb Calorimeter Used to
Determine Energy Content of Food
Bomb Calorimeter Used to
Determine Energy Content of Food
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