Download ISAT 310: Energy Fundamentals

King Fahd University of Petroleum &
Minerals
Mechanical Engineering
Thermodynamics ME 203
BY
Dr. Salem Al-Dini
Mission statements of the department
The department is committed to
■ providing highest quality education in mechanical
engineering,
■ conducting world-class basic and applied research,
■ addressing the evolving needs of industry and society, and
■ supporting the development of more competitive new industry
in the Kingdome of Saudi Arabia
Textbook
Thermodynamics: An Engineering Approach, by Yunus A.
Cengel and Michael A. Boles,
References:
Fundamentals of Engineering Thermodynamics, by Moran, M. J.,
and H. N. Shapiro.
Fundamentals of Thermodynamics, by Sonntag, R. E.,
Borgnakke, C., and Van Wylen, G. J.
Chapter 1
Introduction and Basic
Concepts
Objectives
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Identify the unique vocabulary associated with thermodynamics through
the precise definition of basic concepts to form a sound foundation for the
development of the principles of thermodynamics.
Review the metric SI and the English unit systems that will be used
throughout the text.
Explain the basic concepts of thermodynamics such as system, state, state
postulate, equilibrium, process, and cycle.
Review concepts of temperature, temperature scales, pressure, and
absolute and gage pressure.
Introduce an intuitive systematic problem-solving technique.
Introduction
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Thermodynamics can be defined as the since of energy and entropy
An alternate definition: is the since that deals with heat and work and those
properties of substances that bear a relation to heat and work
The word thermodynamics stems from the Greek words therme (heat) and
dynamics (force).
Thermodynamics is a science that is based in experimental findings
These findings have been formalized into certain laws
Introduction
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Energy cannot be created or destroyed it transforms (conservation of
energy 1st law)
Introduction
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Energy has a quality and a quantity (actual process is decreasing quality
of energy 2nd law)
Application Areas of Thermal-Fluid
Sciences
Macroscopic and Microscopic Views of
Thermodynamics
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As you know, any substance consists of a large number
of molecules. The properties of the substance depend on
the behavior of these molecules.
Consider a gas in a container. The pressure of the gas is a
result of the momentum transfer as the molecules hit the
walls of the container.
However, we do need to know the force exerted by the
molecule on an infinitesimal area on the wall in order to
find the pressure (microscopic approach)
Instead, it will be sufficient to attach a pressure gauge to
the wall (finite area) and read the average pressure
exerted by a large number of molecules on that finite area
(macroscopic approach).
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That is, the macroscopic approach to
thermodynamics is concerned with the
average or overall behavior. This approach
is called sometimes classical
thermodynamics.
On the other hand, the microscopic
approach to thermodynamics, known as
statistical thermodynamics, is concerned
directly with the structure of matter. It is
objective is to find (by statistical means)
the average behavior of the particles
making up a system. This approach is
involved and is not used any more in the
remaining of this course.
The macroscopic approach provides a
direct and easy solution to engineering
problems.
Introduction
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Thermodynamics is both a branch of physics and an engineering science.
The scientist is normally interested in gaining a fundamental
understanding of the physical and chemical behavior of fixed quantities of
matter and then, uses the principles of thermodynamics to relate the
properties of matter.
Engineers are generally interested in studying systems and how they
interact with their surroundings. To facilitate this, engineers extend the
subject of thermodynamics to the study of systems through which matter
flows.
Thermodynamic systems
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An important step in any engineering analysis
is to describe precisely what is being studied.
In mechanics, if the motion of a body is to be
determined (Figure 1), normally the first step
is to define a free body and identify all the
forces exerted on it by other bodies (Figure 2).
Newton’s second law of motion is then
applied.
In thermodynamics, the term system is used to
identify the subject of the analysis (e.g. coffee
in the cup).
Once the system is defined and the relevant
interactions with other systems are identified,
one or more physical laws or relations are
applied.
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The system is whatever we want to study. It may be as simple as a free
body or as complex as an entire chemical refinery. We may want to study a
quantity of matter contained within a closed, rigid-walled tank or we may
want to consider something such as a gas pipeline through which gas
flows.
Everything external to the system is considered to be part of the system’s
surroundings.
The system is distinguished from its
surroundings by a specified boundary
which may be at rest or in motion.
It is essential for the boundary
to be determined carefully before
proceeding with any thermodynamic analysis.
Two basic kinds of systems are distinguished in thermodynamics study.
These are referred to as closed systems and control volumes.
Closed systems (control mass)
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A closed system refers to a fixed quantity of matter.
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A closed system is used when a particular quantity
of matter is under study.
A closed system always contains the same matter.
There can be no transfer of mass across its boundary.
What do we call the system if even energy is not allowed
to cross the boundary?
The figure shows a gas in a piston-cylinder assembly.
Let us consider the gas to be a closed system.
The boundary lies just inside the piston and cylinder
walls, as shown by the dashed lines on the figure.
If the cylinder were placed over a flame, the gas would
expand, raising the piston.
The portion of the boundary between the gas and the
piston moves with the piston.
No mass would cross this or any other part of the
boundary.
Open system (control volume)
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An open system (control volume) is a properly selected region in space.
It encloses a device that involves mass flow such as nozzle, compressor,
turbine.
Flow through such devices is best studied by selecting the region within
the device as the control volume .
Both mass and energy can cross the boundary of the control volume.
There are no concrete rules for the selection of the control volume but
proper choice makes the analysis much easier .
The boundary of the control volume is called boundary surface
The boundary surface can be real or imaginary
A control volume can be fixed in shape and size or it may involve a
moving boundary.
Open Systems (continued)
Open Systems (continued)
Properties of a System
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To describe a system and predict its behavior
requires knowledge of its properties and
how those properties are related.
Properties are macroscopic characteristics of
a system.
Any characteristic of a system is called a
property. Some familiar properties are
pressure P, temperature T, volume V, and
mass m.
Properties describe the state of a system only
when the system is in an equilibrium state.
Not all properties are independent. Density
is a dependent property on pressure and
temperature.
Density as a property
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Density is mass per unit volume;
 = mass/volume (kg/m3)
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Specific gravity: the ratio of the density of a substance to the density of some
standard substance at specified temperature (usually water at 4 oC)
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Specific volume is volume per unit mass.
 = Volume/mass, (m3/kg)
 = 1/ 
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Gases
Water
Volume

Gases
P


Liquids
T
P
Liquids
T
Extensive and Intensive Properties
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Intensive properties are those that are
independent of the size of system, such as
temperature, pressure, and density.
Extensive properties are dependent on the size
(or extent) of the system. Mass m, volume V,
and total energy E are some examples of
extensive properties.
Criteria to differentiate extensive and
intensive properties is illustrated in the Figure.
Extensive properties per unite mass are called
specific properties (i.e. specific volume).
State
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A state is defined as a condition of a substance that can
be described by certain observable macroscopic
properties. (T, P, ,  etc.)
In above figure, the system does not undergo any
change. All properties can be measured throughout the
system. Hence the condition of the system is completely
described. This condition is called state 1.
Now remove some weights. If the value of even one
property changes, then the state will change to different
one (state 2).
The word State refers to the condition of a system as it is
described by its properties.
m = 2kg
T1= 20 °C
V1= 1.5 m3
State 1
m = 2kg
T1= 20 °C
V1= 2.5 m3
State 2
Equilibrium
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Thermodynamics deals with equilibrium states.
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The word equilibrium implies a state of
balance.
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Equilibrium state means that there are no
unbalanced potentials (or driving forces)
within the system.
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A system is said to be in thermodynamic
equilibrium if it maintains thermal,
mechanical, phase, and chemical equilibrium.
Thermal Equilibrium
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Thermal equilibrium means that there is no temperature differential
through the system.
20 °C
30 °C
30 °C
35 °C
32 °C
32 °C
32 °C
40 °C
No thermal equilibrium
32 °C
32 °C
Thermal equilibrium
Mechanical Equilibrium
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Mechanical equilibrium means that there is no change in pressure in the
system.
20 pa
20 pa
20 pa
20 pa
20 pa
(a) Slow compression (quasi-equilibrium)
20 pa
90 pa
20 pa
(b) fast compression (non quasi-equilibrium)
Phase Equilibrium
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Phase equilibrium means that the mass of each phase reaches an
equilibrium level and stays there.
Vacuum
t= 0, P = 0
Water
At t = 0
Vapor, P > 0
Vapor, P = Pv
Water
Water
After some time
After long time
Chemical Equilibrium
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Chemical equilibrium means that the chemical composition of the system
does not change with time
The State Postulate
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We mentioned earlier that a state is described uniquely by measuring a
few of its properties. The remaining properties will assume certain
values. The question here is how much is this “few”?.
The answer depends on how simple or complex our system is.
If we have a system where the gravitational, electrical, magnetic,
motion and surface tension effects are absent, then this system is called
a simple compressible system.
According to what is called “state postulate”, the number of properties
required to completely specify the state of such system is two
independent, intensive properties.
The State Postulate
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If, however, the gravitational effects are important in the simple
compressible system, then the elevation z needs to be specified in addition
to the two properties necessary to fix the state.
The state postulate requires that the two properties are independent of each
other.
Two properties are considered to be independent if one property is varied
while the other one is constant.
Temperature and specific volume are good examples.
You will see, however, in coming units that temperature and pressure are
not always independent of each other. They become dependent during
phase change processes.
Processes and Cycles
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Any change from one equilibrium state to
another is called a process.
Process diagrams are very useful in
visualizing the processes.
The series of states through which a system
passes during a process is called a path
To describe a process completely initial and
final states as well as the path it follows, and
the interactions with the surrounding should be
specified
A process with identical end states is called a
cycle
Process diagrams plotted by employing
thermodynamic properties as coordinates are
very useful in visualizing the processes.
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Isothermal process means a
process at constant T.
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Isobaric process means a
process at constant pressure
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Isochoric process means a
process at constant volume
Quasi-Equilibrium process
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During a quasi-static or quasi-equilibrium
process, the system remains infinitesimally
close to an equilibrium state at all times.
A sufficiently slow process that allow the
system to adjust itself internally so that
properties in one part of the system do not
change any faster than those at other parts.
Compression is very slow and thus
equilibrium is attained at any intermediate
state. Therefore, the intermediate states can
be determined and process path can be
drawn.
It is an idealized process but many process
closely approximate it with negligible error.
Quasi-Equilibrium, Work-Producing Devices
Deliver the Most Work (it is the standard to
which other processes can be compared)
State 2
P
Process path
Intermediate
states
State 1
20
V
20 pa
20 pa
20 pa
20 pa
20 pa
(a) Slow compression (quasi-equilibrium)
Non-Quasi-Equilibrium process
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Compression process is fast and
thus equilibrium can not be
attained.
Intermediate states can not be
determined and the process path
can not be defined. Instead we
represent it as dashed line.
State 2
P
Non-equilibruim
process
90
?
20
State 1
V
20 pa
90 pa
20 pa
(b) Fast compression (non quasiequilibrium)
Forms of Energy
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In absence of magnetic, electric, and surface tension effects, the total energy of a
system consists of the kinetic, potential, and internal energies and is expressed as
E  U  KE  PE
(kJ),
2
mv
me  mu 
 mgz (kJ),
2
or, on a unit mass basis
v2
e  u  ke  pe  u   gz (kJ/kg)
2
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The macroscopic form of energy are those a system possesses as a whole with
respect to some outside reference (i.e. kinetics and potential).
The microscopic forms of energy are those related to the molecular structure of the
system , independent of outside reference frames (i.e. internal).
The change in the total energy E of a stationary system (closed system) is
identical to the change in its internal energy U.
Forms of Energy (continued)
The portion of the internal energy of a system associated
with the
1. kinetic energies of the molecules is called the sensible
energy.
2. phase of a system is called the latent energy.
3. atomic bonds in a molecule is called chemical energy.
4. strong bonds within the nucleus of the atom itself is
called nuclear energy.
eU  235  6.73 1010 kJ / kg
5. Static energy (stored in a system)
6. Dynamic energy: energy interactions at the system
boundary (i.e. heat and work)
Temperature and the Zeroth Law of
Thermodynamics
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The zeroth law of thermodynamics states
that: If two bodies are in thermal
equilibrium with the third body, they are
also in thermal equilibrium with each
other.
The equality of temperature is the only
requirement for thermal equilibrium.
Temperature scales
 
T R   T  F   459.67
T K   T oC  273.15
o
T R   1.8T K 
 
 
T K   T  C 
T R   T  F 
T o F  1.8T oC  32
o
o
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I n thermodynamics it is desirable to have a temperature scale
that is independent of the properties of any substance.
Note: it makes no difference to use K or C in formulas involving
temperature difference. However, you should use Absolute
temperature in formulas involving temperature only like the ideal
gas low.
Dimensions and Units
The seven
fundamental
dimensions and their
units in SI
(International
System).
Dimensions and Units
SI
British System
Conversion
Length
Meter (m)
Foot (ft)
1 ft = 0.3048 m
Time
Second (s)
Second (s)
Mass
Force
Definition
of
Unit force
Temperature
Slug
Pound mass (lbm)
Kg
1 slug = 32.2 lbm
Newton (N)
Pound force (lbf)
2
1 N = (1Kg).(1 m/s )
1 lbf = (1 slug)(1. ft/s2)
Newton (N): is the force Pound force (lbf) is the force
required to give a mass of required to give a mass of 1
1 kg an acceleration of 1 slug an acceleration of 1 ft/s2.
m/s2.
Degree Celsius.(°C)
Degree Fahrenheit (°F)
Absolute Temp.: Kelvin (K).
Absolute Temp.: Rankine (°R)
K = °C + 273.15
°R = °F + 459.67
1 slug =14.59 kg
1 lbm = 0.4536 kg
1 lbf = 4.448 N
°C = (5/9)*(°F –32)
°R = (9/5)*K
°C = (5/9)*(°F –32)
°R = (9/5)*K
Pressure
Pressure is defined as the force exerted by a fluid
per unit area.
Units in SI are Pa=N/m2. The pressure unit Pascal is
too small for pressure encountered in practice.
Therefore, kPa and MPa are commonly used.
Units in British are : psf = lbf/ft2, psi = lbf/in2
You have to convert from psi to psf ( 144 in2 = 1 ft2)
1bar  105 Pa  0.1MPa  100 kPa
1atm  101,325 Pa  101.325 kPa  1.01325 bars  14.696 psi
Pressure (Continued)
Absolute pressure, is
measured relative to
absolute vacuum (i.e.,
absolute zero pressure.)
Gauge pressure, is
measured relative to
atmospheric pressure
Pgage  Pabs  Patm for pressure above Patm 
Pvac  Patm  Pabs for pressure below Patm 
Pressure (continued)
Variation of Pressure with Depth
The pressure variation in a constant density fluid is
given as
Or
P + Z = constant
P1+ Z1 = P2 + Z2
Z is the vertical coordinate ( positive upward).
 is the specific weight of fluids, (N/m3)
  g
For small to moderate distances, the variation of
pressure with height is negligible for gases because
of their low density.
Pressure (continued)
Pressure at a Point
• The pressure at a point in a fluid has the same
magnitude in all direction.
Pressure (continued)
Pressure Variation in horizontal planes
Pressure is constant in
horizontal planes
provided the fluid does
not change. ( this leads
to Pascal’s principle.)
F1 F2
F2 A2
P1  P2 


 .
A1 A2
F1 A1
Noting that P1 = P2, the area ratio A2/A1 is called the ideal mechanical
advantage. Using a hydraulic car jack with A2/A1 = 10, a person can
lift a 1000-kg car by applying a force just 100 kg (= 908 N).
The Manometer
A device based on P + Z
= constant is called a
manometer (Right), and it
is commonly used to
measure small and
moderate pressure
differences.
P2 = Patm + h
Specific gravity
 f g f  f
S  s 


 w g w  w
FIGURE 1–61
Schematic for Example 1–8.
1-17
Barometer and the Atmospheric
Pressure
• The atmospheric pressure
is measured by a device
called a barometer; thus the
atmospheric pressure is
often referred to as the
barometric pressure.
PB   Z B  PC   Z C
PC  Pvapor  0
 PB  Patm   ( Z C  Z B )
  Hg gh
Barometer and the Atmospheric
Pressure (continued)
• The standard atmospheric pressure is
the pressure produced by a column of
mercury 760 mm in height at 0oC. The
unit of mmHG is also called the torr in
honor of Evangelista Torricelli
(1608−1647).
• The atmospheric pressure at a location
is simply the weight of the air above that
location per surface area. Patm changes
with elevation and weather conditions.
The length or the
Patm  760 mmHg torr   101.325 kPa
1torr  133.3 Pa
P1000m  89.88 kPa; P1610m:Denver
cross-sectional area of
the tube has no effect
on the height of the
 83.4 kPa fluid column of a
barometer.
P5000m  54.05 kPa; P10,000m  26.5 kPa
Problem Solving Technique
Step-by-step approach:
1. Problem Statement
2. Schematic
3. Assumptions
4. Physical Laws
5. Properties
6. Calculations
7. Reasoning, Verification, and
Discussion
The assumptions made while solving
an engineering problem must be
reasonable and justifiable.
Problem Solving Technique (continued)
When solving problems,
we will assume the
given information to be
accurate to at least 3
significant digits.
Therefore, if the length
of a pipe is given to be
40 m, we will assume it
to be 40.0 m in order to
A result with more
justify using 3
significant digits in the significant digits than that of
given data falsely implies
final results.
more accuracy.
Examples
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A pressure gage connected to a tank reads
500 kPa. The absolute pressure in the
tank is to be determined .
Pabs
Patm = 94 kPa
500 kPa
Examples
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The vacuum gage connected to a tank
reads 15 kPa at a location where the
barometer reading is 750 mmHg.
Determine the absolute pressure of the
tank. The density of mercury is given to
be  = 13,590 kg/m3.
Pabs
15 kPa
Patm = 750 mmHg
Examples
Examples
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The air pressure in a tank is measured by
an oil manometer. For a given oil-level
difference between the two columns, the
absolute pressure in the tank is to be
determined. The density of oil is given to
be  = 850 kg/m3.
AIR Patm = 98 kPa0.60 m
AIR
Patm = 98 kPa
0.60 m
Examples
730 mmHg
h
755 mmHg