• Thermodynamics : The science of energy.
• Energy : The ability to cause changes.
• The name thermodynamics stems from the Greek words therme (heat) and dynamis (power).
• Conservation of energy principle :
During an interaction, energy can change from one form to another but the total amount of energy remains constant.
• Energy cannot be created or destroyed.
• The first law of thermodynamics : An expression of the conservation of energy principle.
• The first law asserts that energy is a thermodynamic property.
2

• The second law of thermodynamics:
It asserts that energy has quality as well as quantity , and actual processes occur in the direction of decreasing quality of energy.
• Classical thermodynamics : A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles.
• It provides a direct and easy way to the solution of engineering problems and it is used in this text.
• Statistical thermodynamics : A microscopic approach , based on the average behavior of large groups of individual particles.
• It is used in this text only in the supporting role.
3

All activities in nature involve some interaction between energy and matter; thus, it is hard to imagine an area that does not relate to thermodynamics in some manner.
4

5

• Any physical quantity can be characterized by dimensions .
• The magnitudes assigned to the dimensions are called units .
• Fundamental or primary dimensions : mass m , length L , time t , and temperature T
• Secondary of derived dimensions: velocity
V , energy E , and volume V.
• Metric SI system : A simple and logical system based on a decimal relationship between the various units.
•
English system : It has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily.
6

Work = Force

Distance
1 J = 1 N∙m
1 cal = 4.1868 J
1 Btu = 1.0551 kJ
7

A body weighing
60 kgf on earth will weigh only 10 kgf on the moon.
W weight m mass g gravitational
8 acceleration

Specific weight

: The weight of a unit volume of a substance.
9

All equations must be dimensionally homogeneous .
All nonprimary units (secondary units) can be formed by combinations of primary units .
Force units, for example, can be expressed as
They can also be expressed more conveniently as unity conversion ratios as
Unity conversion ratios are identically equal to 1 and are unitless, and thus such ratios (or their inverses) can be inserted conveniently into any calculation to properly convert units.
10

11

12

In thermodynamics, the term
SYSTEM is used to identify what is going to be analyzed.
Once a system is identified, everything external to the system is referred to as the SURROUNDINGS of the system.
The system is seperated from its surroundings by a specified
BOUNDARY, which may be at rest, rigid or deforming, or moving either rigid or deforming.
With a system and its surroundings identified, the relevant interactions with its surroundings are next identified and one or more physical laws are applied.

Thermodynamics Mechanics
Conservation of Mass
Conservation of Momentum
Conservation of Energy
Entropy Balance
Entropy is not conserved
Exergy Balance
Exergy is not conserved
Conservation of Momentum
Newtons Second
Law of Motion
Conservation of Energy
Fluid Mechanics
Mechanics of Deformable
Bodies
Heat Transfer

A system could be as simple as a metal piece.

A system could be as complex as as a powerplant.

• System : A quantity of matter or a region in space chosen for study.
• Surroundings : The mass or region outside the system
•
Boundary : The real or imaginary surface that separates the system from its surroundings.
• The boundary of a system can be fixed or movable .
• Systems may be considered to be closed or open .
•
Closed system (Control mass): A fixed amount of mass, and no mass can cross its boundary
13

A system is referred to as a CLOSED SYSTEM
İf the mass it encloses is fixed both in identity and amount .
Thus, mass is not allowed to cross the boundaries of a closed system.

• Open system ( control volume) : A properly selected region in space.
• It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle.
• Both mass and energy can cross the boundary of a control volume.
• Control surface : The boundaries of a control volume. It can be real or imaginary.
14

A system is referred to as a CONTROL VOLUME
İf it is defined as just a region of space.
Thus, mass is allowed to cross the boundaries of a control volume.

Closed System
Open System
Open System

It is essential for the system to be delineated carefully before proceeding with thermodynamic analysis.
The same physical phenomena often can be analyzed in terms of alternative choices of the system and,therefore, system bondary and the surroundings.
The choice of a particular system depends heavily on the convenience it allows in the subsequent analysis .
In general the choice of a system is governed by two considerations:
What is known about a possible system, particularly at its boundaries.
and the objective of the analysis.

The system boundary encloses the tank, the compressor, and all the piping.
This boundary might be selected if the electrical power input is known, and the objective of the analysis is to determine how long the compressor might operate in order the pressure in the tank is increased to a specified value.
A control volume enclosing only the compressor might be chosen if the condition of the air entering and exiting the compressor is known and the objective is to determine the electric power input.

A system, a closed system or a control volume,
Interacts with its surroundings.
How does it interact ?
Closed System heat transfer work done by or on the matter within the system or simply, the system
It interacts via
Open System heat transfer work done by or on the matter within the control volume or simply, the control volume
Mass transport through the control surface
System-surroundings interaction occurs at the boundary of the system

WHAT HAPPENS AS A RESULT OF INTERACTION?
The overall contition , that is, the state of the mass within the closed system , that is, the closed system , or the mass within the control volume , that is, the control volume , changes.
To be able to predict this change, first the state
-the state of the closed system or the state of the control volume itself should be defined.

The state of a system , a closed system or a control volume , is its overall conditionthe overall condition of matter in itwhich can be characterized by the characteristics of matter in the system .

HOW MATTER IS CHARACTERIZED ?
Matter is characterized in terms of physical characteristics known as properties .
HOW PROPERTIES ARE CHARACTERIZED ?
The answer lies in the structure of matter at the molecular, atomic, and subatomic levels.
The average behavior of the particles constituting the matter is predicted through statistical means and this average behavior is related to the observed macroscopic behavior of matter.
This approach is referred to as the statistical approach and trhermodynamics based on this approach is referred to as
STATISTICAL THERMODYNAMICS.
A highly complex approach !

Characterization of matter, on the other hand, is simplified by hypotesizing that matter is continously distributed throughout the region .
The correctnes of this hypothesis, known as the
CONTINUUM HYPOTHESIS , is inferred from the fact that for an extremely large class of engineering interest the resulting description of the behavior of matter is in aggreement with measured data.
The continuum idealization is valid as long as the size of the sysytem we deal with is large relative to the space between the molecules, the average distance they travel before they collide with each other
– the mean free path.

To have a sense of the distance involved at the molecular level, consider a container filled with oxygen at atmospheric conditions.

There are about 3 x 10 16 molecules of oxygen in the tiny volume of 1 mm 3 at 1 atm pressure and 20 º C
The diameter of the oxygen molecule molecule is about
3 x 10 -10 m and its mass is
5.3 x 10 -26 kg .
The mean free path of oxygen at 1 atm pressure and 20
ºC is 6.3 x 10 -8 m .
That is, an oxygen molecule travels, on average, a distance of 6.3 x 10 -8 m
(about 200 times of its diameter) before it collides with another molecule .

Despite the large gaps between molecules, a substance can be treated as a continuum because of the very large number of molecules even in an extremely small volume .

The continuum model is applicable as long as the characteristic length of the system
(such as its diameter) is much larger than the mean free path of the molecules.
At very high vacuums or very high elevations, the mean free path may become large
(for example, it is about 0.1 m for atmospheric air at an elevation of 100 km).
For such cases the rarefied gas theory should be used, and the impact of individual molecules should be considered.
In this course we will limit our consideration to substances that can be modeled as a continuum.

This approach is referred to as the continuum approach and thermodynamics based on this approach is referred to as
CLASSICAL THERMODYNAMICS

When matter can be treated as
CONTINUA , it is possible to speak of their intensive properties at a point.
Thus at any instant the density ρ at a point is defined as
  m lim
V  V * V where V* is the smallest volume fo which the limit exists.
The volume V* is large enough to contain sufficient numuber of particles for statistical averages to be significant, yet it is the smallest volume for which matter can be considered a continuum and is normally small enough that can be considered a point.

Density defined as such
  m lim
V  V * V can be described mathematically as a continuous function of position and time
  
( x , y , z , t )

The density , or local “mass per unit volume” is a property that does not depend on the extent of matter and at any instant may vary from point to point within the region comprising the matter.
The mass contained in a particular volume V is determed by integration m
 
V
 dv m

1
V

V
 dv
  average
For water at 20 ⁰ C
ρ averaage
= 1000 kg/m 3 , approximately

Let us consider a small area A passing through a point in a fluid at rest.
The fluid on one side of the area exerts a compressive force on it that is normal to the area, F normal
.
An equal but oppositely directed force is exerted on the area by the fluid on the other side.
For a fluid at rest no other forces than these act on the area.
The pressure p at the specifie point is defined as p
 lim
A  A *
F normal
A where A* is the area at the point in the same limiting sense as used in the definition of density.

p
 lim
A  A *
F normal
A
If the area was given new orientations by rotating it around the given point, and the pressure determined for each new orientation, it would be found that the pressure at the point is the same in all directions as long as the fluid isi at rest.
However, the pressure can vary from point to point within a fluid at rest: examples are the variation of atmospheric pressure with elevation and the pressure variation with depth in a body of water.

Properties are considered to be either intensive or extenesive .
Intensive properties are those whose values are independent of the extent and, therefore, the mass of matter.
Typical such properties are temperature, pressure and density.
Extensive properties are those whose values are dependent on the extent and, therefore, the mass of matter.
Typical such properties are total mass, total volume, total momentum.

Extensive properties per unit mass are referred to as specific properties, and are intensive properties .
V,m 3 V/m = v m 3 /kg specific volume , an intensive property
E, Joules E/m = e joules/kg specific energy , an intensive property
At an instant, specific properties may vary from point to point in the region comprising the matter
V = V
(x,y,z,t) e = e (x,y,z,t)

A physical property is a characteristic of matter enclosed in a system – a closed system or an open system.
Can we associate a physical property of matter with the system that encloses that matter and talk about a property of a system ?
YES if the property is an extensive property; you can talk about the volume of a system, mass of a system, energy of a system, and so on.

If the property is an intensive property and the system is a closed system
YES if the system is at an equilibrium state, the intensive properties will be uniform throughout the system.
You can then talk about the density of the system, pressure of the system, temperature of the system, and so on.
YES or NO
NO if the system is
NOT at an equilibrium state , intensive propereties are NOT uniform throughout, and, therefore, a fixed value of any property can not be attributed to the entire system.
ρ = ρ(x,y,z,t)
T = T(x,y,z,t) p = p(x,y,z,t)

If the property is an intensive property and the system is an OPEN SYSTEM
NO
Intensive properties are not uniform throughout the system, in general.

• Property: Any characteristic of a system.
• Some familiar properties are pressure P , temperature T , volume
V , and mass m .
• Properties are considered to be either intensive or extensive .
• Intensive properties: Those that are independent of the mass of a system, such as temperature, pressure, and density.
• Extensive properties: Those whose values depend on the size — or extent —of the system.
• Specific properties: Extensive properties per unit mass.
15

• Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance and view it as a continuous, homogeneous matter with no holes, that is, a continuum .
• The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinuities.
• This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules.
• This is the case in practically all problems.
• In this text we will limit our consideration to substances that can be modeled as a continuum.
16

Density
Specific volume
Specific gravity : The ratio of the density of a substance to the density of some standard substance at a specified temperature
(usually water at 4 °C).
Specific weight : The weight of a unit volume of a substance.
Density is mass per unit volume; specific volume is volume per unit mass.
17

•
Thermodynamics deals with equilibrium states.
• Equilibrium : A state of balance.
• In an equilibrium state there are no unbalanced potentials (or driving forces) within the system.
• Thermal equilibrium : If the temperature is the same throughout the entire system.
•
Mechanical equilibrium: If there is no change in pressure at any point of the system with time.
• Phase equilibrium: If a system involves two phases and when the mass of each phase reaches an equilibrium level and stays there.
• Chemical equilibrium: If the chemical composition of a system does not change with time, that is, no chemical reactions occur.
18

In mechanics , equilibrium implies a condition of balance maintained by the equality of opposing forces.
In thermodynamics , the concept is more far reaching, including not only a balance of forces but also a balance other potentials or driving forces.
Each kind of potential refers to a particular aspect of
COMPLETE EQUILIBRIUM referred to as
THERMODYNAMIC EQUILIBRIUM .
Accordingly, several types fo equilibrium must exist individually to fulfill the condition of complete or thermodynamics equilibrium.
Among these are mechanical equilibrium, thermal equilibrium, phase equilibrium and chemical equilibrium.

A system is in mechanical equilibrium if pressure is the same throughout the entire system .
That is, the system involves no pressure differentials which is the driving force towards a state with uniform pressure throughout.
Pressure, however, may vary within the system with elevation as a result of gravitational effects .
For example, the higher pressure at a bottom layer is balanced by the extra weight it must carry, and, therefore, there is no inbalance of forces.
The variation of pressure as a result of gravity in most thermodynamic systems is relatively small and usually disregarded.

A system is in thermal equilibrium if temperature is the same throughout the entire system .
That is, the system involves no temperature differenetials which is the driving force for heat flow.
If a system involves two phases, the system is in phase equilibrium when the mass of each phase remains the same.
A system is in chemical equilibrium if its chemical composition remains the same.
That is no net chemical reactions occur.

You may test to see if a system is in thermodynamic equilibrium or not by the following procedure:
Isolate the system from its surroundings and watch for changes in its observable properties.
If there are no changes, we conclude that the system was in equilibrium at the moment of isolation.
A system in equilibrium is said to be at an equilibrium state.

• The number of properties required to fix the state of a system is given by the state postulate :
 The state of a simple compressible system is completely specified by two independent, intensive properties.
• Simple compressible system: If a system involves no electrical, magnetic, gravitational, motion, and surface tension effects.
The state of nitrogen is fixed by two independent, intensive properties.
19

Process : Any change that a system undergoes from one equilibrium state to another.
Path : The series of states through which a system passes during a process.
To describe a process completely, one should specify the initial and final states, as well as the path it follows, and the interactions with the surroundings.
Quasistatic or quasi-equilibrium process: When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times.
20

When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasi-static , or a quasi-equilibrium process.
A quasi-equilibrium process can be viewed as a sufficiently slow process that allows 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.

As a consequence of the interaction between a system and its surroundings, properties of matter within the system, and, therefore, state of matter within the system, or the state of the system changes and, the system is said to have undergone a process.

A process is a transformation from one state to another.
The initial state is in equilibrium state, in general.
The final state may or may not be an equilibrium state.
In the majority of the situations, analyzed in elementary thermodynamics, the final state is also an equilibrium state at the end of the system-surroundings interaction.
There is, however, no requirement that a system undergoing a process be in equilibrium during the process.
Some or all of the intervening states may be nonequilibrium states.
For such processes we are limited to knowing the state before the process occurs and the state after the process is completed.

When a gas in a piston-cylinder device is compressed suddenly , the molecules near the face of the piston will not have enough time to escape and they will have to pile up in a small region in front of the piston, thus creating a high-pressure region there .
Because of this pressure difference , the system can no longer be said to be in equilibrium, and this makes the entire process nonquasi-equilibrium
Very fast compression
(nonquasi-equilibrium)

However, if the piston is moved slowly , the molecules will have sufficient time to redistribute and there will not be a molecule pileup in front of the piston .
As a result, the pressure inside the cylinder will always be nearly uniform and will rise at the same rate at all locations .
Since equilibrium is nearly maintained at all times, this is a quasi-equilibrium process.
Slow compression
(quasi-equilibrium)

It should be pointed out that a quasi-equilibrium process is an idealized process and is not a true representation of an actual process.
But many actual processes closely approximate it, and they can be modeled as quasi-equilibrium with negligible error.
Engineers are interested in quasi-equilibrium processes for two reasons.
First , they are easy to analyze
Second , work-producing devices deliver the most work when they operate on quasi-equilibrium processes.
Therefore, quasi-equilibrium processes serve as standards to which actual processes can be compared.

• Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes.
• Some common properties that are used as coordinates are temperature
T , pressure P , and volume V (or specific volume v ).
• The prefix iso is often used to designate a process for which a particularproperty remains constant.
•
Isothermal process : A process during which the temperature T remains constant.
• Isobaric process : A process during which the pressure P remains constant.
• Isochoric (or isometric) process : A process during which the specific volume v remains constant.
• Cycle : A process during which the initial and final states are identical.
21

A system is said to have undergone a cycle if it returns to its initial state at the end of the process.
That is, for a cycle the initial and final states are identical .

A gas, which we define as our system, is restrained at high pressure by a piston that is secured by a pin.
Initially the system is in equilibrium at some pressure and temperatute .

When the pin is removed, the piston is raised and forced abruptly against the stops.
Initial State Final State
The system is in equilibrium: both the pressure and the temperature is uniform throughout
The system is not in equilibrium: neither the pressure nor the temperature is uniform throughout
The system is in equilibrium: both the pressure and the temperature is uniform throughout
In the vicinity of the piston face pressure is lower and there exist a pressure gradient throughout.

Initial State
Final State
Since the piston has been raised a certain amount as the gas expands from the initial state to the final state, some work is done by the system.

Suppose we wish to restore the system to its initial state.
One way of doing this would be to exert a force on the piston and thus compress the gas until the pin can be reinserted in the piston.
Since the pressure on the face of the piston is greater on the return stroke than on the initial stroke, the work done on the gas in this reverse process is greater than the work done by the gas in the initial process.

An amount of heat must be transferred from the gas during the reverse stroke so that the system has the same internal energy as it had originally.
Thus, the system is restored to its initial state, but the surroundings have changed by virtue of the fact that work was required to force the piston down and heat was transferred to the surroundings. The two are not equivalent to each other.
The initial process, therefore, is an irreversible one because it could not be reversed without leaving a change in the surroundings .

Although work done by the surroundings is equal to the heat given to the surroundigs in amount, the two are of different nature: heat cannot be completely converted to work.
Thus, the system is restored to its initial state, but the surroundings have changed by virtue of the fact that work was done by the surroundings to force the piston down and an equal amount of heat was transferred to the surroundings.
The initial process therefore is an irreversible one because it could not be reversed without leaving a change in the surroundings.

Why the pressure on the face of the piston is greater on the return stroke than on the initial stroke?
If the state change were fast
If the state change were slow

Consider a high pressure gas contained in a piston and cylinder assembly.
The piston is constrained from moving and the gaseous system is in equilibrium, that is, it has a unique pressure, temperature, and so on.

When the constraint is removed, the force exerted by the high pressure gas on the face of the piston causes the piston to move outward.
Since the high pressure gas causes a movement of the piston, energy as work has been transferred from the gaseous system to the piston.

The walls and end of the cylinder do not move because the force they exert on the gas is equal to the force exerted by the gas on them. The piston does move since the force it is able to exert on the gas is less than the force exerted on the piston by the gas.
As the piston moves out rapidly, the pressure or force on the face of the piston decreases much more rapidly than the pressure or force the gas exerts against the walls and end of the cylinder.

The gas cannot maintain as high a pressure against the piston as it can against the walls.
The piston is being accelerated and therefore cannot oppose the pressure of the gas to the same extent as the walls and end of the cylinder.

There is, therefore, a pressure gradient in the gaseous system during the process and this gradient will exist until the piston comes to rest and the system reaches equilibrium.
The pressure of the entire system at any instant during the process is simply undefined.

Consider a system consisting of a gas being alternately compressed and expanded in a piston-cylinder assembly.
We assume no friction between the piston and the cylinder walls and no heat transfer with the surroundings.
With a very small increase in the external pressure , the piston would compress the gas slightly.
At each intermediate volume during the compression, the intensive properties T, p, v, etc. would be uniform throughout:
The gas would pass through a series of states
With a small decrease in the external pressure, the piston would slowly move out as the gas expands.
At each intermediate volume of the expansion, the intensive properties of the gas would be at the same uniform values they had at the corresponding step during the compression.
The gas again would pass through a series of equilibrium states .

When the gas volume returned to its initial value, all properties would be restored to their initial values as well.
The work done on the gas during the compression would equal the work done by the gas during the expansion.
There would also be no net change in the surroundings.
This process would then be reversible.

In contrast, if the gas were rapidly compressed , the pressure near the piston face would be higher than that elsewhere in the gas.
Spatial variations in other intensive properties might also occur. The intermediate states of the compression process would therefore not be equilibrium states.
Moreover, even if the gas were restored to its initial state without additional irreversibilities, more work would have been required to compress the gas than would be returned to the surroundings in the subsequent expansion of the gas .
Since a permanent change in the surroundings would occur, this process would, then, be irreversible .

In the reversible expansion of a gas there must be only an infinitesimal difference between the force exerted by the gas and the restraining force, so that the rate at which the boundary moves will be infinitesimal.
That is a reversible expansion process, should, therefore, fulfill the requirements of a quasi-equilibrium process.
However, actual systems have a finite difference in forces, which causes a finite rate of movement of the boundary, and thus such processes are irreversible in some degree.

Note the general interrelation of equilibrium,reversibility and time.
In a reversible process, the deviation from equilibrium is infinitesimal, and, therefore, it occurs at an infinitesimal rate.
Since actual processes proceed at a finite rate, the deviation from equilibrium must be finite, and, therefore, an actual process is necessarily irreversible in some degree.
The greater the deviation from equilibrium, the greater the irreversibility, and more rapidly the process will occur.
İt should also be noted that quasi-equilibrium processes in the absence of friction are reversible ( totally reversible) processes.

Consider a closed system that consists of a gas contained in an adiabatic piston-cylinder device.
The insulation of the cylinder head is such that it may be removed to bring the cylinder into contact with reservoirs to provide heat transfer.

Reversible Isothermal Expansion (process 1-2, T
H
= constant )
Initially (state 1), the temperature of the gas is T
H and the cylinder head is in close contact with a source at temperature T
H
. The gas is allowed to expand slowly, doing work on the surroundings.
As the gas expands, the temperature of the gas tends to decrease. But as soon as the temperature drops by an infinitesimal amount dT, some heat is transferred from the reservoir into the gas, raising the gas temperature to T
H
.
Thus, the gas temperature is kept constant at T
H
.
Since the temperature difference between the gas and the reservoir never exceeds a differential amount dT, this is a reversible heat transfer process. It continues until the piston reaches position 2. The amount of total heat transferred to the gas during this process is Q
H
.

Reversible Adiabatic Expansion (process 2-3, temperature drops from T
H to T
L
).
At state 2, the reservoir that was in contact with the cylinder head is removed and replaced by insulation so that the system becomes adiabatic.
The gas continues to expand slowly, doing work on the surroundings until its temperature drops from T
H to T
L
(state 3). The piston is assumed to be frictionless and the process to be quasi-equilibrium, so the process is reversible as well as adiabatic.

Reversible Isothermal Compression (process 3-4, T
L
= constant).
At state 3, the insulation at the cylinder head is removed, and the cylinder is brought into contact with a sink at temperature T
L
. Now the piston is pushed inward by an external force, doing work on the gas.
As the gas is compressed, its temperature tends to rise. But as soon as it rises by an infinitesimal amount dT, heat is transferred from the gas to the sink, causing the gas temperature to drop to T
L
. Thus, the gas temperature remains constant at T
L
.
Since the temperature difference between the gas and the sink never exceeds a differential amount dT, this is a reversible heat transfer process. It continues until the piston reaches state 4. The amount of heat rejected from the gas during this process is Q
L
.

Reversible Adiabatic Compression (process 4-1, temperature rises from T
L to T
H
).
State 4 is such that when the low-temperature reservoir is removed, the insulation is put back on the cylinder head, and the gas is compressed in a reversible manner, the gas returns to its initial state (state 1).
The temperature rises from T
L to T
H during this reversible adiabatic compression process, which completes the cycle.

The P-V diagram of this cycle is shown below. Remembering that on a P-V diagram the area under the process curve represents the boundary work for quasi-equilibrium (internally reversible) processes, we see that the area under curve 1-2-3 is the work done by the gas during the expansion part of the cycle, and the area under curve 3-4-1 is the work done on the gas during the compression part of the cycle. The area enclosed by the path of the cycle (area
1-2-3-4-1) is the difference between these two and represents the net work done during the cycle.

The terms steady and uniform are used frequently in engineering, and, thus, it is important to have a clear understanding of their meanings.
The term steady implies no change with time .
The opposite of steady is unsteady , or transient .
The term uniform , however, implies no change with location over a specified region.
These meanings are consistent with their everyday use
(uniform properties etc.).

• The term steady implies no change with time . The opposite of steady is unsteady , or transient .
• A large number of engineering devices operate for long periods of time under the same conditions, and they are classified as steady-flow devices .
• Steady-flow process : A process during which a fluid flows through a control volume steadily.
• Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines, pumps, boilers, condensers, and heat exchangers or power plants or refrigeration systems.
During a steadyflow process, fluid properties within the control volume may change with position but not with time.
22

During a steady-flow process, at an instant of time, intensive fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process.
The extensive properties, on the other hand, the volume V, the mass m, and the total energy content E of the control volume, remain constant during a steady-flow process .

• The zeroth law of thermodynamics : If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.
• By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact .
23

If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.
95

If
A is in thermal equilibrium with C, and
B is in thermal equilbrium with C, then
A is also in thermal equilibrium with B

Replacing C with a thermometer we can state two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact.

• All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water: the ice point and the steam point.
P versus T plots of the experimental data obtained from a constant• Ice point : A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm pressure (0 °C or
32 °F).
volume gas thermometer using four
• Steam point : A mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure (100 °C or
212 °F).
different gases at different (but low) pressures.
• Celsius scale : in SI unit system
• Fahrenheit scale : in English unit system
• Thermodynamic temperature scale : A temperature scale that is independent of the properties of any substance.
• Kelvin scale (SI) Rankine scale (E)
• A temperature scale nearly identical to the Kelvin scale is the ideal-gas temperature scale . The temperatures on this scale are measured using a constant-volume gas thermometer .
A constant-volume gas thermometer would read -273.15
°C at absolute zero pressure.
24

Comparison of temperature scales.
Comparison of magnitudes of various temperature units.
• The reference temperature in the original Kelvin scale was the ice point ,
273.15 K, which is the temperature at which water freezes (or ice melts).
• The reference point was changed to a much more precisely reproducible point, the triple point of water (the state at which all three phases of water coexist in equilibrium), which is assigned the value 273.16 K.
25

Pressure : A normal force exerted by a fluid per unit area
Some basic pressure gages.
26

• Absolute pressure : The actual pressure at a given position. It is measured relative to absolute vacuum (i.e., absolute zero pressure).
• Gage pressure : The difference between the absolute pressure and the local atmospheric pressure. Most pressure-measuring devices are calibrated to read zero in the atmosphere, and so they indicate gage pressure.
• Vacuum pressures : Pressures below atmospheric pressure.
Throughout this text, the pressure
P will denote absolute pressure unless specified otherwise.
27

When the variation of density with elevation is known
28

In a room filled with a gas, the variation of pressure with height is negligible.
Pressure in a liquid at rest increases linearly with distance from the free surface.
29

Pascal’s law : The pressure applied to a confined fluid increases the pressure throughout by the same amount.
The area ratio A
2
/ A
1 is called the ideal mechanical advantage of the hydraulic lift.
Lifting of a large weight by a small force by the application of Pascal’s law.
30

It is commonly used to measure small and moderate pressure differences. A manometer contains one or more fluids such as mercury, water, alcohol, or oil.
Measuring the pressure drop across a flow section or a flow device by a differential manometer.
The basic manometer.
31

• Bourdon tube : Consists of a hollow metal tube bent like a hook whose end is closed and connected to a dial indicator needle.
• Pressure transducers : Use various techniques to convert the pressure effect to an electrical effect such as a change in voltage, resistance, or capacitance.
• Pressure transducers are smaller and faster, and they can be more sensitive, reliable, and precise than their mechanical counterparts.
• Strain-gage pressure transducers: Work by having a diaphragm deflect between two chambers open to the pressure inputs.
• Piezoelectric transducers : Also called solidstate pressure transducers , work on the principle that an electric potential is generated in a crystalline substance when it is subjected to mechanical pressure.
32

• Atmospheric pressure is measured by a device called a barometer ; thus, the atmospheric pressure is often referred to as the barometric pressure .
• A frequently used pressure unit is the standard atmosphere , which is defined as the pressure produced by a column of mercury 760 mm in height at 0
°C ( 
Hg
=
13,595 kg/m 3 ) under standard gravitational acceleration ( g = 9.807 m/s 2 ).
The length or the cross-sectional area of the tube has no effect on the height of the fluid column of a barometer, provided that the tube diameter is large enough to avoid surface tension
(capillary) effects.
33

34

1 –18C A large fraction of the thermal energy generated in the engine of a car is rejected to the air by the radiator through the circulating water. Should the radiator be analyzed as a closed system or as an open system? Explain.
1-18C The radiator should be analyzed as an open system since mass is crossing the boundaries of the system.

1 –20C A can of soft drink at room temperature is put into the refrigerator so that it will cool. Would you model the can of soft drink as a closed system or as an open system? Explain.
1-20C A can of soft drink should be analyzed as a closed system since no mass is crossing the boundaries of the system.

1 –26C Define the isothermal, isobaric, isochoric, and adiabatic processes.
1-26C A process during which the temperature remains constant is called isothermal; a process during which the pressure remains constant is called isobaric; a process during which the volume remains constant is called isochoric; a process during which there is no heat transfer to/from system boundary.

What is a steady-flow process?
A process during which a fluid flows through a control volume steadily
(no change with time).

What is the zeroth law of thermodynamics?
If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

Consider two identical fans, one at sea level and the other on top of a high mountain, running at identical speeds. How would you compare ( a ) the volume flow rates and ( b ) the mass flow rates of these two fans?


1 –62 A gas is contained in a vertical, frictionless piston –cylinder device. The piston has a mass of 3.2 kg and a crosssectional area of 35 cm 2 . A compressed spring above the piston exerts a force of 150 N on the piston. If the atmospheric pressure is 95 kPa, determine the pressure inside the cylinder.

1 –64 Both a gage and a manometer are attached to a gas tank to measure its pressure. If the reading on the pressure gage is 80 kPa, determine the distance between the two fluid levels of the manometer if the fluid is ( a ) mercury (

= 13,600 kg/m 3 ) or ( b ) water (

= 1000 kg/m 3 ).

1 –66 A manometer containing oil (

=850 kg/m 3 ) is attached to a tank filled with air. If the oil-level difference between the two columns is 80 cm and the atmospheric pressure is 98 kPa, determine the absolute pressure of the air in the tank.