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ENG 235
ENGINEERING THERMODYNAMICS
COLLEGE OF ENGINEERING
AFE BABALOLA UNIVERSITY
OCTOBER 2015
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COURSE OUTLINE
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Definition of terms and general concepts of systems, surrounding process, temperature
heat, work and energy.
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First law of thermodynamics, application to heat engines
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Entropy and the Second law.
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Ideal gases Joules – thompson’s coefficient
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Equilibrium processes
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Maxwell’s relation
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Thermodynamic function of solution
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P-V-T relationship
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Work from heat pump refrigeration
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INTRODUCTION
Energy forms the backbone of all technological and engineering developments. It powers all
aspect of the economy especially the manufacturing, transportation and communication sectors.
Without energy most productive activities will cease and the economy will be stagnant. Hence it
has always been the primary concern of engineers and technologist to identify and harness all
existing sources of energy and to find means of transforming or converting these energy sources
into usable forms. Attempts are also made to find ways of storing and transporting energy easily
within our present technological frame work.
Presently, the most predominant sources of energy are:
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Natural Gas
Petroleum
Coal
Hydroelectric energy
Nuclear energy
The name thermodynamics is derived from the Greek words “therme” meaning heat and
dynamis meaning power or motion. Thermodynamics essentially means heat power or heat-inmotion. The oxford Dictionary defines thermodynamics as the science of relation between heat
and mechanical energy. It is the study of energy, its transformation from one form to another and
its relation to various states of matter. It is concerned with the scientific principles and means of
converting heat energy from available source into mechanical work. Every engineering operation
in our daily activities involves an interaction between energy and matter and hence the
application of the principles of thermodynamics. For instance, the heat energy obtained due to
the burning of a mixture of fuel and air in the combustion chamber of a motor car is converted to
mechanical work to propel the car. Thermal power station at Egbin, Ikorodu where fossil fuels
are burned in a gas turbine, the resulting heat energy is converted to mechanical work usually in
the form of torque and this is in turn used to drive a generator for the production of electricity.
All the conversion processes involve basically an application of the scientific principles
developed in thermodynamics. And all the thermodynamic processes of conversion are
accomplished through the use of a wide range of engineering plant such as turbines,
compressors, reciprocating engines, condensers which is based on the study of other engineering
sciences such as heat transfer, mechanics of machines, properties of materials and fluid
mechanics.
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APPLICATION OF THERMODYNAMICS
1). All types of vehicles that we use, cars, motorcycles, trucks, ships, aeroplanes, and many other
types work on the basis of second law of thermodynamics and Carnot Cycle. They may be using
petrol engine or diesel engine, but the law remains the same.
2) Refrigerators, deep freezers, industrial refrigeration systems, all types of air-conditioning
systems, heat pumps, etc work on the basis of the second law of thermodynamics.
3) Air and gas compressors, blowers, fans, run on various thermodynamic cycles.
4) One of the important fields of thermodynamics is heat transfer, which relates to transfer of
heat between two media. There are three modes of heat transfer: conduction, convection and
radiation. The concept of heat transfer is used in wide range of devices like heat exchangers,
evaporators, condensers, radiators, coolers, heaters, etc.
5) Thermodynamics also involves study of various types of power plants like thermal power
plants, nuclear power plants, hydroelectric power plants, power plants based on renewable
energy sources like solar, wind, geothermal, tides, water waves etc,
6) Renewable energy is an important subject area of thermodynamics that involves studying the
feasibility of using different types of renewable energy sources for domestic and commercial use.
Thermodynamics involves the study of infinite universe and it indeed has infinite applications.
No other field of study is as closely associated to human life as thermodynamics.
Thermodynamic System
An important concept in thermodynamics is the “system”. A system is the region of the universe
under study. It is a quantity of matter enclosed in a boundary to which attention is directed for a
thermodynamic analysis. A system is separated from the surroundings by a boundary which may
be imaginary or not. The surroundings include everything external to the system, while
boundaries may be fixed or movable. The possible exchanges of work, heat, or matter between
the system and the surroundings take place across this boundary. Two types of exchange can
occur between system and surroundings:
(1) Energy exchange (heat, work, friction, radiation, etc.)
(2) Matter exchange (movement of molecules across the boundary of the system and surrounding
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A gas enclosed in a piston cylinder assembly is an example of a system whose boundaries are the
cylinder walls and the piston. As the piston moves either up, the gas moves, in the process part of
the system boundary also moves (movable boundary). The piston is fit tightly such that the mass
of gas is not allowed to cross the boundary.
Closed system is a system that does not allow transfer of mass but can allow energy transfer
across its boundaries.
Open System permit the flow of both mass and energy across its boundaries, example is the
turbine, the flow of water through a pipe. An open system is also called a control volume and its
boundary is called control surface.
Isolated system is a system that neither energy nor mass flows out of the boundary. It is a system
that has no interaction with its surroundings.
Property, State and thermodynamic Equilibrium
A property of a system is any observable characteristics of the system. The property of a system
should have a definite value when the system is in a particular state. Thermodynamic property is
a point function. The most familiar properties are temperature, pressure, volume and mass. Some
properties are defined in terms of other properties. Example is the specific volume, v, of a
substance which is defined as volume of the substance per unit mass (i.e. v = V/m).
The property of a system may either be extensive or intensive. An extensive property is that
which is dependent on the amount of mass present. If a system is divided into number of parts,
the value of extensive property is the sum total of all parts for which the system has been
divided. E.g volume, V, of the substance.
Intensive property of a system is independent of mass of matter under consideration. Regardless
of the number of parts for which a system is subdivided at any particular point in time, the value
of any intensive property of the system remains the same. E.g density is constant for each of the
individual parts.
In practice, upper case letters are used to denote extensive properties while lower case letters are
for intensive properties. Exception to these rule are Pressure P, and temperature T, which are
intensive properties and mass, m which is an extensive property.
The intensive property of a system is the extensive property divide by the mass of the system. e.g
Specific volume, v.
State
The state of a thermodynamic system is the overall condition of the system at a given point in
time and is defined by listing the properties of the system. For instance, the state of the gas at any
given point in time can be specified by the properties of the gas such as pressure P, temperature
T and specific volume, v.
In thermodynamic, only two independent properties (intensive properties) of a system are
required to describe or specify the state of a closed system.
Thermal Equilibrium
A system is said to be in thermal equilibrium if the state of all bodies composing a
thermodynamic state does not change for a long period of time. If there is no external
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spontaneous factor, a system cannot be out of the state of equilibrium. When the property of a
system is defined, it is understood that the system is in equilibrium.
If a system is in thermal equilibrium, the temperature will be same throughout the system.
If a system is in mechanical equilibrium, there is no tendency for the pressure to change. In a
single phase system, if the concentration is uniform and there is no tendency for mass transfer or
diffusion, the system is said to be in chemical equilibrium.
A system which is simultaneously in thermal, mechanical, and chemical equilibrium is said to be
in thermal equilibrium.
Processes and Cycles
A system undergoes a process when it changes from one state to another. The path of the process
is the series of state through which the system passes during the process. A process is described
by specifying the initial and final equilibrium states and the path of the process. In
thermodynamics, property diagram are extensively used to locate the path of a process. A
property diagram is a graphical representation of the changes in the properties that occur between
the initial and final states.
A system is said to undergo a cycle when it passes through a series of processes such that its
final state is identical in all respect to its initial state.
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Energy, Work and Heat
Energy is defined as the capability or ability to do work with the ultimate aim of achieving some
desired change or effect. The forms of energies that are essential for proper understanding of
thermodynamics are internal energy, potential energy, kinetic energy, heat and work
Internal Energy
The internal energy is associated with the molecular structure and molecular activity of a system.
It is the energy stored within a system as a result of continuous internal motion of molecules.
The molecule as a whole can move in x, y and z directions with respective components of
velocities and hence possesses kinetic energy. it is a property, it value cannot be directly
measured. The microscopic modes of energy are due to the internal structure of the matter and
hence sum of all microscopic modes of energy is called the internal energy.
Bulk kinetic energy (KE) and potential energy (PE) are considered separately and the other
energy of control mass as a single property (U).
The total energy possessed by the body is given by:
E = KE + PE + U
Work
Whenever a system interacts with its surroundings, it can exchange energy in two ways- work
and heat. In mechanics, work is defined as the product of the force and the displacement in the
direction of the force.
Work done when a spring is compressed or extended:
According to Hooke's law, Spring force = - k (x – x0)
Where k is the spring constant, x0 is the equilibrium position, and x is the final position. The
negative sign shows that the direction of the spring force is opposite the direction of the
displacement from x0.
The external force is equal in magnitude but opposite in sign to the spring force, so
External force (force of your hands) = k (x –x0).
Now, we want to calculate the work done when we stretch the spring from position 1 to position
2.
W = F dx = k (x – x0) d(x-x0) = 1/2 k [(x2-x0)2 - (x1-x0)2]
Work done when a volume is increased or decreased
Consider a gas in a container with a movable piston on top. If the gas expands, the piston moves
out and work is done by the system on the surroundings. However, if the gas inside contracts, the
piston moves in and work is done by the surroundings on the system. Why would the gas inside
contract or expand?
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It would if the external pressure, Pex, and the internal pressure, Pin, were different.
For expansion process; Let P = Pressure of the fluid on the piston face
A = Crossectional area of the piston
F = Force exerted by the fluid on the piston due to Pressure P;
Since F = PA;
Assuming after an infinitesimal lapse of time, for piston assembly; the piston has moved a
distance dx due to Pressure P; the workdone dW is given by;
dW = force X distance
dW = PA X dx
dW = P(Adx)
But Adx = dV = small element of volume
Therefore;
dW = PdV
The total amount of workdone W12 , by the fluid during the expansion process from state 1 to
state 2 is obtained by intergrating;
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2
∫1 𝑑𝑊 = ∫1 𝑃𝑑𝑉
2
W12 = ∫1 𝑃𝑑𝑉
The integral represents the area under the curve on a pressure versus volume diagram. Therefore
the work depends on the path followed and work is a path function and hence not a property of
the system.
The thermodynamic definition of work is “Work is said to be done if part of the boundary of a
system undergoes a displacement under the action of a force. Mechanical work is the workdone
by the action of a force in moving the boundary of a system.
Since work is energy exchange between a system and its surroundings, there is a need to adapt a
sign convention for work done during any thermodynamic process.
If work is done by the system on the surroundings e.g when a system expands within a pistoncylinder and pushes the piston outward, the work is said to be positive.
If work is done on the system by the surroundings e.g when a force is applied to a piston to
compress the system inside the cylinder, the work is said to be negative.
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Heat
Heat like work, is a form of energy. The energy transfer between a system and its surroundings is
called heat if it occurs by virtue of the temperature difference across the boundary.
The two modes of energy transfer – work and heat- depend on the choice of the system.
Heat energy moves from a hotter body to a colder body upon contact of the two bodies.
If two bodies at different temperatures are allowed to remain in contact, the system of two bodies
will eventually reach a thermal equilibrium (they will have the same temperature). Heat is a
transient phenomenon which can be possessed or stored in a system, however it can be converted
to some other form of energy after crossing the boundary of the system. Heat is not a property of
the system (it is incorrect to say heat in a system or heat of a system) but heat supplied or
removed from a system, the amount of heat flow to or from a system during a thermodynamics
process cannot be determined from the end states alone but depends on how the system has
changed from one state to another.. In SI system, the unit of heat is the Joule (J). Heat is a path
function, the heat transfer during a specific process say from state 1 to state 2 is denoted by Q12;
2
Q12 = ∫1 𝑑𝑄
These show that heat is directional in nature and flows from a region of high temperature to a
region of low temperature. The sign convention of heat flow is;
Heat flow into or supplied to a system is positive
Heat flow from or rejected by a system is negative
Reversible and Irreversible Processes
A reversible process occurs when a system changes its state and passes through a continuous
series of thermodynamic states such that both the system and its surroundings can be restored to
their original state. For a system to undergo a reversible process, it must have passed through
series of thermodynamic equilibrium state points that can be joined up to form a continuous line
on a diagram of properties.
Irreversible processes are practical processes that cannot be restored to their original states. An
irreversible process is represented by a dotted line joining the end states on the property diagram.
LAWS OF THERMODYNAMICS
Thermodynamics is an empirical science and is thus based on experimental observation. The
principles of thermodynamics are summarized in the form of four laws known as zeroth, first,
second, and the third laws of thermodynamics.
The zeroth law of thermodynamics was first formulated by R.H. Fowler in 1931. It states how a
body is in thermal equilibrium with a third body. If a hot body X is brought into physical contact
with a cold body Y while all other bodies are isolated from them, then heat will flow from X to
Y until the two bodies attain the same temperature. The two bodies are in thermal equilibrium
with each other or have attained equality of temperature.
Zeroth law of thermodynamics states that if two bodies are separately in thermal equilibrium
with a third body, then the three bodies are all in thermal equilibrium with each other. It is useful
concept in determining if two bodies are in thermal equilibrium without the need of bringing
them in contact with each other. All that is needed is a third body called the thermometer.
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FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics is the thermodynamic expression of the law of conservation of
energy which states that “energy can not be created or destroyed but can be change in form or
that “the energy of the universe is constant”. When this law is stated with particular reference to
heat and work energy, it is called the first law of thermodynamics. The law may also be stated
thus;
“When a closed system is taken through a thermodynamic cycle, then the net heat
supplied to the system from its surroundings is equal to the net work done by the system
on its surroundings.
In symbol;
∮ 𝑑𝑄 = ∮ 𝑑𝑊
(∮ 𝑚𝑒𝑎𝑛𝑠 𝑖𝑛𝑡𝑒𝑟𝑔𝑟𝑎𝑡𝑖𝑜𝑛 𝑎𝑙𝑜𝑛𝑔 𝑎 𝑐𝑦𝑐𝑙𝑒 𝑝𝑎𝑡ℎ 𝑜𝑟 𝑠𝑢𝑚𝑚𝑎𝑡𝑖𝑜𝑛 𝑟𝑜𝑢𝑛𝑑 𝑡ℎ𝑒 𝑐𝑦𝑐𝑙𝑒)
Thus according to first law, for a system to operate in a cycle and produce a net amount of work,
some heat must be supplied. It is impossible to construct a device which when set in motion will
produce work continuously without any source of heat input from the surroundings.
An imaginary device that delivers work continuously without absorbing energy from the
surroundings is called a Perpetual Motion machine of the first kind. Since the device has to
deliver work continuously, it has to operate on a cycle. If such a device does not absorb energy
from its surroundings ∫dQ =0. From the first law, it can be observed that ∫dW =0, if ∫ dQ = 0.
Therefore such a device is impossible from first law of thermodynamics.
Property X
Property Y
Considering a cycle composed of four processes;
Thus Q12 + Q23 + Q34 + Q41 = W12 + W23 + W34 + W41
The algebraic sum of the net heat transfer for the cycle is equal to the algebraic sum of the net
work transfer for the cycle.
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