CPE 201: Engineering Thermodynamics (2nd Part)

Resource Person: Dr J.G. Akinbomi

1) Basics of Engineering Thermodynamics
(
 Intensive and extensive properties
(a) Definition of Thermodynamics  Importance of Thermodynamic properties
 Classical (or macroscopic) thermodynamics
 Statistical (or microscopic) thermodynamics (4) Thermodynamic Property
(b) Laws of Thermodynamics Relations
 Zeroth Law of Thermodynamics  Derivation of four fundamental property relations
 First Law of Thermodynamics  Derivation of Maxwell’s equations using
 Second Law of Thermodynamics principle of criterion of exactness
 Third Law of Thermodynamics
(5) A Two-phase System
(c) Thermodynamic Systems, States and
 Characteristics of a wet vapour
Processes  Derivation of an expression for the specific
 Types of Thermodynamic Systems (isolated, closed, volume of a wet vapour
open and adiabatic systems)  Derivation of Clapeyron equation from phase-
 Thermodynamic State change principle of pure substances
 Thermodynamic Processes (adiabatic, isenthalpic,
isentropic, isobaric, isochoric / isomeric and (6)Steam Tables
isothermal process
 Saturated Steam Tables
 Equilibrium process
 Superheated Steam Tables
(2) Gibb’s Phase Rule
(3) Thermodynamic Properties
 1. Basics of Engineering Thermodynamics
(A )Definition of Thermodynamics (a) Classical (mascroscopic) thermodynamics
Thermodynamics is the science that is Classical thermodynamics concentrates on the
concerned with energy as a property of matter net changes affecting a system without
and the energy transformation into heat and considering the detailed changes occurring
work, as well as, the principles of energy within the system and the enclosing boundaries.
transformation in macroscopic systems It does not require knowledge of the behaviour
of the individual particles of a system but it is
The principles of energy transformation are concerned with the relationships between bulk
general restrictions that apply to all energy properties of matter that we see directly. Nothing
transformations and are known as laws of is examined at the atomic or molecular level.
thermodynamics
The goal here is to describe the average
These laws describe the bounds within which behaviour of macroscopic properties (such as
energy transformations are observed to occur. temperature, pressure, internal energy, etc.)
Approaches/Methods of Studying rather than the microscopic details of each.
Thermodynamics (b) Statistical (or microscopic) thermodynamics
There are two methods of Studying Statistical thermodynamics considers the
Thermodynamics: detailed changes occurring to and within the
• molecules inside the system. It seeks to each other.
explain those bulk properties in terms of
constituent atoms. It is important to note that Zeroth Law of
• Statistical thermodynamics provides a Thermodynamics defines Temperature (T).
quantitative link between the properties of
the microscopic particles and the behaviour (ii) First Law of Thermodynamics
of the bulk material. In statistical approach, The First Law of Thermodynamics is an
systems are described in terms of
microscopic variables(positions, velocities, expression of the principle of conservation of
etc, of all particles in the system). energy. It states that energy can be
transformed (changed from one form to
(B) Laws of Thermodynamics another) but cannot be created or destroyed.
Energy can be stored within systems in
Thermodynamics is principally based on a set
of four laws which are universally valid when various macroscopic forms including kinetic
applied to systems that fall within the energy, gravitational potential energy and
constraints implied by each. internal energy.
Thermodynamic laws include : Energy can also be transformed from one form
(i) Zeroth Law of Thermodynamics to another and transferred between systems.
It states that ‘if two systems are each in For closed, energy can be transferred by work
thermal equilibrium with a third system, they and heat transfer. The total amount of energy
are also in thermal equilibrium with each
is conserved in all transformations and
It is important to know that First Law of is reversible, the total entropy of the system
Thermodynamics defines Energy (U). plus its surroundings must be constant.
The second law asserts that processes occur in
(iii) Second Law of Thermodynamics a certain direction but not in the reverse
direction. The law states that heat cannot
The second law is an expression of the spontaneously flow from a colder location to a
universal principle of decay in nature that over hotter location.
time, differences in temperature, pressure, etc., For example, a cup of hot water left in the
tend to even out in a physical system that is open eventually cools, but a cup of cold
isolated from the outside world. Entropy is a coffee left in the open never gets hot by itself.
measure of how much this process has The Kelvin-Planck statement of the second
progressed. law of thermodynamics states that ‘it is
impossible by a cyclic process (the word
The second law requires that the entropy of an cyclic requires that the system be restored
isolated system either increases or, in the limit, periodically to its original state) to convert the
heat absorbed by a system completely into
where the system has reached an equilibrium work done by the system.
state, remains constant. The limitations of the first law of
For a closed (but not isolated) system, it thermodynamics (in the sense that it does not
requires that an entropy decreases in either the indicate the extent to which energy can be
converted and also whether a certain process
system or its surroundings be more than is permissible or not) have invariably led to
compensated by an entropy increase in the the introduction of the second law of
other part. In other words, where the process thermodynamics.
It is important to note that second law of It is important to know that the third
thermodynamics defines ‘Entropy’ law of thermodynamics gives
(iv) Third Law of Thermodynamics ‘numerical vale of entropy’.
The law states that ‘as a system approaches For a reversible process change in
absolute zero (-273.15 C or 0 K), all processes entropy maybe calculated by Equation
cease and the entropy of the system approaches
a minimum value. Alternate statement of the 1:
dQ
third law is that ‘it is impossible to reach the dS  (1)
T
absolute zero of temperature by any finite
number of processes.
The third law is a statistical law of nature where ST is the total entropy of the
regarding entropy and the impossibility of system and T is the absolute
reaching absolute zero of temperature. temperature of the system.
This law provides an absolute reference point
for the determination of entropy.
The entropy determined relative to this point is
the absolute entropy.
(C) Thermodynamic Systems, States and (iv) Closed System
Processes
This is a system that allows only energy (as
(i) Thermodynamic System heat and/or work) to pass through the
System is the subject of the investigation. boundaries but matter cannot pass through .
It is a specified quantity of matter and/ or a The mass of the system is constant/
region that can be separated from anything
else (surroundings) by a well-defined (v) Open System
surface This is a system that allows both energy and
(ii) Boundary matter to pass through its boundary.
Boundary is the imaginary envelope which (vi) Adiabatic system
encloses a system an separates it from its This id a system that does not allow heat to
surroundings pass through its boundaries. Work can be
(iii) Isolated System performed on or by the system.
This is a system that does not interact in (vi) Thermodynamic State
any way with its surroundings. In an State is the condition of a system at any instant
isolated system, no energy and no matter of time. The state at a given instant of time is
can pass through the boundaries of the described by the properties of the system . A
system property is any quantity whose numerical
value depends on the state but not the history (d) Isobaric process: a process that occurs at
of the system constant pressure
(vii) Thermodynamic Processes (e) Isochoric process/Isometric process: a
A thermodynamic process is the succession of process that occurs at constant volum
thermodynamic states that a system passes (f) Isothermal process: a process that occurs at
through as it goes from an initial state to a a constanr temperature.
final state.
(2) Gibbs Phase Rule
Several commonly studied thermodynamic A phase is a form of matter that is
processes are: homogeneous in chemical composition and
(a) Adiabatic process: a process that occurs physical state. Homogeneity in physical
without loss or gain of energy by heat structure means that the matter is all solid, or
all liquid, or all vapour( or equivalently all
(b) Isenthalpic process: a process that occurs
gas).
at a constant enthalpy
A system can contain one or more phases. For
(c) Isentropic process: it is a reversible
example, a system of liquid water and water
adiabatic process that occurs at a constant
vapour (steam) contains two phases.
entropy
A pure substance (substance that is uniform where
and invariable in chemical composition) can F is the degree of freedom,
exist in more than one phase but its chemical
composition must be the samw in each phase. π is the number of phases and
For example, if liquid water and water vapour N is the number of chemical species
form a system with two phases, the system can The phase rule gives the number of
be regarded as a pure substance because each independent variables that must be arbitrarily
phase has the same composition. fixed to establish the intensive state of any
It is most important to be able to predict the system. i.e. , the degrees of freedom (F) of the
effect of change in pressure, temperature or system.
composition; on the number of phases that can The phase – rule variables are intensive
exist in a system that is in equilibrium. properties, which are independent of the extent
The possible variations are predicted from of the system and of the individual phases.
Gibbs; phase rule which states that: Thus the phase rule gives the same
information for a large system as for a small
one and for different relative amounts of the
phases present
F  2   N (2) Note
Various phases can coexist but they must be in
equilibrium for the phase rule to apply
Example 1
The number of phases, 𝜋 = 2. There are two
What is the degree of freedom of each of phases (One liquid and one vapour)
the following isolated systems:
(a)Liquid water in the equilibrium with it is FIND:
vapour. F = number of degrees of freedom
(b)Liquid water in the equilibrium with a
mixture of water vapour and nitrogen F  2   N
Phase Rule:
(c)A liquid solution of alcohol in water in
equilibrium with its vapour
F = 2 -2 + 1 = 1.
Solution
(a) This results is in agrrement with the well-
known fact that for a given pressure, wate has
KNOWN: but one boiling point. Temperature or
pressure, but not both, may be specified for a
The number of chemical species, N =1 system consisting of water in equilibrium
with its vapour.
 (b) KNOWN: • Now temperature and pressure may be
independently varied , but once they are
N = the number of chemical species =2. fixed the system described can exist in
There are two chemical species, equilibrium only at a particular composition
of the vapour phase.
𝜋 = number of phases =2. There are two
phase liquid water and nitrogen gas
(c)KNOWN
- FIND
The number of chemical species = 2. There
F = Number of degree of freedom are two chemical species
Thus F = 2 – 𝜋+ N The number of phases, 𝜋 = 2
= 2 -2 + 2= 2 FIND:
F = Number of degree of freedom
• Thus F = 2- 𝜋 + N
The addiction of an inert gas to a system of
water in equilibrium with its vapour changes =2-2+2=2
the characteristics of the system.
The phase-rule variables are temperature,
pressure and the phase compositions. The • Assignment
composition variables are either the mass or Find the number of degree of freedom
mole fractions of the species in a phase , and of an isolated system consisting of a
they must sum to unity for each phase. Thus boiling saturated solution of a salt in
fixing the mole fraction of the water in the water with excess salt crystals present
liquid phase automatically fixed the mole
fraction of the alcohol.