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THERMODYNAMICS
AND
HEAT TRANSFER
Presented By:
Manish Kumar
ISM Dhanbad
What is Thermodynamics?
Thermodynamics is a branch of science which deals
with energy interaction and its effect on the system
and surrounding.
2
System, surroundings and boundary
 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.
3
Type of system
(isolated system)
 Isolated system – neither
mass nor energy can cross
the selected boundary
 Example (approximate): well-
insulated thermos bottle, Universe
5
Type of system
(Closed system)
 Closed system – only energy
can cross the selected
boundary
 Examples: piston cylinder
arrangement without valve
6
Type of system
(Open system)
 Open system – both mass and
energy can cross the selected
boundary
 Example: piston cylinder
arrangement with valves
7
Properties of a system
Properties of a system is defined as a characteristic of a system that is in
equilibrium.
Properties may be intensive or extensive.
 Intensive – Are independent of the amount of mass:
e.g: Temperature, Pressure, and Density,
 Extensive – varies directly with the mass
e.g: mass, volume, energy, enthalpy
Specific properties – The ratio of any extensive property of a system to that
of the mass of the system is called an average specific value of that property
(also known as intensives property)
Properties of a system
State, Equilibrium and Process
 Thermodynamic equilibrium -
system that maintains thermal,
mechanical, phase and chemical
equilibriums.
Important point w.r.t to properties
1.Properties are point function.
2.Properties are independent of past history.
Heat: Heat is mode of heat transfer which takes place by the virtue of
temperature difference.
Q= mc t
C= specific heat is the energy required to change the temp of unit mass of a
substance by a unit. Specific heat at constant pressure(Cp) is greater than
constant heat at constant volume(Cv) because Cp includes boundary work
along with internal energy where as Cv include only internal energy.
Work: Work is said to be done by the system if the entire effect of the
system on the surrounding can be converted into raising of weight though
the weight may not be raised.
Important point w.r.t Heat & Work
Interaction
1.Heat added to the system is taken as positive and heat rejected by
the system is taken as negative whereas work done on the system
is taken as negative and work done by the system is taken as
positive.
2.Heat and work are path function.
3.Heat and work are transient phenomena.
4.Heat and work are boundary phenomena.
The prefix iso- is often used to designate a process for which a particular property
remains constant.
State, Equilibrium and Process
Isobaric process: A process during which the pressure P remains constant.
Pressure is Constant (ΔP = 0)
12
State, Equilibrium and Process
 Process – change from one
equilibrium state to another.
Process Property held
constant
isobaric pressure
isothermal temperature
isochoric volume
isentropic entropy
Isothermal process: A process during
which the temperature T remains
constant.
.
Isochoric (or isometric) process: A process during which the specific volume v
remains constant
Process Property held
constant
isobaric pressure
isothermal temperature
isochoric volume
isentropic entropy
14
Types of Thermodynamics Processes
 Reversible process - it is defined as a process
that, once having take place it can be reversed
following the same path as that of forward. In
doing so, it leaves no change in the system or
surrounding.
 Irreversible process - a process that cannot
return both the system and surrounding to their
original conditions
15
Types of Thermodynamics Processes
 Adiabatic process - a process that has no heat transfer
into or out of the system. It can be considered to be
perfectly insulated.
 Isentropic process - a process where the entropy of the
fluid remains constant.
 Polytropic process - when a gas undergoes a reversible
process in which there is heat transfer, it is represented
with a straight line, PVn
= constant.
16
Zeroth Law of Thermodynamics
“ If two bodies are in thermal equilibrium with a third
body separately, there are also in thermal equilibrium
with each other.”
What is Pure Substances?
17
 A substance that has a fixed
chemical composition throughout
the mixture is called a pure
substance.
Gibbs phase rule :
P+F = C+2
P= number of phases
C= number of components
F= Degree of freedom
18
Phases of A Pure Substance
 The substances exist in different phases, e.g. at
room temperature and pressure, copper is solid
and mercury is a liquid.
 It can exist in different phases under variations
of condition.
 There are 3 Principal phases
• solid
• Liquid
• gas
Each with different molecular structures.
19
s
This constant
pressure heating
process can be
illustrated as:
20
Property Diagram
21
Saturated and Sub-cooled Liquids
 If a substance exists as a liquid at the
saturation temperature and pressure,
it is called a saturated liquid
 If the temperature of the liquid is
lower than the saturation
temperature for the existing
pressure, it is called either a
subcooled liquid or a compressed
liquid
22
 If a substance exists entirely as
vapor at saturation temperature, it
is called saturated vapor.
 When the vapor is at a temperature
greater than the saturation
temperature, it is said to exist as
superheated vapor.
 The pressure and temperature of
superheated vapor are independent
properties, since the temperature
may increase while the pressure
remains constant
Saturated and Superheated Vapors
23
Quality
 When a substance exists as part liquid and part vapor at
saturation conditions, its quality (x) is defined as the
ratio of the mass of the vapor to the total mass of both
vapor and liquid.
 The quality is zero for the saturated liquid and one for
the saturated vapor (0 ≤ x ≤ 1)
 For example, if the mass of vapor is 0.2 g and the mass
of the liquid is 0.8 g, then the quality is 0.2 or 20%.
x
mass
mass
m
m m
saturated vapor
total
g
f g
= =
+
24
Important Definition
o Critical point - the temperature and pressure above which there
is no distinction between the liquid and vapor phases.
o Triple point - the temperature and pressure at which all three
phases can exist in equilibrium.
o Sublimation - change of phase from solid to vapor.
o Vaporization - change of phase from liquid to vapor.
o Condensation - change of phase from vapor to liquid.
o Fusion or melting - change of phase from solid to liquid.
25
26
Ideal Gas Law
 Robert Boyle formulates a well-known law that states the pressure of a
gas expanding at constant temperature varies inversely to the volume,
or
constant2211 == VPVP
 As the result of experimentation, Charles concluded that the pressure of
a gas varies directly with temperature when the volume is held
constant, and the volume varies directly with temperature when the
pressure is held constant, or
2
1
2
1
2
1
2
1
T
T
P
P
or
T
T
V
V
==
27
 By combining the results of
Charles' and Boyle's
experiments, the following
relationship can be obtained
 The constant in the above
equation is called the ideal gas
constant and is designated by
R; thus the ideal gas equation
becomes
 In order to make the equation
applicable to all ideal gas, a
universal gas constant RU is
introduced
constant=
T
Pv
mRTPVorRTPv ==
M
R
R U
=
28
 For example the ideal gas constant for air, Rair
KkgkJ
M
R
R
air
airU
air ./2871.0
96.28
3144.8
)(
)(
===
 The amount of energy needed to raise the temperature of a unit of
mass of a substance by one degree is called the specific heat at
constant volume Cv for a constant-volume process and the specific
heat at constant pressure Cp for a constant pressure process. They
are defined as
P
P
v
v
T
h
Cand
T
u
C 





∂
∂
=





∂
∂
=
29
 Using the definition of enthalpy (h = u + Pv) and writing the
differential of enthalpy, the relationship between the specific heats
for ideal gases is
 The specific heat ratio, k is defined as
v
P
C
C
k =
P V
P V
h u Pv
dh du RT
C dt C dt RdT
C C R
= +
= +
= +
= +
30
 For ideal gases u, h, Cv, and Cp are functions of temperature alone.
The Δu and Δh of ideal gases can be expressed as
)( 1212 TTCuuu v −=−=∆
)( 1212 TTChhh P −=−=∆
First Law of Thermodynamics
31
 The First Law is usually referred to as the Law of Conservation
of Energy, i.e. energy can neither be created nor destroyed, but
rather transformed from one state to another.
 For a closed system undergoing a cycle net heat interaction =
net work transfer.
Q W U
Q W
net net cycle
net net
− =
=
∆
32
Energy Balance for Closed System
Heat
Work
z
Closed
System
Reference Plane, z = 0

V
or
E E Ein out system− = ∆
33
 According to classical thermodynamics
Q W Enet net system− = ∆
 The total energy of the system, Esystem, is given as
E Internal energy Kinetic energy Potential energy
E U KE PE
= + +
= + +
 The change in stored energy for the system is
∆ ∆ ∆ ∆E U KE PE= + +
 The first law of thermodynamics for closed systems then can be
written as
Q W U KE PEnet net− = + +∆ ∆ ∆
34
Boundary Works
3
2
4
51
P
V
35
No Value of n Process Description Result of IGL
1 ∞ isochoric constant volume (V1
= V2
)
2 0 isobaric constant pressure (P1
= P2
)
3 1 isothermal constant temperature
(T1
= T2
)
4 1<n< γ polytropic -none-
5 γ isentropic constant entropy (S1
= S2
)
According to a law of constant=n
VP
2
2
1
1
T
P
T
P
=
2
2
1
1
T
V
T
V
=
2211 VPVP =
1
2
1
1
2
2
1
−






=





=
n
n
n
T
T
V
V
P
P
36
 Various forms of work are expressed as follows
Process Boundary Work
isochoric
isobaric
isothermal
polytropic
isentropic
0)( 1212 =−= VVPW
)( 1212 VVPW −=
1
2
1112 ln
V
V
VPW =
n
VPVP
W
−
−
=
1
1122
12
37
 Second law is useful:
 Used for predicting the direction of processes,
 determining the best theoretical performance of cycles, engines
and other devices.
Second Law of Thermodynamics
It is impossible to develop a cyclic device which converts low grade
energy into high grade energy completely
38
Second Law of Thermodynamics
Kelvin-Planck statement
 No heat engine can have a
thermal efficiency 100
percent.
 It is impossible to create a
cyclic device which
produces work by
exchanging heat with a
single reservoir.
39
Thermal Efficiency
 Represent the magnitude of the energy wasted in order
to complete the cycle.
 A measure of the performance that is called the
thermal efficiency.
 Can be expressed in terms of the desired output and
the required input
ηth =
Desired Result
Required Input
 For a heat engine the desired result is the net work
done and the input is the heat supplied to make
the cycle operate.
40
The thermal efficiency is always less than 1 or less than
100 percent.
ηth
net out
in
W
Q
=
,
W W W
Q Q
net out out in
in net
, = −
≠
where
41
 Applying the first law to the cyclic heat engine
Q W U
W Q
W Q Q
net in net out
net out net in
net out in out
, ,
, ,
,
− =
=
= −
∆
 The cycle thermal efficiency may be written as
ηth
net out
in
in out
in
out
in
W
Q
Q Q
Q
Q
Q
=
=
−
= −
,
1
42
 A thermodynamic temperature scale related to the heat
transfers between a reversible device and the high and low-
temperature reservoirs by
Q
Q
T
T
L
H
L
H
=
 The heat engine that operates on the reversible Carnot
cycle is called the Carnot Heat Engine in which its
efficiency is
ηth rev
L
H
T
T
, = −1
43
Heat Pumps and Refrigerators
 A device that transfers heat from a low
temperature medium to a high temperature one is
the heat pump.
 Refrigerator operates exactly like heat pump
except that the desired output is the amount of
heat removed out of the system
 The index of performance of a heat pumps or
refrigerators are expressed in terms of the
coefficient of performance.
44
45
COP
Q
W
Q
Q Q
HP
H
net in
H
H L
= =
−,
COP
Q
W
R
L
net in
=
,
46
Carnot Cycle
Process Description
1-2 Reversible isothermal heat addition at
high temperature
2-3 Reversible adiabatic expansion from high
temperature to low temperature
3-4 Reversible isothermal heat rejection at
low temperature
4-1 Reversible adiabatic compression from low
temperature to high temperature
47
48
 The thermal efficiencies of actual and reversible heat
engines operating between the same temperature limits
compare as follows
 The coefficients of performance of actual and reversible
refrigerators operating between the same temperature limits
compare as follows
49
Entropy
 The 2nd law states that process occur in a certain
direction, not in any direction.
 It often leads to the definition of a new property called
entropy, which is a quantitative measure of disorder
for a system.
 Entropy can also be explained as a measure of the
unavailability of heat to perform work in a cycle.
 This relates to the 2nd law since the 2nd law predicts
that not all heat provided to a cycle can be
transformed into an equal amount of work, some heat
rejection must take place.
50
Entropy Change
 The entropy change during a reversible process is defined
as
 For a reversible, adiabatic process
dS
S S
=
=
0
2 1
 The reversible, adiabatic process is called an isentropic
process.
51
Entropy Change and Isentropic Processes
The entropy-change and isentropic relations for a process
can be summarized as follows:
i. Pure substances:
Any process: Δs = s2 – s1 (kJ/kg⋅K)
Isentropic process: s2 = s1
ii. Incompressible substances (liquids and solids):
Any process: s2 – s1 = cav T2/T1 (kJ/kg
Isentropic process: T2 = T1
52
iii. Ideal gases:
a) constant specific heats (approximate treatment):
s s C
T
T
R
v
v
v av2 1
2
1
2
1
− = +, ln ln
2 2
2 1 ,
1 1
ln lnp av
T P
s s C R
T P
− = −
for isentropic process
2 1
1 2.
k
s const
P v
P v=
   
=   
   
for all process
HEAT TRANSFER
Difference between thermodynamics and
heat transfer analysis:-
In thermodynamics we deals with reversible path and find out the heat
interaction during a process or from one initial point to the final point but in
heat transfer analysis we focus on the rate of heat transfer , here we find out
heat transfer per unit time whereas in thermodynamics we find the amount of
heat transfer during a process and have nothing to do with the time required
for that heat transfer to occur. Therefore in thermodynamics we calculate heat
interaction in joules and in heat transfer it comes out to be in joules per
second that is watt.
HEAT TRANSFER
Heat transfer is the flow of heat energy from a hot
substance to a colder substance.
MODE OF HEAT TRANSFER.
Heat can be transferred by THREE methods:
 By CONDUCTION
 By CONVECTION
 By RADIATION
HEAT TRANSFER BY CONDUCTION
Heat has traveled through the metal rod by a process known
as CONDUCTION.
It is a mode of heat transfer which generally occurs in solid due to
temperature difference by molecular lattice vibrational energy transfer
and also by free electron transfer. The reason behind all electrically good
conductors are also in general good conductors of heat is due to the
presence of free electons. E.g all metals
Exception to above statement is Diamond which is non metal and
having highest conductivity i.e 2300W/mk due to crystalline molecular
lattice arrangement.
Gases also conduct heat by molecular momentum transfer when
high speed high temp molecule collide with lo speed low temp
molecule but in general it is very bad conductor of heat.
Thermal conductivity can be defined as: The number of heat units
(Joules) flowing per unit of time (1 second) through a cross-sectional
area (1 square metre) when the temperature falls by one degree (1
Kelvin or 1 degree Celsius) per unit length (1 metre) of its path.
The units of thermal conductivity are, therefore w/mk
units of thermal conductivity are:
Watts per meter per o
C or K (W m-1
K-1
)
K VALUES
The k values for most materials have been determined in the
laboratory. The thermal conductivities of some materials used in
industry are shown below.
MATERIAL THERMAL CONDUCTIVITY k (Wm-1
K-1
)
 Solids – metals
Copper 390
Aluminum 202
Steel 55
 Solids – nonmetals
Magnesia 0.07
Cork 0.043
Glass 1.09
Asbestos 0.16
Brick (alumina) 3.1
 Fluids
Water 0.62
Benzene 0.16
Air 0.024
CONDUCTION OF HEAT THROUGH SINGLE
WALLS
The rate of flow Φ (J S-1
) from the hot side to the cold side will
depend on the following factors:-
The thermal conductivity k (W m-1
K-1
)
The area A (m2),
 The difference in temperature T1 – T2 (K) (T1 – T2 is referred to as
ΔT).
 The thickness d (metres)
Refractory wall,
thermal conductivity, k,
(W m-1
K-1
)
Thickness of wall
d (m)
cold side temperature
T2
K
hot side temperature
T1
K
area of wall over
which heat flows
A (m2
)
heat flow by conduction through wall (J/s)
If Φ represents the heat flow in J S-1
(or watts), then
Φ = k A (T1 – T2) / d
Where:
Φ rate of heat flow (in J S-1
)
k thermal conductivity (in W m-1
K-1
)
A area over which heat is passing (in m2
)
T1 hot face temperature (in K)
T2 cold face temperature (in K)
d thickness or distance between
hot face and cold face (in m)
This is the way in which heat energy travels through liquids and
gases. (FLUIDS)
CONVECTION
It is the mode of heat transfer which generally occurs
between solid surface and the surrounding fluid due to
temperature difference associated with macroscopic bulk
motion of fluid transporting thermal energy.
CALCULATION OF THE HEAT TRANSFER RATE
BY CONVECTION
When the fluid outside the solid surface is in forced or natural
convective motion, the expression of the rate of heat transfer from
the solid to the fluid, or vice versa, is as follows:
qc = hc A (Ts – Tf)
where:
qc =rate of heat transfer convection in J/s or W
A =Area of heat transfer, m2
Ts = The temperature of the solid surface, K
Tf =The average temperature of the fluid, K
hc =The convection heat transfer coefficient, W/m2
.K
The coefficient hc is a function of the system
geometry, fluid properties, flow velocity, and
temperature difference. In many cases, empirical
correlation are available to predict this
coefficient, since it often cannot be predicted
theoretically.
NATURAL AND FORCED CONVECTION
Convection currents occur when fluids come into contact with hot
surfaces. The convection currents produced give rise to a
MIXING ACTION, which helps to transfer heat throughout the
body of the fluid. The process of heat transfer by NATURAL
CIRCULATION is called NATURAL CONVECTION.
Movement of the fluid is caused only by the differences in
density, The rate of heat transfer by convection can be improved
by agitating the fluid as it is heated.
The rate of heat transfer by convection will be greater if
propeller is used to mix or agitate the fluids. Heat
transfer produced in this way, i.e. by FORCING a fluid
to move over a heating surface, is referred to as
FORCED CONVECTION.
Generally speaking, with forced convection there is:
more rapid circulation.
more turbulence and,
more rapid heat transfer.
Baffles fitted to the heating vessel will produce more
turbulence, improved mixing and hence more rapid heat
transfer by convection.
NATURAL AND FORCED CONVECTOR
HEATERS (EXAMPLE)
These are similar in principle to the domestic heater.
(a) shows cold air circulating over the tubes of a heat
exchanger. Heated air then moves upwards and away
from the heating surfaces. As this happens, cold air is
drawn in near the base of the exchanger and, in its turn,
becomes heated.
(b) shows a fan forcing cold air over the tubes of the
heat exchanger. This enables very much larger volumes
of air to be heated than can be heated by natural
circulation, i.e. natural convection, alone.
Fan assisted convector heaters can be fitted with a
number of smaller diameter tubes so increasing the area
of the hot surface available for heat transfer for a given
unit size. This, in turn, will increase the volume of cold
air that can be heated in a given time. The extra force
required to push the air through the bank of tubes is
provided by the fan.
RADIATION consists of invisible energy waves, which are
able to pass across a space. Unlike heat transfer by conduction
and convection, heat transfer by radiation does not require any
material to be present between the hot part and the cold part of
the system. Heat can travel by radiation across a vacuum. For
example, heat energy from the sun travels across the empty
space beyond the earth’s atmosphere. Scientists call these
invisible heat waves ELECTROMAGNETIC WAVES.
Electromagnetic waves travel across a space very rapidly, 3 ×
108
m s-1
.
CALCULATING HEAT TRANSFER BY RADIATION
As you will recall, the factors which affect the rate of heat transfer by radiation
from a body are:
◦ Its temperature T2 (K)
◦ Temperature of surroundings T1 (K)
◦ Its area A (m2
)
◦ Its emissivity ε
Scientists have combined these factors and produced a formula from which the
rate of heat transfer by radiation, Φ (J s-1
or W), can be calculated. This is given
below.
Φ = 5.7 × 10-8
× ε × A × (T2
4
– T1
4
)
Where:
◦ Φ is the heat radiated from the hot surface (W)
◦ ε is the emissivity, from reference books.
◦ A is the surface area radiating heat (m2
)
◦ T2 is the surface temperature (K)
◦ T1 is the surrounding temperature (K)

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Thermodynamics and Heat Transfer

  • 2. What is Thermodynamics? Thermodynamics is a branch of science which deals with energy interaction and its effect on the system and surrounding.
  • 3. 2 System, surroundings and boundary  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.
  • 4. 3 Type of system (isolated system)  Isolated system – neither mass nor energy can cross the selected boundary  Example (approximate): well- insulated thermos bottle, Universe
  • 5. 5 Type of system (Closed system)  Closed system – only energy can cross the selected boundary  Examples: piston cylinder arrangement without valve
  • 6. 6 Type of system (Open system)  Open system – both mass and energy can cross the selected boundary  Example: piston cylinder arrangement with valves
  • 7. 7 Properties of a system Properties of a system is defined as a characteristic of a system that is in equilibrium. Properties may be intensive or extensive.  Intensive – Are independent of the amount of mass: e.g: Temperature, Pressure, and Density,  Extensive – varies directly with the mass e.g: mass, volume, energy, enthalpy
  • 8. Specific properties – The ratio of any extensive property of a system to that of the mass of the system is called an average specific value of that property (also known as intensives property) Properties of a system State, Equilibrium and Process  Thermodynamic equilibrium - system that maintains thermal, mechanical, phase and chemical equilibriums.
  • 9. Important point w.r.t to properties 1.Properties are point function. 2.Properties are independent of past history. Heat: Heat is mode of heat transfer which takes place by the virtue of temperature difference. Q= mc t C= specific heat is the energy required to change the temp of unit mass of a substance by a unit. Specific heat at constant pressure(Cp) is greater than constant heat at constant volume(Cv) because Cp includes boundary work along with internal energy where as Cv include only internal energy. Work: Work is said to be done by the system if the entire effect of the system on the surrounding can be converted into raising of weight though the weight may not be raised.
  • 10. Important point w.r.t Heat & Work Interaction 1.Heat added to the system is taken as positive and heat rejected by the system is taken as negative whereas work done on the system is taken as negative and work done by the system is taken as positive. 2.Heat and work are path function. 3.Heat and work are transient phenomena. 4.Heat and work are boundary phenomena.
  • 11. The prefix iso- is often used to designate a process for which a particular property remains constant. State, Equilibrium and Process Isobaric process: A process during which the pressure P remains constant. Pressure is Constant (ΔP = 0)
  • 12. 12 State, Equilibrium and Process  Process – change from one equilibrium state to another. Process Property held constant isobaric pressure isothermal temperature isochoric volume isentropic entropy
  • 13. Isothermal process: A process during which the temperature T remains constant. . Isochoric (or isometric) process: A process during which the specific volume v remains constant Process Property held constant isobaric pressure isothermal temperature isochoric volume isentropic entropy
  • 14. 14 Types of Thermodynamics Processes  Reversible process - it is defined as a process that, once having take place it can be reversed following the same path as that of forward. In doing so, it leaves no change in the system or surrounding.  Irreversible process - a process that cannot return both the system and surrounding to their original conditions
  • 15. 15 Types of Thermodynamics Processes  Adiabatic process - a process that has no heat transfer into or out of the system. It can be considered to be perfectly insulated.  Isentropic process - a process where the entropy of the fluid remains constant.  Polytropic process - when a gas undergoes a reversible process in which there is heat transfer, it is represented with a straight line, PVn = constant.
  • 16. 16 Zeroth Law of Thermodynamics “ If two bodies are in thermal equilibrium with a third body separately, there are also in thermal equilibrium with each other.”
  • 17. What is Pure Substances? 17  A substance that has a fixed chemical composition throughout the mixture is called a pure substance. Gibbs phase rule : P+F = C+2 P= number of phases C= number of components F= Degree of freedom
  • 18. 18 Phases of A Pure Substance  The substances exist in different phases, e.g. at room temperature and pressure, copper is solid and mercury is a liquid.  It can exist in different phases under variations of condition.  There are 3 Principal phases • solid • Liquid • gas Each with different molecular structures.
  • 21. 21 Saturated and Sub-cooled Liquids  If a substance exists as a liquid at the saturation temperature and pressure, it is called a saturated liquid  If the temperature of the liquid is lower than the saturation temperature for the existing pressure, it is called either a subcooled liquid or a compressed liquid
  • 22. 22  If a substance exists entirely as vapor at saturation temperature, it is called saturated vapor.  When the vapor is at a temperature greater than the saturation temperature, it is said to exist as superheated vapor.  The pressure and temperature of superheated vapor are independent properties, since the temperature may increase while the pressure remains constant Saturated and Superheated Vapors
  • 23. 23 Quality  When a substance exists as part liquid and part vapor at saturation conditions, its quality (x) is defined as the ratio of the mass of the vapor to the total mass of both vapor and liquid.  The quality is zero for the saturated liquid and one for the saturated vapor (0 ≤ x ≤ 1)  For example, if the mass of vapor is 0.2 g and the mass of the liquid is 0.8 g, then the quality is 0.2 or 20%. x mass mass m m m saturated vapor total g f g = = +
  • 24. 24 Important Definition o Critical point - the temperature and pressure above which there is no distinction between the liquid and vapor phases. o Triple point - the temperature and pressure at which all three phases can exist in equilibrium. o Sublimation - change of phase from solid to vapor. o Vaporization - change of phase from liquid to vapor. o Condensation - change of phase from vapor to liquid. o Fusion or melting - change of phase from solid to liquid.
  • 25. 25
  • 26. 26 Ideal Gas Law  Robert Boyle formulates a well-known law that states the pressure of a gas expanding at constant temperature varies inversely to the volume, or constant2211 == VPVP  As the result of experimentation, Charles concluded that the pressure of a gas varies directly with temperature when the volume is held constant, and the volume varies directly with temperature when the pressure is held constant, or 2 1 2 1 2 1 2 1 T T P P or T T V V ==
  • 27. 27  By combining the results of Charles' and Boyle's experiments, the following relationship can be obtained  The constant in the above equation is called the ideal gas constant and is designated by R; thus the ideal gas equation becomes  In order to make the equation applicable to all ideal gas, a universal gas constant RU is introduced constant= T Pv mRTPVorRTPv == M R R U =
  • 28. 28  For example the ideal gas constant for air, Rair KkgkJ M R R air airU air ./2871.0 96.28 3144.8 )( )( ===  The amount of energy needed to raise the temperature of a unit of mass of a substance by one degree is called the specific heat at constant volume Cv for a constant-volume process and the specific heat at constant pressure Cp for a constant pressure process. They are defined as P P v v T h Cand T u C       ∂ ∂ =      ∂ ∂ =
  • 29. 29  Using the definition of enthalpy (h = u + Pv) and writing the differential of enthalpy, the relationship between the specific heats for ideal gases is  The specific heat ratio, k is defined as v P C C k = P V P V h u Pv dh du RT C dt C dt RdT C C R = + = + = + = +
  • 30. 30  For ideal gases u, h, Cv, and Cp are functions of temperature alone. The Δu and Δh of ideal gases can be expressed as )( 1212 TTCuuu v −=−=∆ )( 1212 TTChhh P −=−=∆
  • 31. First Law of Thermodynamics 31  The First Law is usually referred to as the Law of Conservation of Energy, i.e. energy can neither be created nor destroyed, but rather transformed from one state to another.  For a closed system undergoing a cycle net heat interaction = net work transfer. Q W U Q W net net cycle net net − = = ∆
  • 32. 32 Energy Balance for Closed System Heat Work z Closed System Reference Plane, z = 0  V or E E Ein out system− = ∆
  • 33. 33  According to classical thermodynamics Q W Enet net system− = ∆  The total energy of the system, Esystem, is given as E Internal energy Kinetic energy Potential energy E U KE PE = + + = + +  The change in stored energy for the system is ∆ ∆ ∆ ∆E U KE PE= + +  The first law of thermodynamics for closed systems then can be written as Q W U KE PEnet net− = + +∆ ∆ ∆
  • 35. 35 No Value of n Process Description Result of IGL 1 ∞ isochoric constant volume (V1 = V2 ) 2 0 isobaric constant pressure (P1 = P2 ) 3 1 isothermal constant temperature (T1 = T2 ) 4 1<n< γ polytropic -none- 5 γ isentropic constant entropy (S1 = S2 ) According to a law of constant=n VP 2 2 1 1 T P T P = 2 2 1 1 T V T V = 2211 VPVP = 1 2 1 1 2 2 1 −       =      = n n n T T V V P P
  • 36. 36  Various forms of work are expressed as follows Process Boundary Work isochoric isobaric isothermal polytropic isentropic 0)( 1212 =−= VVPW )( 1212 VVPW −= 1 2 1112 ln V V VPW = n VPVP W − − = 1 1122 12
  • 37. 37  Second law is useful:  Used for predicting the direction of processes,  determining the best theoretical performance of cycles, engines and other devices. Second Law of Thermodynamics It is impossible to develop a cyclic device which converts low grade energy into high grade energy completely
  • 38. 38 Second Law of Thermodynamics Kelvin-Planck statement  No heat engine can have a thermal efficiency 100 percent.  It is impossible to create a cyclic device which produces work by exchanging heat with a single reservoir.
  • 39. 39 Thermal Efficiency  Represent the magnitude of the energy wasted in order to complete the cycle.  A measure of the performance that is called the thermal efficiency.  Can be expressed in terms of the desired output and the required input ηth = Desired Result Required Input  For a heat engine the desired result is the net work done and the input is the heat supplied to make the cycle operate.
  • 40. 40 The thermal efficiency is always less than 1 or less than 100 percent. ηth net out in W Q = , W W W Q Q net out out in in net , = − ≠ where
  • 41. 41  Applying the first law to the cyclic heat engine Q W U W Q W Q Q net in net out net out net in net out in out , , , , , − = = = − ∆  The cycle thermal efficiency may be written as ηth net out in in out in out in W Q Q Q Q Q Q = = − = − , 1
  • 42. 42  A thermodynamic temperature scale related to the heat transfers between a reversible device and the high and low- temperature reservoirs by Q Q T T L H L H =  The heat engine that operates on the reversible Carnot cycle is called the Carnot Heat Engine in which its efficiency is ηth rev L H T T , = −1
  • 43. 43 Heat Pumps and Refrigerators  A device that transfers heat from a low temperature medium to a high temperature one is the heat pump.  Refrigerator operates exactly like heat pump except that the desired output is the amount of heat removed out of the system  The index of performance of a heat pumps or refrigerators are expressed in terms of the coefficient of performance.
  • 44. 44
  • 45. 45 COP Q W Q Q Q HP H net in H H L = = −, COP Q W R L net in = ,
  • 46. 46 Carnot Cycle Process Description 1-2 Reversible isothermal heat addition at high temperature 2-3 Reversible adiabatic expansion from high temperature to low temperature 3-4 Reversible isothermal heat rejection at low temperature 4-1 Reversible adiabatic compression from low temperature to high temperature
  • 47. 47
  • 48. 48  The thermal efficiencies of actual and reversible heat engines operating between the same temperature limits compare as follows  The coefficients of performance of actual and reversible refrigerators operating between the same temperature limits compare as follows
  • 49. 49 Entropy  The 2nd law states that process occur in a certain direction, not in any direction.  It often leads to the definition of a new property called entropy, which is a quantitative measure of disorder for a system.  Entropy can also be explained as a measure of the unavailability of heat to perform work in a cycle.  This relates to the 2nd law since the 2nd law predicts that not all heat provided to a cycle can be transformed into an equal amount of work, some heat rejection must take place.
  • 50. 50 Entropy Change  The entropy change during a reversible process is defined as  For a reversible, adiabatic process dS S S = = 0 2 1  The reversible, adiabatic process is called an isentropic process.
  • 51. 51 Entropy Change and Isentropic Processes The entropy-change and isentropic relations for a process can be summarized as follows: i. Pure substances: Any process: Δs = s2 – s1 (kJ/kg⋅K) Isentropic process: s2 = s1 ii. Incompressible substances (liquids and solids): Any process: s2 – s1 = cav T2/T1 (kJ/kg Isentropic process: T2 = T1
  • 52. 52 iii. Ideal gases: a) constant specific heats (approximate treatment): s s C T T R v v v av2 1 2 1 2 1 − = +, ln ln 2 2 2 1 , 1 1 ln lnp av T P s s C R T P − = − for isentropic process 2 1 1 2. k s const P v P v=     =        for all process
  • 54. Difference between thermodynamics and heat transfer analysis:- In thermodynamics we deals with reversible path and find out the heat interaction during a process or from one initial point to the final point but in heat transfer analysis we focus on the rate of heat transfer , here we find out heat transfer per unit time whereas in thermodynamics we find the amount of heat transfer during a process and have nothing to do with the time required for that heat transfer to occur. Therefore in thermodynamics we calculate heat interaction in joules and in heat transfer it comes out to be in joules per second that is watt.
  • 55. HEAT TRANSFER Heat transfer is the flow of heat energy from a hot substance to a colder substance. MODE OF HEAT TRANSFER. Heat can be transferred by THREE methods:  By CONDUCTION  By CONVECTION  By RADIATION
  • 56. HEAT TRANSFER BY CONDUCTION Heat has traveled through the metal rod by a process known as CONDUCTION. It is a mode of heat transfer which generally occurs in solid due to temperature difference by molecular lattice vibrational energy transfer and also by free electron transfer. The reason behind all electrically good conductors are also in general good conductors of heat is due to the presence of free electons. E.g all metals Exception to above statement is Diamond which is non metal and having highest conductivity i.e 2300W/mk due to crystalline molecular lattice arrangement. Gases also conduct heat by molecular momentum transfer when high speed high temp molecule collide with lo speed low temp molecule but in general it is very bad conductor of heat.
  • 57. Thermal conductivity can be defined as: The number of heat units (Joules) flowing per unit of time (1 second) through a cross-sectional area (1 square metre) when the temperature falls by one degree (1 Kelvin or 1 degree Celsius) per unit length (1 metre) of its path. The units of thermal conductivity are, therefore w/mk
  • 58. units of thermal conductivity are: Watts per meter per o C or K (W m-1 K-1 )
  • 59. K VALUES The k values for most materials have been determined in the laboratory. The thermal conductivities of some materials used in industry are shown below. MATERIAL THERMAL CONDUCTIVITY k (Wm-1 K-1 )  Solids – metals Copper 390 Aluminum 202 Steel 55  Solids – nonmetals Magnesia 0.07 Cork 0.043 Glass 1.09 Asbestos 0.16 Brick (alumina) 3.1  Fluids Water 0.62 Benzene 0.16 Air 0.024
  • 60. CONDUCTION OF HEAT THROUGH SINGLE WALLS The rate of flow Φ (J S-1 ) from the hot side to the cold side will depend on the following factors:- The thermal conductivity k (W m-1 K-1 ) The area A (m2),  The difference in temperature T1 – T2 (K) (T1 – T2 is referred to as ΔT).  The thickness d (metres) Refractory wall, thermal conductivity, k, (W m-1 K-1 ) Thickness of wall d (m) cold side temperature T2 K hot side temperature T1 K area of wall over which heat flows A (m2 ) heat flow by conduction through wall (J/s)
  • 61. If Φ represents the heat flow in J S-1 (or watts), then Φ = k A (T1 – T2) / d Where: Φ rate of heat flow (in J S-1 ) k thermal conductivity (in W m-1 K-1 ) A area over which heat is passing (in m2 ) T1 hot face temperature (in K) T2 cold face temperature (in K) d thickness or distance between hot face and cold face (in m)
  • 62. This is the way in which heat energy travels through liquids and gases. (FLUIDS) CONVECTION It is the mode of heat transfer which generally occurs between solid surface and the surrounding fluid due to temperature difference associated with macroscopic bulk motion of fluid transporting thermal energy.
  • 63. CALCULATION OF THE HEAT TRANSFER RATE BY CONVECTION When the fluid outside the solid surface is in forced or natural convective motion, the expression of the rate of heat transfer from the solid to the fluid, or vice versa, is as follows: qc = hc A (Ts – Tf) where: qc =rate of heat transfer convection in J/s or W A =Area of heat transfer, m2 Ts = The temperature of the solid surface, K Tf =The average temperature of the fluid, K hc =The convection heat transfer coefficient, W/m2 .K
  • 64. The coefficient hc is a function of the system geometry, fluid properties, flow velocity, and temperature difference. In many cases, empirical correlation are available to predict this coefficient, since it often cannot be predicted theoretically.
  • 65. NATURAL AND FORCED CONVECTION Convection currents occur when fluids come into contact with hot surfaces. The convection currents produced give rise to a MIXING ACTION, which helps to transfer heat throughout the body of the fluid. The process of heat transfer by NATURAL CIRCULATION is called NATURAL CONVECTION. Movement of the fluid is caused only by the differences in density, The rate of heat transfer by convection can be improved by agitating the fluid as it is heated.
  • 66. The rate of heat transfer by convection will be greater if propeller is used to mix or agitate the fluids. Heat transfer produced in this way, i.e. by FORCING a fluid to move over a heating surface, is referred to as FORCED CONVECTION. Generally speaking, with forced convection there is: more rapid circulation. more turbulence and, more rapid heat transfer. Baffles fitted to the heating vessel will produce more turbulence, improved mixing and hence more rapid heat transfer by convection.
  • 67. NATURAL AND FORCED CONVECTOR HEATERS (EXAMPLE) These are similar in principle to the domestic heater.
  • 68. (a) shows cold air circulating over the tubes of a heat exchanger. Heated air then moves upwards and away from the heating surfaces. As this happens, cold air is drawn in near the base of the exchanger and, in its turn, becomes heated. (b) shows a fan forcing cold air over the tubes of the heat exchanger. This enables very much larger volumes of air to be heated than can be heated by natural circulation, i.e. natural convection, alone. Fan assisted convector heaters can be fitted with a number of smaller diameter tubes so increasing the area of the hot surface available for heat transfer for a given unit size. This, in turn, will increase the volume of cold air that can be heated in a given time. The extra force required to push the air through the bank of tubes is provided by the fan.
  • 69. RADIATION consists of invisible energy waves, which are able to pass across a space. Unlike heat transfer by conduction and convection, heat transfer by radiation does not require any material to be present between the hot part and the cold part of the system. Heat can travel by radiation across a vacuum. For example, heat energy from the sun travels across the empty space beyond the earth’s atmosphere. Scientists call these invisible heat waves ELECTROMAGNETIC WAVES. Electromagnetic waves travel across a space very rapidly, 3 × 108 m s-1 .
  • 70. CALCULATING HEAT TRANSFER BY RADIATION As you will recall, the factors which affect the rate of heat transfer by radiation from a body are: ◦ Its temperature T2 (K) ◦ Temperature of surroundings T1 (K) ◦ Its area A (m2 ) ◦ Its emissivity ε Scientists have combined these factors and produced a formula from which the rate of heat transfer by radiation, Φ (J s-1 or W), can be calculated. This is given below. Φ = 5.7 × 10-8 × ε × A × (T2 4 – T1 4 ) Where: ◦ Φ is the heat radiated from the hot surface (W) ◦ ε is the emissivity, from reference books. ◦ A is the surface area radiating heat (m2 ) ◦ T2 is the surface temperature (K) ◦ T1 is the surrounding temperature (K)