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Chapter 1
Introduction And Basıc
Concepts
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Objectives
• Understand how thermodynamics and heat transfer are related to each other.
• Distinguish thermal energy from other forms of energy, and heat transfer from
other forms of energy transfer.
• Perform general energy balances as well as surface energy balances.
• Understand the basic mechanisms of heat transfer, which are conduction,
convection, and radiation, and Fourier's law of heat conduction, Newton's law of
cooling, and the Stefan–Boltzmann law of radiation.
• Identify the mechanisms of heat transfer that occur simultaneously in practice.
• Develop an awareness of the cost associated with heat losses.
• Solve various heat transfer problems encountered in practice.
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1-1 Thermodynamics And Heat Transfer
1
Heat: The form of energy that can be transferred from one system to
another as a result of temperature difference.
Thermodynamics is concerned with the amount of heat transfer as a
system undergoes a process from one equilibrium state to another.
Heat Transfer deals with the determination of the rates of such energy
transfers as well as variation of temperature.
The transfer of energy as heat is always from the higher-temperature
medium to the lower-temperature one.
Heat transfer stops when the two mediums reach the same temperature.
Heat can be transferred in three different modes:
conduction, convection, radiation
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1-1 Thermodynamics And Heat Transfer
Figure 1–1
We are normally interested in how long
it takes for the hot coffee in a thermos
bottle to cool to a certain temperature,
which cannot be determined from a
thermodynamic analysis alone.
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Figure 1–2
Heat flows in the direction
of decreasing temperature.
4
1-1 Thermodynamics And Heat Transfer
3
Application Areas of Heat Transfer
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Figure 1–3
Some application areas of heat transfer.
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1-1 Thermodynamics And Heat Transfer
Historical Background
4
Kinetic theory: Treats molecules as tiny balls
that are in motion and thus possess kinetic
energy.
Heat: The energy associated with the random
motion of atoms and molecules.
Figure 1–4
In the early 19th century, heat was
thought to be an invisible fluid
called caloric that flowed from
warmer bodies to cooler ones.
Caloric theory: Heat is a fluidlike substance
called the caloric that is a massless, colorless,
odorless, and tasteless substance that can be
poured from one body into another.
It was only in the middle of the nineteenth
century that we had a true physical
understanding of the nature of heat.
Careful experiments of the Englishman James
P. Joule published in 1843 convinced the
skeptics that heat was not a substance after all,
and thus put the caloric theory to rest.
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1-2 Engineering Heat Transfer
1
Heat transfer equipment such as heat exchangers, boilers, condensers, radiators, heaters,
furnaces, refrigerators, and solar collectors are designed primarily on the basis of heat
transfer analysis.
The heat transfer problems encountered in practice can be considered in two groups: (1)
rating and (2) sizing problems.
The rating problems deal with the determination of the heat transfer rate for an existing
system at a specified temperature difference.
The sizing problems deal with the determination of the size of a system in order to transfer
heat at a specified rate for a specified temperature difference.
An engineering device or process can be studied either experimentally (testing and taking
measurements) or analytically (by analysis or calculations).
The experimental approach has the advantage that we deal with the actual physical system,
and the desired quantity is determined by measurement, within the limits of experimental
error. However, this approach is expensive, timeconsuming, and often impractical.
The analytical approach (including the numerical approach) has the advantage that it is fast
and inexpensive, but the results obtained are subject to the accuracy of the assumptions,
approximations, and idealizations made in the analysis.
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1-2 Engineering Heat Transfer
2
Modeling in Engineering
Figure 1–6
Mathematical modelling of
physical problems.
Figure 1–7
Modeling is a powerful engineering
tool that provides great insight and
simplicity at the expense of some
accuracy.
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8
1-3 Heat And Other Forms Of Energy
1
Energy can exist in numerous forms such as:
• thermal
• mechanical
• kinetic
• potential
• electrical
• magnetic
• chemical
• nuclear
Their sum constitutes the total energy E (or e on a unit mass basis) of a system.
The sum of all microscopic forms of energy is called the internal energy of a
system.
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1-3 Heat And Other Forms Of Energy
2
• Internal energy: May be viewed as the sum of the kinetic and potential
energies of the molecules.
• Sensible heat: The kinetic energy of the molecules.
• Latent heat: The internal energy associated with the phase of a system.
• Chemical (bond) energy: The internal energy associated with the atomic
bonds in a molecule.
• Nuclear energy: The internal energy associated with the bonds within the
nucleus of the atom itself.
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1-3 Heat And Other Forms Of Energy
3
• In the analysis of systems that
involve fluid flow, we frequently
encounter the combination of
properties u and Pv.
• The combination is defined as
enthalpy (h = u + Pv).
• The term Pv represents the flow
energy of the fluid (also called
the flow work).
Figure 1–8
The internal energy u represents the
microscopic energy of a nonflowing
fluid, whereas enthalpy h represents the
microscopic energy of a flowing fluid.
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1-3 Heat And Other Forms Of Energy
4
Specific Heats of Gases,
Liquids, and Solids
Specific heat: The energy required to raise the
temperature of a unit mass of a substance by one
degree.
Two kinds of specific heats:
• specific heat at constant volume cv .
Figure 1–9
Specific heat is the energy required to raise
the temperature of a unit mass of a
substance by one degree in a specified way.
• specific heat at constant pressure cp.
The specific heats of a substance, in general,
depend on two independent properties such as
temperature and pressure.
At low pressures all real gases approach ideal gas
behavior, and therefore their specific heats depend
on temperature only.
Figure 1–10
The specific heat of a substance changes
with temperature.
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1-3 Heat And Other Forms Of Energy
5
1 kJ/kg. C  1 J/g. C  1 KJ/kg.K  1 J/g.k
du  cv dT
and
u  cv ,avg T and
u  mcv ,avg T and
dh  cpdT
h  cp ,avg T (J/g)
H  mcp ,avg T (J)
• Incompressible substance: A substance
whose specific volume (or density) does not
change with temperature or pressure.
• The constant-volume and constant-pressure
specific heats are identical for
incompressible substances.
• The specific heats of incompressible
substances depend on temperature only.
ΔU = mcavg ΔT
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(J)
Figure 1–11
The cv and cp values of
incompressible substances are
identical and are denoted by c.
13
1-3 Heat And Other Forms Of Energy
t
Q   Qdt
Energy Transfer
0
6
 J
When Q is constant:
Q  Qt  J
Energy can be transferred to or from a given mass by two
mechanisms:
heat transfer and work.
Heat transfer rate: The amount of heat transferred per unit time.
q
Q
A
 w/m 
2
Heat flux: The rate of heat transfer per unit area normal to the
direction of heat transfer.
Power: The work
done per unit time.
q
Figure 1–12
The sensible and latent forms of internal energy can
be transferred as a result of a temperature difference,
and they are referred to as heat or thermal energy.
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Q 24W

 4W / m2
2
A 6m
Figure 1–13
Heat flux is heat transfer per
unit time and per unit area, and
is equal to q  Q / A when Q
is uniform over the area A.
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1-4 The First Law Of Thermodynamics
1
The first law of thermodynamics (conservation of energy principle) states that
energy can neither be created nor destroyed during a process; it can only change
forms.
 Total energy   Total energy   Change in the 

 
 

 entering the    Leaving the    total energy of 
 system
  system
  the system


 
 

Ein  Eout

Net energy transfer by
heat, work,and mass
Ein  Eout
Net energy transfer by
heat, work,and mass
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
Esystem
J 
The energy balance for any system
undergoing any process in the rate form.
Change in internal,kinetic,
Potential, etc., energies
dEsystem / dt
The net change (increase or decrease)
in the total energy of the system during
a process is equal to the difference
between the total energy entering and
the total energy leaving the system
during that process.
(W)
Rate of Change in internal
kinetic,potential,etc.,energies
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1-4 The First Law Of Thermodynamics
steady, rate form :

Ein
Rate of net energy transfer in
by heat,work, and mass
2
Eout
Rate of net energy transfer out
by heat,work, and mass
In heat transfer problems it is convenient
to write a heat balance and to treat the
conversion of nuclear, chemical,
mechanical, and electrical energies into
thermal energy as heat generation.
Qin  Qout  Egen  Ethermal,system
Net hear tranmsfer
Figure 1–15
In steady operation, the rate of energy
transfer to a system is equal to the rate
of energy transfer from the system.
Heat
generation
J 
Change in thermal
energy of the system
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1-4 The First Law Of Thermodynamics
3
Energy Balance for Closed Systems (Fixed Mass)
A closed system consists of a fixed mass.
The total energy E for most systems
encountered in practice consists of the internal
energy U.
This is especially the case for stationary
systems since they don’t involve any changes
in their velocity or elevation during a process.
Starionary closed system:
Ein  Eout  U  mcv T
Figure 1–16
In the absence of any work
interactions, the change in the energy
content of a closed system is equal to
the net heat transfer.
J 
Stationary closed system, no work:
Q  mcv T
J 
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1-4 The First Law Of Thermodynamics
4
Energy Balance for Steady-Flow Systems
A large number of engineering devices such
as water heaters and car radiators involve
mass flow in and out of a system, and are
modeled as control volumes.
Most control volumes are analyzed under
steady operating conditions.
The term steady means no change with time
at a specified location.
Mass flow rate: The amount of mass flowing
through a cross section of a flow device per
unit time.
Volume flow rate: The volume of a fluid
flowing through a pipe or duct per unit time.
m  pVAc  kg / s 
m
v  VAc =
m3 / s 

p
Q  mh  mc p T
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 kJ/s 
Figure 1–17
The mass flow rate of a fluid at a cross section is
equal to the product of the fluid density, average
fluid velocity, and the cross-sectional area.
Figure 1–18
Under steady conditions, the net rate of energy
transfer to a fluid in a control volume is equal
to the rate of increase in the energy of the fluid
stream flowing through the control volume.
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1-4 The First Law Of Thermodynamics
5
Surface Energy Balance
A surface contains no volume or mass,
and thus no energy. Thereore, a surface
can be viewed as a fictitious system
whose energy content remains constant
during a process.
Surface energy balance :
Ein  Eout
This relation is valid for both steady and
transient conditions, and the surface energy
balance does not involve heat generation
since a surface does not have a volume.
Figure 1–19
Energy interactions at the outer wall
surface of a house.
Q1  Q1  Q3
When the directions of interactions are not known, all energy interactions can be assumed
Ein  0.
to be towards the surface, and the surface energy balance can be expressed as

Note that the interactions in opposite direction will end up having negative values, and
balance this equation.
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1-5 Heat Transfer Mechanisms
Heat as the form of energy that can be transferred from one system to another as
a result of temperature difference.
A thermodynamic analysis is concerned with the amount of heat transfer as a
system undergoes a process from one equilibrium state to another.
The science that deals with the determination of the rates of such energy
transfers is the heat transfer.
The transfer of energy as heat is always from the higher-temperature medium to
the lower-temperature one, and heat transfer stops when the two mediums reach
the same temperature.
Heat can be transferred in three basic modes:
• Conduction.
• Convection.
• Radiation.
All modes of heat transfer require the existence of a temperature difference.
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1-6 Conduction
1
Conduction: The transfer of energy from the more
energetic particles of a substance to the adjacent
less energetic ones as a result of interactions
between the particles.
In gases and liquids, conduction is due to the
collisions and diffusion of the molecules during
their random motion.
In solids, it is due to the combination of vibrations
of the molecules in a lattice and the energy
transport by free electrons.
The rate of heat conduction through a plane layer is
proportional to the temperature difference across
the layer and the heat transfer area, but is inversely
proportional to the thickness of the layer.
Rate of heat conduction 
Qcond  kA
 Area  Temperature difference 
T1  T2
T
  kA
x
x
Thickness
W
Figure 1–23
Heat conduction through a
large plane wall of thickness
x and area A.
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1-6 Conduction
2
When x  0
Qcond  kA
dT
dx
Fourier’s law of
heat conduction
Thermal conductivity, k: A measure of
the ability of a material to conduct heat.
Temperature gradient dT / dx :
The slope of the temperature curve on a T-x
diagram.
Heat is conducted in the direction of decreasing
temperature, and the temperature gradient
becomes negative when temperature decreases
with increasing x. The negative sign in the
equation ensures that heat transfer in the
positive x direction is a positive quantity.
Figure 1–24
The rate of heat conduction through
a solid is directly proportional to its
thermal conductivity.
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1-6 Conduction
3
Figure 1–26
In heat conduction analysis, A
represents the area normal to the
direction of heat transfer.
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1-6 Conduction
4
Thermal Conductivity
Thermal conductivity: The rate of heat
transfer through a unit thickness of the
material per unit area per unit temperature
difference.
The thermal conductivity of a material is a
measure of the ability of the material to
conduct heat.
A high value for thermal conductivity
indicates that the material is a good heat
conductor, and a low value indicates that the
material is a poor heat conductor or
insulator.
k
L
Q
A T1  T2 
Figure 1–28
A simple experimental setup to
determine the thermal conductivity of a
material.
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1-6 Conduction
Table 1-1
The thermal conductivities
of some materials at room
temperature
*Multiply by 0.5778 to
convert to Btc/h.ft.°F.
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5
Material
K,W/m.K*
Diamond
2300
Silver
429
Copper
401
Gold
317
Aluminum
237
Iron
80.2
Mercury (l)
8.54
Glass
0.78
Brick
0.72
Water (l)
0.607
Human skin
0.37
Wood (oak)
0.17
Helium (g)
0.152
Soft rubber
0.13
Glass fiber
0.043
Air (g)
0.026
Urethane, rigid foam
0.026
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1-6 Conduction
6
The range of
thermal
conductivity of
various materials
at room
temperature.
Figure 1–29
The range of thermal conductivity of
various materials at room temperature.
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1-6 Conduction
7
The thermal conductivities of gases such as air vary by a
factor of 104 from those of pure metals such as copper.
Pure crystals and metals have the highest thermal
conductivities, and gases and insulating materials the lowest.
Table 1-2
Figure 1–30
The mechanisms of heat
conduction in different
phases of a substance.
The thermal conductivity of an alloy is usually
much lower than the thermal conductivity of
either metal of which it is composed
Pure Metal or
K, W/m.k
Alloy
at 300 k
Copper
401
Nickel
91
Constantan
(55% cu, 45% Ni)
23
Copper
401
Aluminum
237
Commercial bronze
(90% Cu, 10% A1)
52
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1-6 Conduction
8
Table 1-3
Thermal conductivities of materials
vary with temperature
Figure 1–31
The variation of the thermal conductivity of
various solids, liquids, and gases with temperature.
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The variation of the thermal
conductivity of various solids,
liquids, and gases with
temperature.
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1-6 Conduction
9
Table 1-4: The thermal diffusivities of
some materials at room temperature
Thermal Diffusivity
Material
a, m 2 / s *
cp Specific heat, J/kg·°C: Heat capacity per unit mass
Silver
149 106
cp Heat capacity, J/m3 . C: Heat capacity per unit
Gold
127 106
volume
2
 Thermal diffusivity, m /s: Represents how fast
heat diffuses through a material
Copper
113 106
Aluminum
97.5 106
Iron
22.8 106
Heat conduction
k


Heat storage
pc p
m / s
4.7 106
Marble
1.2 106
Ice
1.2 106
Concrete
0.75 106
Brick
0.52 106
Heavy Soil (dry)
0.52 106
Glass
0.34 106
Glass Wool
0.23 106
Water (l)
0.14 106
Beef
0.14 106
Wood (oak)
0.13 106
2
A material that has a high thermal conductivity or a
low heat capacity will obviously have a large
thermal diffusivity.
The larger the thermal diffusivity, the faster the
propagation of heat into the medium.
A small value of thermal diffusivity means that heat
is mostly absorbed by the material and a small
amount of heat is conducted further.
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Mercury (l)
29
1-7 Convection
1
Convection: The mode of energy transfer
between a solid surface and the adjacent
liquid or gas that is in motion, and it
involves the combined effects of
conduction and fluid motion.
The faster the fluid motion, the greater the
convection heat transfer.
In the absence of any bulk fluid motion,
heat transfer between a solid surface and
the adjacent fluid is by pure conduction.
Figure 1–34
Heat transfer from a hot surface to air
by convection.
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1-7 Convection
2
Forced convection: If the fluid is
forced to flow over the surface by
external means such as a fan, pump,
or the wind.
Natural (or free) convection: If the
fluid motion is caused by buoyancy
forces that are induced by density
differences due to the variation of
temperature in the fluid.
Figure 1–35
The cooling of a boiled egg by
forced and natural convection.
Heat transfer processes that involve change of phase of a fluid are also considered to be
convection because of the fluid motion induced during the process, such as the rise of the
vapor bubbles during boiling or the fall of the liquid droplets during condensation.
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1-7 Convection
Qconv  hAs Ts  T 
h
As
Ts
T
W
3
Newton’s law of cooling
convection heat transfer coefficient, W/m 2 . C.
the surface area through which convection heat transfer takes place.
the surface temperature.
the temperature of the fluid sufficiently far from the surface.
The convection heat transfer coefficient h
is not a property of the fluid.
It is an experimentally determined
parameter whose value depends on all the
variables influencing convection such as,
Table 1-5: Typical values of convection
heat transfer coefficient
Type of Convection
h, W/m 2 .k *
Free convection of gases
2-25
• the surface geometry.
Free convection of liquids
10-1000
• the nature of fluid motion.
Forced convection of gases
25-250
• the properties of the fluid.
Forced convection of liquids
50-20,000
• the bulk fluid velocity.
Boiling and condensation
2500-100,000
*Multiply by 0.176 to convert to Btu/h.ft 2 . F.
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1-8 Radiation
1
• Radiation: The energy emitted by matter in the form of electromagnetic waves
(or photons) as a result of the changes in the electronic configurations of the
atoms or molecules.
• Unlike conduction and convection, the transfer of heat by radiation does not
require the presence of an intervening medium.
• In fact, heat transfer by radiation is fastest (at the speed of light) and it suffers
no attenuation in a vacuum. This is how the energy of the sun reaches the
earth.
• In heat transfer studies we are interested in thermal radiation, which is the
form of radiation emitted by bodies because of their temperature.
• All bodies at a temperature above absolute zero emit thermal radiation.
• Radiation is a volumetric phenomenon, and all solids, liquids, and gases emit,
absorb, or transmit radiation to varying degrees.
• However, radiation is usually considered to be a surface phenomenon for
solids.
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1-8 Radiation
2
Table 1-6: Emissivities of some materials at 300 k
Qemit,max   AsTs4
 W  Stefan–Boltzmann law
  5.670 108 W/m 2 .K 4 Stefan–Boltzmann constant
Blackbody: The idealized surface that emits
radiation at the maximum rate.
Qemit   AsTs4  W 
Radiation emitted by real surfaces
Emissivity  : A measure of how closely a
surface approximates a blackbody for which
 = 1 of the surface. 0   1.
Figure 1–38
Blackbody radiation represents the maximum
amount of radiation that can be emitted from a
surface at a specified temperature.
© McGraw-Hill Education
Material
Emissivity
Aluminum foil
0.07
Anodized aluminum
0.82
Polished copper
0.03
Polished gold
0.03
Polished silver
0.02
Polished stainless steel
0.17
Black paint
0.98
White paint
0.90
White paper
0.92−0.97
Asphalt pavement
0.85−0.93
Red brick
0.93−0.96
Human skin
0.95
Wood
0.82−0.92
Soil
0.93−0.96
Water
0.96
Vegetation
0.92−0.96
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1-8 Radiation
3
Absorptivity : The fraction of the radiation energy incident on a surface that is
absorbed by the surface. 0   1.
A blackbody absorbs the entire radiation incident on it ( = 1).
Kirchhoff’s law: The emissivity and the absorptivity of a surface at a given
temperature and wavelength are equal.
Qabsorbed   Qincident (W)
Figure 1–39
The absorption of radiation incident on an opaque surface of absorptivity .
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1-8 Radiation
4
Net radiation heat transfer: The
difference between the rates of
radiation emitted by the surface and the
radiation absorbed.
The determination of the net rate of
heat transfer by radiation between two
surfaces is a complicated matter since it
depends on,
When a surface is completely enclosed by a
much larger (or black) surface at
temperature Tsurr separated by a gas (such as
air) that does not intervene with radiation,
the net rate of radiation heat transfer
between these two surfaces is given by
4
Qrad   As Ts4  Tsurr
 W
• the properties of the surfaces.
• their orientation relative to each
other.
• the interaction of the medium
between the surfaces with radiation.
Radiation is usually significant relative
to conduction or natural convection, but
negligible relative to forced convection.
Figure 1–40
Radiation heat transfer between a surface
and the surfaces surrounding it.
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1-8 Radiation
5
When radiation and convection occur simultaneously
between a surface and a gas:
Qtotal  hcombined As Ts  T   W 
Combined heat transfer coefficient hcombined
Includes the effects of both convection and radiation
4
Qtotal  Qconv  Qrad  hconv As Ts  Tsurr    As Ts4  Tsurr

Qtotal  hcombined As Ts  T   W 
2
hcombined  hconv  hrad  hconv   Ts  Tsurr  Ts2  Tsurr

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1-9 Simultaneous Heat Transfer
Mechanisms
Heat transfer is only by conduction in opaque solids, but by
conduction and radiation in semitransparent solids.
A solid may involve conduction and radiation but not
convection. A solid may involve convection and/or
radiation on its surfaces exposed to a fluid or other
surfaces.
Heat transfer is by conduction and possibly by radiation in
a still fluid (no bulk fluid motion) and by convection and
radiation in a flowing fluid.
In the absence of radiation, heat transfer through a fluid is
either by conduction or convection, depending on the
presence of any bulk fluid motion.
Convection = Conduction + Fluid motion
Heat transfer through a vacuum is by radiation.
Most gases between two solid surfaces do not interfere with
radiation.
Liquids are usually strong absorbers of radiation.
Figure 1–42
Although there are three mechanisms
of heat transfer, a medium may
involve only two of them
simultaneously.
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38
1-10 Aerogel—a Remarkable
Superınsulatıng Materıal
1
Aerogels—often called “frozen smoke”—are lightweight
and have an ethereal blue haze.
An aerogel in your hand feels like a piece of air—but it is a
solid.
What makes them so interesting is their unique nanoporous
structure.
This structure is formed through the sol-gel process,
whereby a sol (a particulate-containing solution) undergoes
a polymerization reaction to form a nanoporous wet gel.
The key to making an aerogel is to carefully dry the wet
gel.
Supercritical extraction is commonly used to make
aerogels.
Aerogels can be made from many different precursor
materials.
The most common type of aerogel is the silica aerogel.
© McGraw-Hill Education
Figure 1–48
A silica aerogel insulates a
flower from the heat of a
Bunsen burner.
39
1-10 Aerogel—a Remarkable
Superınsulatıng Materıal
2
When an aerogel is made by the sol-gel
method, it forms a nanoporous structure
comprised of clusters of silica spheres on the
order of 5–10 nm that connect and form long,
chain-like connecting structures with pores
that range from 5 to 100 nm in size.
Silica aerogels are very porous: 90–99
percent air
Have low density:
0.05  0.4 g/cm3
Large surface areas:
Figure 1–49
Schematic representation of
an aerogel nanostructure.
500  1400 m 2 /g
Highly translucent and have low thermal
conductivity:
0.01  0.3 w/m.k
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40
1-10 Aerogel—a Remarkable
Superınsulatıng Materıal
3
Convection through an aerogel is hindered by the small pore size.
At low temperatures radiation is not significant.
The primary method for heat transfer through an aerogel is by conduction in both the solid
and gaseous phases.
Solid phase conduction is related to the thermal conductivity of the solid and the amount of
that solid.
Since aerogels are so porous, there is very little solid material, and what is there follows a
tortuous path with many dead ends (formed by the original sol-gel process).
For these reasons, the thermal conductivity of an aerogel is very low.
Aerogels are commercially available in an impregnated blanket form, as granules, or as
monolithic pieces.
They are used in a variety of applications, including comet dust collectors, oil spill cleanup material, and chemical sensors.
The most common application for aerogels is as insulation where, for example, they are
used in daylighting products (insulating windows), high-performance clothing, and pipe
insulation.
© McGraw-Hill Education
41
1-10 Aerogel—a Remarkable
Superınsulatıng Materıal
4
Heat loss through a 6-mm -thick single-pane glass window
30  10  C = 133.3W
ΔT 
W 
Q = kA
= 1.0
0.25m
×
0.16m

 
Δx  m.K 
0.006 m

o
If we could make the window pane out of a monolithic aerogel under slight
vacuum, the heat loss would be:
30  10  C = 1.3W
ΔT 
W 
Q = kA
=  0.01
0.25m
×
0.16m



Δx 
m.K 
0.006 m

o
© McGraw-Hill Education
42
1-11 Prevention Through Design
1
The first of the fundamental canons of ethics for engineers is to “hold paramount the
safety, health, and welfare of the public” when fulfilling their professional duties.
In 2007, the National Institute for Occupational Safety and Health (NIOSH)
launched the National Prevention through Design (PtD) initiative, with the mission
to prevent or reduce work-related injuries, illnesses, and fatalities by including
prevention considerations in all circumstances that impact individuals in the work
places.
As such, the concept of PtD involves applying the means of reducing risks and
preventing hazards in the design of equipment, tools, processes, and work facilities.
The concepts of PtD can also be rationally applied to preventing failures and
damages of devices, products, and systems. Since such failures and damages are
often led to negative impacts on the environment, profitability, and ultimately the
society at large.
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43
1-11 Prevention Through Design
2
Within the context of heat and mass transfer, the
PtD concepts can be presented along with the
physical mechanisms involved and practical
applications.
Issues such as prevention of thermal burn, fire
hazard, and thermal failure in systems are topics
that can relate the concepts of PtD with the basic
science of heat and mass transfer.
The process of solving heat and mass transfer
problems, along with the application of PtD
concepts, involves incorporating prescribed PtD
criteria, the prevention of burn injury, fire hazard,
or system failure, to the solutions.
To successfully arrive at a solution that satisfies
prescribed PtD criteria requires the understanding
of how the physical mechanisms of heat and mass
transfer interrelate with the concepts of PtD.
Figure 1–50
Process of solving problems with
application of PtD concepts.
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© McGraw-Hill Education
44
1-12 Engineerıng Codes And Standards
1
Engineering codes and standards are ubiquitous.
When we consider any given device that we use daily, the path, from the design to the
prototype and finally to the production of the final product is guided by standards and
regulated by codes.
A product that has been manufactured to meet certain standards tells the user about the
product’s quality and that it has met certain levels of scrutiny set forth by those standards.
In essence, engineering standards communicate to the user that the product is expected to
function safely and meets quality objectives.
At the industrial level, engineering codes and standards provide efficiency in engineering
processes (from design to manufacturing), maintain safety and uniformity of products that
allows interchangeability, and improve commerce.
Engineering codes and standards are evolutionary—they are continually revised to reflect
technical advances, such as new technologies, new designs, new procedures, and new
materials.
© McGraw-Hill Education
45
1-12 Engineerıng Codes And Standards
2
The development of engineering codes and standards is rooted in the engineers’ code of
ethics to “hold paramount the safety, health, and welfare of the public” in their professions
(Code of Ethics for Engineers, 2007).
The founding of the American Society of Mechanical Engineers (ASME) in 1880 provided
a forum for engineers, designers, and machine shop managers to discuss and collaborate to
solve issues brought on by the industrial age.
Within four years, the first ASME standard, “Code for the Conduct of Trials of Steam
Boilers,” was issued.
This was the first ASME Performance Test Code (PTC). Today, there are close to 50 ASME
PTCs, and they cover components, systems, and facilities.
Performance test codes are used by manufacturers to ensure the reliability of their products,
and users can use the PTCs to protect themselves from products that function poorly.
© McGraw-Hill Education
46
1-12 Engineerıng Codes And Standards
3
Some of the very first engineering codes and standards were needed because of
engineering failures that caused the loss of life and property.
In 1905, a boiler exploded in a shoe factory in Brockton, Massachusetts. In the next year,
a similar incident happened in another shoe factory in Lynn, Massachusetts.
Within a decade, the first edition of the Boiler and Pressure Vessel Code (BPVC) was
issued by the ASME in 1914 and published in 1915.
Today, the ASME BPVC has been adopted by the United States and Canada, and it is
being used in over 100 countries around the world.
A recent study done by ASME Vision 2030 Project reveals that almost 50 percent of
engineers in their early career are unfamiliar with engineering codes and standards.
As companies and manufacturers expand operations globally, there is a greater need to
harmonize codes and standards across jurisdictions and disciplines.
The need for engineers to know about codes and standards is growing, and having this
knowledge allows engineers to innovate and manufacture competitive products.
© McGraw-Hill Education
47
1-12 Engineerıng Codes And Standards
4
In the subject of heat and mass transfer, there are several engineering codes and standards
that are relevant.
These codes and standards have been issued and published by professional associations,
such as the ASME and the ASHRAE, and standards organizations, such as ASTM
International, the ANSI, and the ISO.
For instance, the 2015 ASME Boiler & Pressure Vessel Code, Section IV (ASME BPVC
.IV-2015) provided the permitted O-ring materials and their temperature limits for water
tube attachments in hot water boilers.
Another example is the standard specification issued by ASTM International for
polyethylene insulation used on electrical cable (ASTM D1351-14, 2014), which is
important as overheating can rapidly deteriorate the insulation.
Extreme temperatures and thermal cycles can cause materials to deteriorate and thereby
affect the functionality of components.
The knowledge of heat and mass transfer, along with adherence to the relevant codes and
standards, allow engineers to analyze, design, and build components and systems to
function within the design conditions.
© McGraw-Hill Education
48
1-13 Problem-solving Technique
1
Step 1: Problem Statement
Step 2: Schematic
Step 3: Assumptions and Approximations
Step 4: Physical Laws
Step 5: Properties
Step 6: Calculations
Step 7: Reasoning, Verification, and Discussion
© McGraw-Hill Education
49
1-13 Problem-solving Technique
Figure 1–53
A step-by-step approach can
greatly simplify problem solving.
2
Figure 1–54
The assumptions made while solving
an engineering problem must be
reasonable and justifiable.
Access the text alternative for slide images.
© McGraw-Hill Education
50
1-13 Problem-solving Technique
Figure 1–55
The results obtained from an
engineering analysis must be checked
for reasonableness.
© McGraw-Hill Education
3
Figure 1–56
Neatness and organization are highly
valued by employers.
51
1-14 Engineering Software Packages
1
All the computing power and the
engineering software packages available
today are just tools, and tools have
meaning only in the hands of masters.
Hand calculators did not eliminate the
need to teach our children how to add or
subtract, and sophisticated medical
software packages did not take the place
of medical school training.
Neither will engineering software
packages replace the traditional
engineering education. They will simply
cause a shift in emphasis in the courses
from mathematics to physics.
© McGraw-Hill Education
Figure 1–57
An excellent word-processing program
does not make a person a good writer;
it simply makes a good writer a better
and more efficient writer.
52
1-14 Engineering Software Packages
2
Equation Solvers
Despite its simplicity, Excel is commonly used in solving systems of equations in
engineering as well as finance. It enables the user to conduct parametric studies,
plot the results, and ask “what if ” questions. It can also solve simultaneous
equations if properly set up.
Engineering Equation Solver (EES) is a program that solves systems of linear or
nonlinear algebraic or differential equations numerically.
It has a large library of built-in thermodynamic property functions as well as
mathematical functions.
Unlike some software packages, equation solvers do not solve engineering
problems; they only solve the equations supplied by the user.
Therefore, the user must understand the problem and formulate it by applying
any relevant physical laws and relations.
© McGraw-Hill Education
53
1-15 Accuracy, Precision, And Significant
Digits
1
Accuracy error (inaccuracy): The value of
one reading minus the true value. In general,
accuracy of a set of measurements refers to
the closeness of the average reading to the
true value. Accuracy is generally associated
with repeatable, fixed errors.
Precision error: The value of one reading
minus the average of readings. In general,
precision of a set of measurements refers to
the fineness of the resolution and the
repeatability of the instrument. Precision is
generally associated with unrepeatable,
random errors.
Significant digits: Digits that are relevant
and meaningful.
Figure 1–60
Illustration of accuracy versus
precision. Shooter A is more precise,
but less accurate, while shooter B is
more accurate, but less precise.
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© McGraw-Hill Education
54
1-15 Accuracy, Precision, And Significant
Digits
2
Number
Table 1-7:
Significant
digits
Exponential
Notation
Number of
Significant Digits
12.3
1.23×101
3
123, 000
1.23×105
3
0.00123
1.23×10-3
3
40, 300
4.03×104
3
4.0300×104
5
40,300
5.600×10-3
4
0.0056
5.6×10-3
2
0.006
6.×10-3
1
0.005600
In engineering calculations, the information given is not
known to more than a certain number of significant
digits, usually three digits.
Consequently, the results obtained cannot possibly be
accurate to more significant digits.
Reporting results in more significant digits implies
greater accuracy than exists, and it should be avoided.
Figure 1–61
A result with more significant
digits than that of given data
falsely implies more accuracy.
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© McGraw-Hill Education
55
Summary
1
Thermodynamics and Heat Transfer.
• Application areas of heat transfer.
• Historical background.
Engineering Heat Transfer.
• Modeling in engineering.
Heat and Other Forms of Energy.
• Specific heats of gases, liquids, and solids.
• Energy transfer.
The First Law of Thermodynamics.
• Energy balance for closed systems (Fixed Mass).
• Energy balance for steady-flow systems.
• Surface energy balance.
Heat Transfer Mechanisms.
© McGraw-Hill Education
56
Summary
2
Conduction:
• Fourier’s law of heat conduction.
• Thermal Conductivity.
• Thermal Diffusivity.
Convection:
• Newton’s law of cooling.
Radiation:
• Stefan–Boltzmann law.
© McGraw-Hill Education
57
Summary
3
Simultaneous Heat Transfer Mechanisms.
Aerogel – A Remarkable Superinsulating Material.
Prevention through Design.
Engineering Codes and Standards.
Problem Solving Technique.
Engineering Software Packages.
Accuracy, Precision and Significant Digits.
© McGraw-Hill Education
58
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