This document discusses the three main modes of heat transfer: conduction, convection, and radiation. Conduction involves the transfer of heat through direct contact of particles. Convection refers to the transfer of heat by the movement of fluids such as gases and liquids. Radiation involves the emission of electromagnetic waves, usually between surfaces separated by a gas or vacuum. Specific examples are provided to illustrate key concepts for each mode of heat transfer, including how conduction occurs in metals, convection currents in fluids, and radiation between the sun and earth.

1. Dr.ibtihalO.Alkarim

2. Heat vs. Temperature
Heat is the actual energy…
Temperature is the measure of
average kinetic energy of particles in
a substance.
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3. HeatisNOTTemperature
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4. What is Heat transfer?
This is the movement of thermal
energy from a substance at a
higher temperature to another at
a lower temperature.
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5. Heat Transfer
Heat always moves from a warmer place to
a cooler place.
Hot objects in a cooler room will cool to
room temperature.
Cold objects in a warmer room will heat up
to room temperature.
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6. Heat transfer
heat transfer in three method
Heat
transfer
conduction
convection
radiation
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7. Dr.ibtihalO.Alkarim
Heat Transfer Modes
Conduction
• transfer of heat due to random molecular or atomic motion
within a material (aka diffusion)
• most important in solids
Convection
• transfer of heat between a solid surface and fluid due to
combined mechanisms of a) diffusion at surface; b) bulk fluid
flow within boundary layer
Radiation
• transfer of heat due to emission of electromagnetic waves,
usually between surfaces separated by a gas or vacuum

8. Conduction
Conduction heat transfer is the flowing of
heat energy from a high-temperature object to
a lower-temperature object.
Dr.ibtihalO.Alkarim

9. Conduction
When you heat a metal strip at one end, the heat
travels to the other end.
As you heat the metal, the particles vibrate, these
vibrations make the adjacent particles vibrate, and so on
and so on, the vibrations are passed along the metal and
so is the heat. We call this? Conduction
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10. Metalsaredifferent
The outer electrons of metal atoms
drift, and are free to move.
When the metal is heated, this
‘sea of electrons’ gain
kinetics energy and transfer it
throughout the metal.
Insulators, such as wood and plastic, do not have this ‘sea
of electrons’ which is why they do not conduct heat as well
as metals.
Dr.ibtihalO.Alkarim

11. Material Thermal
conductivity k
(W.m-1.K-1)
diamond 2450
Cu 385
Al 205
Brick 0.2
Glass 0.8
Body fat 0.2
Water 0.6
Wood 0.2
Styrofoa
m
0.01
Thermal conductivity, k
property of the material
kdiamond very high: perfect heat sink, e.g.
for high power laser diodes
khuman low: core temp relatively
constant (37 oC)
kair very low: good insulator
* home insulation
* woolen clothing
* windows double
glazing
Metals – good conductors:
electrons transfer energy from
hot to cold
Dr.ibtihalO.Alkarim

12. Whydoesmetalfeelcolderthanwood,iftheyareboth
atthesametemperature?
Metal is a conductor, wood is an insulator. Metal
conducts the heat away from your hands. Wood
does not conduct the heat away from your hands as
well as the metal, so the wood feels warmer than
the metal.
Dr.ibtihalO.Alkarim

13. For conduction between two plane surfaces (eg heat loss through
the wall of a house) the rate of heat transfer is
energy
transferred
through
slab
Q
TH
TC
L
H CQ T T
k A
t L
 


dQ dT
k A
dt dx
 
Thermal conductivity k (W.m-1.K-1)
steady-state
heat current H = dQ/dt
Q QA
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14. Dr.ibtihalO.Alkarim
Heat Transfer Modes - Conduction
Steady-state heat conduction through a plane wall:
L
TT
kq
L
TT
dx
dT
dx
dT
k
dx
dT
kq
x
x
2112
,
constantthenconstant,if
constant






L
T1 T2
x
q (T1>T2)
k

15. Dr.ibtihalO.Alkarim
Heat Transfer Modes - Conduction
Example: What thickness of plate glass would yield the same
heat flux as 3.5 of glass-fiber insulation with the same S-S
temperature difference (T1-T2) ?

16. Convection
Convection takes place when heated
molecules move from one place to another,
taking the heat with them. Convection is
common in both the atmosphere, as well as in
the oceans.
Convection is the primary way that heat moves
through gases and liquids.
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17. Convection
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18. Convection
What happens to the particles in a liquid or a
gas when you heat them?
The particles spread out and
become less dense.
This effects fluid movement.What is a fluid?A liquid or gas.
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19. Fluid movement
Cooler, more dense, fluids
sink through warmer, less
dense fluids.
In effect, warmer liquids and gases rise
up.
Cooler liquids and gases sink.
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20. Watermovement
Hot water
rises
Cooler
water sinks
Convection
current
Cools at the
surface
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21. Coldairsinks
Where is the
freezer
compartment
put in a fridge?
Freezer
compartment
It is put at the top,
because cool air
sinks, so it cools the
food on the way
down.
It is warmer at
the bottom, so
this warmer air
rises and a
convection
current is set up.
Dr.ibtihalO.Alkarim

22. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html
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23. Sea & Land Breezes, Monsoons
http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html
35 oC 20 oC 17 oC11 oC
What is the role of
heat capacity, c of water and soil?
Dr.ibtihalO.Alkarim

24. Dr.ibtihalO.Alkarim
Heat Transfer Modes - Convection
Rate equation (Newton, 1700) is known as Newton’s law of
“cooling”:
where q” = heat flux normal to surface
q = heat rate from or to surface As
Ts = surface temperature
T = freestream fluid temperature
As = surface area exposed to fluid
h = convection heat transfer coefficient
(W/m2-K)
)(or)(   TThAqTThq sss
Fluid flow, T
As
Ts (>T)
q

25. Dr.ibtihalO.Alkarim
Heat Transfer Modes - Convection
The convection heat transfer coefficient (h)
• is not a material property
• is a complicated function of the many parameters that
influence convection such as fluid velocity, fluid properties,
and surface geometry
• is often determined by experiment rather than theory
• will be given in most HW problems until we reach Chapter 6

26. Dr.ibtihalO.Alkarim
Heat Transfer Modes - Convection
Types of Convection
• Forced convection: flow caused by an external source such as a
fan, pump, or atmospheric wind
• Free (or natural) convection: flow induced by buoyancy forces
such as that from a heated plate
• Phase change convection: flow and latent heat exchange
associated with boiling or condensation

27. Radiation
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28. Thethirdmethodofheattransfer
How does heat energy get
from the Sun to the Earth? There are no particles between
the Sun and the Earth so it
CANNOT travel by conduction or
by convection.
?
RADIATION
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29. RADIATION
Energy transferred by electromagnetic waves
All materials radiate thermal energy in amounts
determined by their temperature, where the energy is
carried by photons of light in the infrared and visible
portions of the electromagnetic spectrum.
Thermal radiation wavelength ranges:
IR ~ 100 - 0. 8 m
Visible ~ 0.8 - 0.4 m 800 – 400 nm
UV ~0.4 - 0.1 m
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30. Ludwig Boltzmann
(1844-1906)
All objects above absolute zero emit radiant
energy and the rate of emission increases and
the peak wavelength decreases as the
temperature of object increases
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31. Power absorbed by surface of an object
A, a
Qabs
4abs
abs s
dQ
P Aa T
dt
 
Ts
Absorption & Stefan-Boltzmann Law
Incident radiation (INTENSITY I -
energy passing through a square metre
every second
Iinc = P / A Iabs = a Iinc
• Surface Area, A
• Absorption coefficient, a = 0 to 1
• Stefan-Boltzmann constant
σ = 5.67 x 10-8 W.m-2.K-4
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32. Power radiated from the surface of an object
A, e, T
Qrad
4rad
rad
dQ
P Ae T
dt
 
net rad absPP P 
Emission & Stefan-Boltzmann Law
Pnet > 0 net heat transfer out of system
• Surface Area, A
• Emissivity, e = 0 to 1
• Stefan-Boltzmann constant
σ = 5.67 x 10-8 W.m-2.K-4
Dr.ibtihalO.Alkarim

33. A blackbody absorbs all the radiation incident upon it and emits the max possible
radiation at all wavelengths
(e = a = 1)
A graybody is a surface that absorbs a certain proportion of the energy of a
blackbody, the constant being constant over the entire band of wavelengths
(0  e = a < 1)
emissivity e
absorption coefficient (absorptivity) a
Stefan-Boltzmann constant  = 5.6710-8 W.m-2.K-4
Dr.ibtihalO.Alkarim