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FoE_ Heat Transfer.ppt
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Transcript of FoE_ Heat Transfer.ppt
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ENERGY/REE
Foundations of Energy
Heat Transfer
Tapas Mallick
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ENERGY/REE
Overview
Heat Transfer
Conduction
Heat transfer along a bar
Heat transfer across composite wall
Convection
Radiation
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ENERGY/REE
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Heat Transfer Mechanism:
Conduction
This is the intermolecular transfer of heat from
one part of a solid, liquid or gaseous body toanother part of the same body or from one
body to another body in physical contact,
without being displaced of the parts of the
body.
Hot ColdHeat Transfer from a hot solid body to cold
solid body, in physical contact.
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Heat Transfer Along a Bar
4
dx
dTkq
dx
dTkA
deltax
deltaTkAQ
Heat Transfer rateHeat flux
Fouriers Law of heatConduction
L
Ta Tb
Q
Thermal conductivity
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ENERGY/REE
Thermal Conductivity
5
11
2
KWm
m
Km
Watts
dX
dTA
Qk
dx
dTkAQ
Thermal Conductivity Unit: Wm-1K-1
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Thermal Conductivity
For any single material, k increases as vapour to
liquid to amorphous solid to solid Gases: thermal conductivity is very small, e.g. Air
0.0242 W/mK; Hydrogen: 0.167 W/mK
Liquid: thermal conductivity is small, e.g. Water:
0.569W/mK; Benzene: 0.159 W/mK
Solid: High thermal conductivity; e.g. Ice:
2.25W/mK; Steel 45.3W/mK; Copper 388 W/mK
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Heat Transfer Across Composite
Wall
series thermal Resistance
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Heat Transfer Rate
A B C
xA xB xC
TA TB TC
T
QT1 T2 T3 T4
433221
TTx
AkTT
x
AkTT
x
AkQ
C
C
B
B
A
A
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CBA
C
C
B
B
A
A
C
C
B
B
A
A
RRR
TT
Ak
x
Ak
x
Ak
x
TTQ
TT
x
AkTT
x
AkTT
x
AkQ
4141
433221
Where, R=x/(kA) is the thermal resistance
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Composite Wall:
Parallel Resistance
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A TA
B TB
C TC
x
Q
AA
AB
AC
AQ
BQ
CQ
Heat Transfer Rate
T1 T2
Ak
x
T
Ak
x
T
Ak
x
TQ
C
C
C
B
B
B
A
A
A
CBA RRRTQ
111
or
Where, R=x/(kA) is the thermal resistance
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Conduction
Summary
10
Through solid (mainly)
Q(dot) = - k A T / x = U A T= A T / R Two layers:
Resistance adds up
Two surfaces next to each other:
U-value and area add
Same as electrical network of resistors
(Ohms law)
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Example - 1
The wall of an industrial furnace is constructed from
0.15m thick fireclay brick having a thermal conductivity
of 1.7 W/m.K. Measurements made during steady-stateoperation reveal temperatures of 1400 and 1150 K at
the inner and outer surfaces, respectively. What is the
rate of heat loss through a wall that is 0.5 m by 3 m on a
side?
Answer: 4250 W
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Convection
This is the transfer of heat within fluids by
actual physical movement of the molecules Natural convection
The driving force is a difference of densities
Forced convection
The driving force is a mechanical mixing
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Cold
SolidHot Air
Hot air blowing over a cold solid will
heat up the solid after some time.
G /
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Convection
Heat is moved by fluid motion
Depends on geometries, temperatures, fluidproperties, and fluid velocities
Ignore all that and define a heat transfer
coefficient:
Q= hA T
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Radiation
It is the transfer of heat as radiant energy from a
hot body in all directions, even without amedium.
14
SUN Earth
Heat transfer from Sun to Earth takes place by
radiant heat transfer through space
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Radiation
Each object emits electromagnetic radiationaccording to its temperature
Blackbody radiation
Q= A (T14T2
4)
(T14T2
4)= (T12+ T2
2) (T12T2
2)
= (T12
+ T22
) (T1+ T2) T h= (T1
2+ T22) (T1+ T2)
Q= hA T
= Emissivity
Stephen Boltzmanns Constant
Here T1and T2are arbitrary
temperatures and T1>>T2
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Example - 2
An uninsulated steam pipe passes through a room in
which the air and walls are at 25C. The outside
diameter of the pipe is 70mm, and its surfacetemperature and emissivity are 200C and 0.8,
respectively. What are the surface emissive power and
irradiation? If the coefficient associated with free
convection heat transfer from the surface to the air is 15W/m2. K, what is the rate of heat loss from the surface
per unit length of pipe?
Answer: Emissive power = 2270 W/m2
Irradiation = 447 W/m2
.Rate of heat loss = 998 W/m
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Example - 3
The hot combustion gases of a furnace are separated
from the ambient air and its surroundings, which are at
25C, by a brick wall 0.15 m thick. The brick has athermal conductivity of 1.2 W/m . K and a surface
emissivity of 0.8. Under steady-state conditions an outer
surface temperature of 100C is measured. Free
convection heat transfer to the air adjoining the surface ischaracterised by a convection coefficient of h = 20
W/m2.K. What is the brick inner surface temperature?
Answer: 352C
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Types of heat transfer
Through a solid object:
Conduction
By a moving fluid:
Convection
Through light:
Radiation
dx
dT
x
T kAkAQ ~
ThATTAQ ~4241
ThAQ ~dcomplicatetoo