chapter 1 introduction to heat transfer
TRANSCRIPT
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CHAPTER 1
INTRODUCTION TO
HEAT TRANSFER
CHE 463 HEAT TRANSFER
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In this chapter we will learn
What is heat transfer?
How is heat transferred?
Why heat transfer is important?
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Definition:
Heat transfer is thermal energy transferthat is induced by a temperature
difference (or gradient)
Modes of Heat Transfer:
(i) Conduction Heat Transfer
- Occurs when a temperature gradient exists through
a solid or a stationary fluid (fluid/gas)
(ii) Convection Heat Transfer- Occurs within a moving fluid or between a solid surface
and a moving fluid when they are at different temperatures
(iii) Thermal radiation
- Heat transfer between two surfaces (that are not in
contact)
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Modes of Heat Transfer: Examples
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Example: Design of a container
A closed container filled with hot coffee is in a room
whose air and walls are at a fixed temperature.
Identify all heat transfer processes that contribute tocooling of the coffee. Comment on features that
would contribute to a superior container design.
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Transfer of energy from the more energetic to less
energetic particles of a substance by collisions between
atoms and/or molecules.
Atomic and molecular activity random molecular
motion (diffusion)
T1
x
x
o
T2
qx
T1>T2
T2
CONDUCTION
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Consider a brick wall, of thickness L=0.3 m which in a cold
winter day is exposed to a constant inside temperature,
T1=20C and a constant outside temperature, T2=-20C.
T1=20C
T2= -20C
L=0.3 mx
T
qx
Wall Area, A
Under steady-
state conditions
the temperature
varies linearly as a
function of x.
The rate of
conductive heat
transfer in the x-
direction depends
on
L
TTqx
21"
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The proportionality constant is a transport property,
known as thermal conductivity k (units W/m.K)
LTk
LTTkqx 21"
For the brick wall, k=0.72 W/m.K (assumed constant),therefore qx= 96 W/m
2
How would this value change if instead of the brickwall we had a piece of polyurethane insulating foam
of the same dimensions? (k=0.026 W/m.K)
qx is the heat flux(units W/m2 or (J/s)/m2), which is
the heat transfer rate in the x-direction per unit area
perpendicular to the direction of transfer.
The heat rate, qx (units W=J/s) through a plane wall
of area A is the product of the flux and the area: qx=
qx. A
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In the general case the rate of heat transfer in thex-directionis expressed in terms of the Fourier law:
dx
dTkqx
"
T1(high)
T2 (low)
x
qx
x1 x2
Minus sign because heat flows from high to low T
- For a linear profile
0)(
)(
12
12
xx
TT
dx
dT
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Energy transfer by sum of molecular diffusion (as in
conduction) and macroscopic (advection) movement.
Convection: transport by random motion ofmolecules and by bulk motion of fluid.
Advection: transport due solely to bulk fluid motion.
Types of convection:
Forced convection: Caused by external means such as
by fan, pump and atmospheric winds
Natural (free) convection: Flow induced by buoyancyforces which arises from density differences caused by
temperature variations in the fluid
Boiling and condensation: Latent heat exchange is
associated with phase changes
CONVECTION
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Air at 20C blows over a hot plate, which is maintained
at a temperature Ts=300C and has dimensions 20x40
cm.
CT 20
q
CTS300
Air
The convective heat flux is proportional to
TTq Sx
"
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The proportionality constant is the convection heat
transfer coefficient, h (W/m2.K)
)("
TThq SxNewtons law of Cooling
For air h=25 W/m2.K, therefore the heat flux is qx=
7,000 W/m2
How would this value change if instead of blowing
air we had still air (h=5 W/m2.K) or flowing water
(h=50 W/m2.K)
The heat rate, is qx= qx. A = qx. (0.2 x 0.4) = 560 W.
The heat transfer coefficient depends on surfacegeometry, nature of the fluid motion, as well as fluid
properties. For typical ranges of values, see Table 1.1
textbook.
In this solution we assumed that heat flux is positive
when heat is transferred from the surface to the fluid
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Thermal radiation is energy emitted by matter
Energy is transported by electromagnetic waves (or
photons). Can occur from solid surfaces, liquids and gases.
Does not require presence of a medium
Surface at Ts
Surroundings at Tsur
Eqemitted "Gqincident
"
Emissive power E is
the radiation emitted
by the surface
Irradiation G is the rate
of incident radiationper unit area of the
surface, originating
from its surroundings
Radiation
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For an ideal radiator, or blackbody:
4sbemitted TEq Stefan-Boltzmann law
where Ts is the absolute temperature of the surface
(K) and is the Stefan-Boltzmann constant,
( = 5.67x10-8 W/m2.K4)
For a real surface:
4"semitted TEq
is the emissivity
10
The irradiation G, originating from the surroundings is:
4"surincident TGq
is the absorptivity
For a grey surface, =
10 a
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The net radiation heat transfer from the
surface, per unit area is
)( 44" sursrad TTq
The net radiation heat exchange can be
also expressed in the form:
)( sursrrad TTAhq
where ))(( 22 surssursr TTTTh
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For a control surface:
T
x
T1
T2
T
qcondqrad
qconv 00
"""
radconvcond
outin
qqq
or
EE