Chapter 1
Basics of Heat Transfer
Heat Transfer Mechanisms
144C The thermal conductivity of a material is the rate of heat transfer through a unit thickness of
the material per unit area and per unit temperature difference. The thermal conductivity of a
material is a measure of how fast heat will be conducted in that material.
145C The mechanisms of heat transfer are conduction, convection and radiation. Conduction is 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. Convection is the mode of energy transfer
between a solid surface and the adjacent liquid or gas which is in motion, and it involves combined
effects of conduction and fluid motion. Radiation is 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.
146C
In solids, conduction is due to the combination of the vibrations of the molecules in a lattice
and the energy transport by free electrons. In gases and liquids, it is due to the collisions of the
molecules during their random motion.
147C
The parameters that effect the rate of heat conduction through a windowless wall are the
geometry and surface area of wall, its thickness, the material of the wall, and the temperature
difference across the wall.
148C
Conduction is expressed by Fourier's law of conduction as
Q
kA
dT
dx
cond
= 
where
dT/dx
is
the temperature gradient,
k
is the thermal conductivity, and
A
is the area which is normal to the
direction of heat transfer.
Convection is expressed by Newton's law of cooling as
)
(
∞

=
T
T
hA
Q
s
s
conv
where h is the
convection heat transfer coefficient,
A
s
is the surface area through which convection heat transfer
takes place,
T
s
is the surface temperature and
T
∞
is the temperature of the fluid sufficiently far
from the surface.
Radiation is expressed by StefanBoltzman law as
)
(
4
4
surr
s
s
rad
T
T
A
Q

=
εσ
where
ε
is the
emissivity of surface,
A
s
is the surface area,
T
s
is the surface temperature,
T
surr
is average
surrounding surface temperature and
σ
=
×

567
10
8
.
W / m .K
2
4
is the StefanBoltzman constant.
149C
Convection involves fluid motion, conduction does not. In a solid we can have only
conduction.
150C
No. It is purely by radiation.
151C
In forced convection the fluid is forced to move by external means such as a fan, pump, or
the wind.
The fluid motion in natural convection is due to buoyancy effects only.
152C
Emissivity is the ratio of the radiation emitted by a surface to the radiation emitted by a
blackbody at the same temperature.
Absorptivity is the fraction of radiation incident on a surface
that is absorbed by the surface. The Kirchhoff's law of radiation states that the emissivity and the
absorptivity of a surface are equal at the same temperature and wavelength.
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 Spring '11
 yehiakhraishe
 Heat, Heat Transfer, Steady operating conditions, qconv

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