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Chapter 1 (cont)
Heat Transfer
Nur Aklis
Mechanical EngineeringMuhammadiyah ni!ersity of "urakarta
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#efinition
$ %hen t&o 'odies are at different temperatures
thermal energy transfers from the one &ith
higher temperature to the one &ith lo&er
temperature Heat al&ays transfers from hot tocold
$ Heat transfer is a study of the e*change of
thermal energy through a 'ody or 'et&een
'odies &hich occurs &hen there is a
temperature difference
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Mechanisms of heat transfer
$ There are three mechanisms of heat
transfer
+ Conduction
+ Con!ection
+ ,adiation
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Conduction
$ Conduction is at transfer through solids orstationery
fluids
$ -ne of e*ample of conduction is &hen you touch
a hot o'.ect the heat you feel is transferred
through your skin 'y conduction
$ Mechanism of heat conduction
+ /n solids0 lattice !i'ration and particle collision
+ /n fluids 0 molecular collisions
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Figure Conduction 'y lattice !i'ration
FigureConduction 'y particle collision
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$ Lattice vibration /n solids atoms are 'ound analogousto
springs as sho&n in igure %hen there is atemperature difference
in the solid the hot side of the
solid e*periences more !igorous atomic mo!ementsThe !i'rations
are transmitted through the springs to thecooler side of the
solid
$ Particle collisionmetals ha!e free electrons &hich arenot
'ound to any particular atom and can freely mo!e
a'out the solid The electrons in the hot side of the solidmo!e
faster than those on the cooler side This scenariois sho&n in
Figure As the electrons undergo a series ofcollisions the faster
electrons gi!e off some of theirenergy to the slo&er
electrons
$ Conduction through electron collision is more effecti!ethan
through lattice !i'ration2 this is &hy metalsgenerally are
'etter heat conductors than ceramicmaterials &hich do not ha!e
many free electrons
$ Mechanism of heat conduction in li3uid like
paticlecollision
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,ate of Heat Conduction
$ The rate of heat conduction is depend on
+ 4eometry of medium
+ Thickness
+ Material of medium
+ Temperature difference
$ The rate of conduction (3) through a plane layer
isproportional to the temperature difference across thelayer and
the heat transfer area 'ut is in!erselyproportional to the
thickness of the layer
$ ourier proposed the la& of materials as2356k7T
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A A
dx
T1
T2
q
8 *
dx
dTkAq =
%here
q 5 rate of heat conduction (&att)
dT9d* 5 gradient of temperature
k5thermal conducti!ity of material (&att9moC)
A5area (m:)
The negati!e sign represents that heat flo&
/n the direction opposite to the gradient temperature
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Con!ection
$ Con!ection uses the motion of fluids totransfer heat
$ /t is occur 'et&een a solid surface and
ad.acent fluid
$ Heat is first transferred from hot 'lock to
the ad.acent layer of air 'y conduction and
then carried a&ay from the surfacecon!ection
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Hot block
qcond
Air fow
Velocity o air Temperature
A Ts
T;
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There are t&o kinds of con!ection
$ Natural con!ection is caused 'y 'uoyancy
forces that are induced 'y density differences
due the !ariation of temperature in fluid
$ orce con!ection is &hen the fluid flo& o!er thesurface
'y e*ternal such as fan pump or &ind
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$ Con!ection head transfer depends on
+ #ynamic !iscosity
+ Thermal conducti!ity+ #ensity
+ "pecific heat
+ luid !elocity
+ 4eometry+ ,oughness
+ Type of fluid flo&
)( = TThAQ scon
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,adiation
$ All o'.ect emit electromagnetic radiation and this
radiation carries energy ,adiation heat transfer occurs&hen
the emitted radiation strikes another 'ody and is
a'sor'ed
$ The po&er radiated from surface A at temperature T is
gi!en 'y the stefan6
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$ /f o'.ect is at temperature T and its
surroundings are at temperature To the
net rate of energy loss is
$
)( 44 ToTAP =
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E*ample
$ A &indo&s is 1: m high and 8> m &ide The
glass is ? mm thick and has thermal conducti!ity
8@ %9moC The temperature of the inner face is
1: oC and that of the outer surface is 61: oC%hat is the rate of
flo& through the &indo&
$ T&o sla' of thickness B1 and B: and area A are
in thermal contact &ith their surface
temperatures T1 and T: respecti!ely %hat is thetemperature at
the interface 'et&een t&o sla'
%hat is the rate of heat flo&
