Heat Transfer Mechanisms

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Heat Transfer Mechanisms

• Dr. AA
• Department of Chemical Engineering
• University Teknology Malaysia

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Conduction

• Dr. AA
• Department of Chemical Engineering
• University Teknology Malaysia

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Heat Conduction

• Key Question
• How does heat pass through different materials?

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Heat Transfer

• The science of how heat flows is called heat
  transfer.
• There are three ways heat transfer works
  conduction, convection, and radiation.
• Heat flow depends on the temperature difference.

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Thermal Equilibrium

• Two bodies are in thermal equilibrium with each
  other when they have the same temperature.
• In nature, heat always flows from hot to cold
  until thermal equilibrium is reached.

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Heat Conduction

• Conduction is the transfer of heat through
  materials by the direct contact of matter.
• Dense metals like copper and aluminum are very
  good thermal conductors.

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Heat Conduction

• A thermal insulator is a material that conducts
  heat poorly.
• Heat flows very slowly through the plastic so
  that the temperature of your hand does not rise
  very much.

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Heat Conduction

• The ability to conduct heat often depends more on
  the structure of a material than on the material
  itself.
• Solid glass is a thermal conductor when it is
  formed into a beaker or cup.
• When glass is spun into fine fibers, the trapped
  air makes a thermal insulator.

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Thermal Conductivity

• The thermal conductivity of a material describes
  how well the material conducts heat.

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Thermal Conductivity

• Heat conduction in solids and liquids works by
  transferring energy through bonds between atoms
  or molecules.

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Heat Conduction Equation
Area of cross section (m2)
Thermal conductivity (watts/moC)
PH k A (T2 -T1) L
Heat flow (watts)
Temperature difference (oC)
Length (m)
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Variables for conduction
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Convection

• Dr. AA
• Department of Chemical Engineering
• University Teknology Malaysia

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Convection

• Key Question
• Can moving matter carry thermal energy?

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Convection

• Convection is the transfer of heat by the motion
  of liquids and gases.
• Convection in a gas occurs because gas expands
  when heated.
• Convection occurs because currents flow when hot
  gas rises and cool gas sink.
• Convection in liquids also occurs because of
  differences in density.

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Convection

• When the flow of gas or liquid comes from
  differences in density and temperature, it is
  called free convection.
• When the flow of gas or liquid is circulated by
  pumps or fans it is called forced convection.

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Convection

• Convection depends on speed.
• Motion increases heat transfer by convection in
  all fluids.

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Convection

• Convection depends on surface area.
• If the surface contacting the fluid is increased,
  the rate of heat transfer also increases.
• Almost all devices made for convection have fins
  for this purpose.

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Forced Convection

• Both free and forced convection help to heat
  houses and cool car engines.

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Heat Convection Equation
Area contacting fluids (m2)
Heat transfer coefficient (watts/m2oC)
PH h A (T2 -T1)
Heat flow (watts)
Temperature difference (oC)
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Radiation

• Dr. AA
• Department of Chemical Engineering
• University Teknology Malaysia

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Radiant Heat

• We do not see the thermal radiation because it
  occurs at infrared wavelengths invisible to the
  human eye.
• Objects glow different colors at different
  temperatures.

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26.3 Radiant Heat

• A rock at room temperature does not glow.
• The curve for 20C does not extend into visible
  wavelengths.
• As objects heat up they start to give off visible
  light, or glow.
• At 600C objects glow dull red, like the burner
  on an electric stove.

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Radiant Heat

• As the temperature rises, thermal radiation
  produces shorter-wavelength, higher energy light.
  
• At 1,000C the color is yellow-orange, turning to
  white at 1,500C.
• If you carefully watch a bulb on a dimmer switch,
  you see its color change as the filament gets
  hotter.
• The bright white light from a bulb is thermal
  radiation from an extremely hot filament, near
  2,600C.

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Radiant Heat

• The graph of power versus wavelength for a
  perfect blackbody is called the blackbody
  spectrum.

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Radiation striking a solid surface has one of
three fates
1. 2. 3. How are these properties related
?
Absorption absorptivity (a)
Transmission transmissivity (t)
Reflection reflectivity (z)
a t z 1
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Two special cases require definition
If all of the energy is either reflected or
absorbed (no transmitted radiation), we define
the body as If all of the energy striking a
surface is absorbed, we define the body
as For heat transfer calculations, we often
assume that the properties a, t, and r are
independent of wavelength. When this assumption
is made we say that we have gray surfaces.
Opaque a z 1
Black body a 1
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Let us return to the subject of radiation emitted
by a surface.
Total emissive power is defined as the total
amount of energy leaving the surface per unit
time per unit area W energy/area-time
Btu/hr-ft2 or W/m2 Note Emissive power is a
function of wavelength. The important
wavelengths for heat transfer are 0.5 -50 µm.
For temperatures above 1500F, the important
wavelength range is between 0.5 and 5 µm . In
our analysis, we will use the average values
over all wavelengths.
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Emissivity
The emissivity is the ratio of the emissive power
of a surface compared to the maximum emissive
power. How does the emissivity relate
to the absorptivity (a) at thermal
equilibrium? Although this strictly applies at
thermal equilibrium, we normally assume that it
applies at all temperatures.
e a
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Stefan-Boltzman Law
Finally, we must ask how the emissive power of a
black body is related to temperature. The answer
is provided by the Stefan Boltzman Law. W
sT4 where s 0.1714 x 10-8 Btu/hr-ft2-R4
(Stefan-Boltzman constant) 5.676 x 10-8
W/m2-K4 For an object that is not a black body
(i.e., not a perfect radiator), we can write the
following expression T is absolute
temperature
W esT4
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Heat Transfer Equation
To calculate the heat transfer rate by radiation,
we must include terms for energy output and
energy received from the surroundings. Energy
Energy output input Making the usual
assumption that e a, and multiplying by area
yields This is the expression for an
object totally enclosed by surroundings at T8.
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View factors
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View Factor
Previously, we found that for a body totally
enclosed by its surroundings, the net rate of
heat transfer by thermal radiation is given by
the following expression q esA(Ts4 -
T24) The equation for q given above is one of
the most important and commonly used results,
however, it does not cover all situations.
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View Factor
Suppose we have two surfaces at temperature T1
and T2, but both are finite in area, and neither
surface is completely enclosed by the other.
  An example might be the floor and ceiling of a
room. Only a fraction of the energy leaving the
ceiling strikes the floor and vice versa.
To account for this incomplete exchange of
energy, we define the view factor, F12   F12
fraction of energy leaving A1 reaching A2
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View Factor
The calculation of view factors is a
straightforward exercise in calculus as shown in
the figure on the preceding page. For each point
on the surface A1, we consider rays of thermal
energy emanating out equally in all directions.
The fraction of these rays (actually, the total
solid angle) which strikes A2 gives the fraction
of energy reaching that surface. Integrating
over all points on surface A1 and averaging gives
the view factor F12. The following relationship
is true A1F12 A2F21
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What is the energy transfer rate from 1 to 2 and
vice versa?
q1-gt2 q2-gt1
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Analytical Expression of View Factor
Case 1 Differential surface parallel to a finite
rectangular surface
where XL1/D and YL2/D
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Analytical Expression of View Factor
Case 2 Plane circular surface with common
central normal
where B b/a and C c/a
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Analytical Expression of View Factor
Case 3 Plane element A1 to sphere of radius r2
normal to centre of element passes through centre
of sphere