MECHANISMS OF HEAT TRANSFER

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MECHANISMS OF HEAT TRANSFER
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• HEAT TRANSFER form of energy transfer due to
  temperature difference
• higher temperature lower temperature
• stops when two mediums reach the same temperature
• BASIC MECHANISMS OF HEAT TRANSFER
• CONDUCTION
• CONVECTION
• RADIATION

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CONDUCTION Transfer of energy from more
energetic particles of a substance to the
adjacent less energetic ones as a result of
interaction between the particles
Mechanism of heat conduction

• In gases and liquids due to collisions and
  diffusions of the molecules during their random
  motion
• In solids combination of vibrations of the
  molecules in a lattice and the energy transport
  by free electrons.

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RATE OF CONDUCTION

• Geometry of medium
• Thickness
• Material of the medium
• Temperature difference

The rate of heat conduction through a plane layer
is proportional to the temperature difference
across the layer and the heat transfer area, but
is inversely proportional to the thickness of the
layer
Rate of heat conduction
(area)(temperature difference)
Thickness
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k thermal conductivity of the material, measure
of the ability of a material to conduct heat
Fouriers Law of Heat Conduction
Temperature gradient (Negative when temperature
decreases with increasing x)
Heat is conducted in the direction of decreasing
temperature. Heat transfer area A is always
normal to the direction of heat transfer
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For copper
Note that the negative sign in Fouriers law
ensures that heat transfer in the positive x
direction is a positive quantity
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THERMAL CONDUCTIVITY
The rate of heat transfer through a unit
thickness of the material per unit area per unit
temperature difference Kinetic theory thermal
conductivity of gases is proportional to the
square root of the absolute temperature and
inversely proportional to the square root of the
molar mass. independent of pressure in a wide
range In liquids decreases with increasing
temperature and also with increasing molar
mass In solids depends upon lattice vibrational
waves and the free flow electrons
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The range of thermal conductivity of various
materials at room temperature

• ability to conduct heat
• different for each material
• At room temperature
• k 0.608 W/m 0 C for water
• k 80.2 W/m 0 C for iron

Which one conducts heat better ?
The ones having low thermal conductivities are
insulators
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The thermal conductivities of materials vary with
temperatures
The thermal conductivities of certain solids
exhibit dramatic increases at temperatures near
absolute zero, when these solids become super
conductors.
For example Copper T ( K) k (W/m. 0 C) 20 20
000 300 401
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THERMAL DIFFUSIVITY
Thermal diffusivity is a material property which
represents how fast heat diffuses through a
material. The ratio of heat conducted through a
material to the heat stored per unit volume
Thermal diffusivity Specific heat
High k or low CP large thermal diffusivity The
larger the thermal diffusivity, the faster the
propagation of heat into the medium. A small
value of thermal diffusivity means that heat is
mostly absorbed by the material and a small
amount of heat will be conducted further.
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CONVECTION
Heat transfer between a solid surface and the
adjacent fluid that is in motion and it involves
the combined effects of conduction and fluid
motion
Heat is first transferred from hot block to the
adjacent layer of air by conduction and then
carried away from the surface by convection
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If the fluid is forced to flow over the surface
by external means such as a fan, pump or the
wind, heat is transferred by forced convection
whereas if the fluid is caused by buoyancy forces
that are induced by density differences due to
the variation of temperature in fluid it is
called natural or free convection
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NEWTONS LAW OF COOLING
Rate of convection heat transfer
temperature difference
Convection heat transfer coefficient in W/m2.0
C Surface area through which convection heat
transfer takes place Surface temperature Temperatu
re of the fluid sufficiently far from the surface

• h is not a property of the fluid, experimentally
  determined, depends on
• surface geometry
• nature of fluid motion
• Properties of fluid
• Bulk fluid velocity

Note Fluid temperature at the surface equals the
surface temperature of the solid
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RADIATION

• 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
• All bodies at a temperature above absolute zero
  emit thermal radiation
• Does not require an intervening medium
• Fastest (at the speed of light)
• Possible also in vacuum
• Example energy of sun reaching the earth
• Thermal radiation form of radiation emitted by
  bodies because of their temperature
• different from other forms of electromagnetic
  radiation X-rays, gamma rays, microwaves, and
  television waves that are not related with
  temperature

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STEFAN-BOLTZMANN LAW
The maximum rate of radiation that can be emitted
from a surface at an absolute temperature is
Stefan-Boltzman constant 5.6710-8 W/m2.K4
Black body an idealized surface that emits
radiation at this maximum rate Black body
radiation radiation emitted by blackbodies Real
surfaces emit less radiation
For black bodies
For real bodies
Emissivity of the surface
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Radiation heat transfer between a surface and the
surfaces around it
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Combined heat transfer coefficient includes
effects of both convection and radiation in such
an example and conduction heat transfer may be
neglected.
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Although there are three mechanisms of heat
transfer, a medium may involve only two of them
simultaneously
Solids conduction and radiation Fluids
conduction and radiation (no motion) convection
and radiation (in motion) conduction and
convection (no radiation)
Heat transfer through a vacuum is by radiation
only
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HEAT TRANSFER BETWEEN TWO PLATES