Basic Modes of Heat Transfer
Md. Al Amin Hossain
Lecturer (ME), Textile, GUB
1
Heat Transfer
• The basic requirement for heat transfer is the presence
of a temperature difference.
• The second law requires that heat
will be transferred in the direction
of decreasing temperature.
• The temperature difference is the driving force for heat
transfer.
• The rate of heat transfer in a certain direction depends
on the magnitude of the temperature gradient in that
direction.
• The larger the temperature gradient, the higher the rate
of heat transfer.
2
Application Areas of Heat Transfer
3
Heat Transfer Mechanisms
• Heat can be transferred in three basic modes:
– conduction,
– convection,
– radiation.
• All modes of heat
transfer require the
existence of a temperature difference.
• All modes are from the high-temperature
medium to a lower-temperature one.
4
Conduction
• 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.
• Conduction can take place in solids,
liquids, or gases
– In gases and liquids conduction is due to
the collisions and diffusion of the
molecules during their random motion.
– In solids conduction is due to the
combination of vibrations of the
molecules in a lattice and the energy
transport by free electrons.
5
Conduction
where the constant of proportionality k is the
thermal conductivity of the material.
In differential form
which is called Fourier’s law of heat conduction.
6
Thermal Conductivity
• The thermal conductivity of a material is a
measure of the ability of the material to conduct
heat.
• High value for thermal conductivity
good heat conductor
• Low value
poor heat conductor or insulator.
7
Thermal diffusivity
(1-23)
• The thermal diffusivity represents how fast heat
diffuses through a material.
• Appears in the transient heat conduction analysis.
• A material that has a high thermal conductivity or a
low heat capacity will have a large thermal diffusivity.
• The larger the thermal diffusivity, the faster the
propagation of heat into the medium.
8
Convection
Convection = Conduction + Advection
(fluid motion)
• Convection is the mode of energy transfer between a
solid surface and the adjacent liquid or gas that is in
motion.
• Convection is commonly classified into three
sub-modes:
– Forced convection,
– Natural (or free) convection,
– Change of phase (liquid/vapor,
solid/liquid, etc.)
9
Convection
• The rate of convection heat transfer is expressed by
Newton’s law of cooling as
• h is the convection heat transfer coefficient in W/m2°
C.
• h depends on variables such as the
surface geometry, the nature of fluid
motion, the properties of the fluid,
and the bulk fluid velocity.
10
Radiation
• Radiation is the 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.
• Heat transfer by radiation does not require the presence of
an intervening medium.
• In heat transfer studies we are interested in thermal
radiation (radiation emitted by bodies because of their
temperature).
• Radiation is a volumetric phenomenon. However, radiation
is usually considered to be a surface phenomenon for
solids that are opaque to thermal radiation.
11
Radiation - Emission
• The maximum rate of radiation that can be emitted from a
surface at a thermodynamic temperature Ts (in K or R) is given
by the Stefan–Boltzmann law as
• σ =5.670X10-8 W/m2·K4 is the Stefan–Boltzmann constant.
• The idealized surface that emits radiation at this maximum rate
is called a blackbody.
• The radiation emitted by all real surfaces is less than the
radiation emitted by a blackbody at the same temperature, and
is expressed as
• ε is the emissivity of the surface.
12
Radiation - Absorption
• The fraction of the
radiation energy incident
on a surface that is
absorbed by the surface is
termed the absorptivity α.
• Both ε and α of a surface depend on the temperature
and the wavelength of the radiation.
13
