Heat Transfer
- Heat Transfer Definition
- Rate of Heat Transfer
- Types of Heat transfer
- Stefan Boltzmann Law
- Heat Transfer Coefficient
- Heat Transfer Examples
- Thermal Conductivity of Air
Heat Transfer ::: The
particles of matter have the capability to be vibrated, translate, or rotate
which is because of the kinetic energy of particles. The measurement of the
kinetic energy of the particles is done by temperature. The kinetic energy of rotation,
translation or vibration increases with increasing temperature. The flow of
energy takes place from a higher to a lower temperature. The temperature
difference between the higher and lower temperature objects is the reason for
transfer of heat. This process is continued to get the thermal equilibrium
condition.
A simple example of heat transfer is cooling of a hot tea
cup; similar warming of a cold can. The transfer of heat is from the hot to the
cold water till both samples get the same temperature. Let’s discuss about the
process of transfer of heat and its three types of mechanism, some examples and
law based on
Heat Transfer Definition
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Heat is the form of energy. We also know that there is a
transfer of heat only if there is a temperature difference.
Heat Transfer is defined as:
The transmission of energy from one region to another region
as a result of the temperature difference between them.
or
The Heat transfer taking place between the bodies of
different temperature.
Rate of Heat Transfer
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The rate at which heat is transferred or conducted through a
substance is directly proportional to the
1.Area of the surface (A) perpendicular to the flow of heat
and
2.The temperature gradient $\frac{\Delta T}{x}$ along the
path of heat transfer.
For a one dimensional steady state heat transfer Rate of
Heat Transfer is expressed by Fourier equation:
Q = kA $\frac{\Delta T}{\Delta x}$.
where,
K = Thermal Conductivity depends on the material
A = Area of the surface
$\frac{\Delta
T}{\Delta x}$ = Temperature gradient for small change in temperature with
respect to distance.
Use below widget to calculate the rate of heat transfer
according to the formula.
Types of Heat transfer
There are three modes of Heat transfer:
1.Conduction
2.Convection
3.Radiation
Conduction:
Conduction is a process where the heat transfer takes place
between the two solid bodies in contact, two regions of the same solid body.
This will happen because of the hot, vibrating, and rapidly moving molecules
transfer the heat to their neighboring atoms.
Convection:
The convection is a type of heat transfer where the heat
transfer takes place through a medium and the medium may be liquid or the gas.
The heat transfer takes place by the movement of fluid from one place to
another. The heat transfer here is due to the bulk motion of the fluid.
Convection is described by the Newton’s law of cooling; the
law states that the rate of heat loosed by a body is proportional to the
difference in temperatures between the body and its surroundings.
Radiation:
The Third mode of energy transfer is Radiation Heat
Transfer. The transfer of heat from hot body to a cold body with any material
medium for propagation.
Every object in the universe is made up of atoms and
molecules. These atoms and molecules vibrate due to thermal energy present in
them. Every object emits electromagnetic radiations because of the thermal
vibrations of these atoms and molecules. In case of energy transfer, the
radiation conversion of radiated electromagnetic energy to thermal energy takes
place. Radiation Heat Transfer can also be termed as transfer of energy through
waves.
Thermal Radiation: This is a kind of Electromagnetic
Radiation which is emitted due to the heat of the material .every material is
made up of small atoms and molecules,and these atoms and molecules are made up
of charge particles.so the movement of these charge particles are responsible
for the emission of electromagnetic radiation. Every material which is hot
emits thermal radiation but the intensity is not same .intensity of the
radiation depends on temperature of the body that how hot it is.when body becomes
hot then the internal charges of the material starts emitting the radiation in
the form of waves, this radiation is known as thermal radiation and the waves
emitted are known as electromagnetic waves.
The waves emitted are of different frequency,the variation
in the frequency is because of the difference in the degree of hotness of the
material.sun also emits thermal radiation due to the extremely hot gases
present in it,and this radiation heats the Earth.This thermal radiation is of
great importance in the Thermodynamics.It is just a kind of conversion of heat
energy in the form of electromagnetic waves.Warmer body emits more heat than
the colder body.
Stefan Boltzmann Law
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Stefan Boltzmann Law states that the emissive power of a (black)
body is proportional to the forth power of the absolute temperature.
Mathematically, it is given as
p = $\sigma AεT^{4}$where ,
Ρ= Power radiated in watts by the substance,
$\sigma$ = Stefan-Boltzmann constant,
$\sigma$ = 5.669 6 x10-8 W/m2K4,
A = surface area of the substance in meters,
ε = emissivity constant, 0<= ε<=1
T = Surface temperature in K
The value of ε depends on the properties of the substance
surface from which the radiations are to be transmitted.
Heat Transfer Coefficient
Heat Transfer Coefficient is defined as the ratio of heat
lost due to the flow to the product of area and temperature difference. It is
used to measure the transfer of heat by convection or in the phase change
(generally between fluid and a solid).
h = $\frac{Q}{A \Delta T}$Where,
Q = Heat flow in input or lost heat flow, J/s = W
h = Heat transfer coefficient, W/m2/K.
A = Heat transfer surface area, m2
$\Delta$ T = difference in temperature between the solid
surface and surrounding fluid area,
Heat flux is the heat flowing per unit area - $\frac{Q}{A}$.
Convective Heat Transfer coefficient of Air - hair= 10 to
100 W/(m2K).
Heat Transfer coefficient of Water - hwater = 500 to 10,000
W/(m2K).
Heat Transfer Example
Examples of Heat Transfer through Conduction:
Take one long piece of metal. Put first end of this metal in
the flame. Gradually, you will find that the temperature of the other end of
the metal in your hand starts increasing. Energy gets transferred from the
first end under flame to the second end of the metal in your hand by
conduction.
Examples of Heat Transfer through Natural Convection:
Natural convection is the convection which occurs naturally
due to the bouncy effect. When water is heated in a pot then the particles,
atoms or molecules of the water which are in contact with the pot gain energy.
Kinetic energy of these particles gets increased. As a result, density of these
particles decreases and they start moving upward towards the open surface of
the port. Cool water which is in contact with the air in the port starts
sinking downward and convection current gets established. Cool water in the
upper surface is denser than the hot water in contact with the surface of the
pot. Convection Currents flow in circular fashion. Therefore, heat is
transferred by the movement of water (fluids).
Forced convection: The convection that takes place by the
force that is the force created from the fans, stirrers, pumps and etc. Examples
of Heat Transfer through Radiation:
Radiations coming out
from the burner of the electric stove or toaster coils. Transfer of heat energy
from Sun to Earth or to us takes place due to the electromagnetic rays emitted
by the sun. Another example is the heating of food inside the microwave.
Thermal Conductivity of Air
The property of the material that indicates its ability to
conduct heat is termed as thermal conductivity and is denoted by the letter
'k'. Thermal conductivity is measured in terms of Watt per Kelvin per metre (
W.K-1m-1 ). We multiply by a temperature difference in Kelvin and an area of
the material in square metres and divide it by the thickness of the material in
metres, the thermal conductivity will predict the heat loss in Watt through
that particular material.
what is Thermal conductivity? The reciprocal of thermal
conductivity is thermal resistivity usually measured in Kelvin metres per Watt
(K.m.W-1). The thermal conductance and the reciprocal property, thermal
resistivity can be described when we deal with a known amount of material.
Thermal conductance is quantity of heat that passes in unit time through a
plate of particular area and thickness when its opposite faces have a
temperature difference of one Kelvin. For a plate of thermal conductivity k,
area A and thickness L, thermal conductance is given by $\frac{kA}{L}$ measured
in W.K-1
Thermal conductivity depends on many properties of the
materials, mainly on it's structure and temperature for example pure
crystalline substances. Air and other gases are good insulators, in the absence
of convention.Therefore many insulation materials function simply by having a
large number of filled pockets which prevent large-scale convection. Thermal
conductivity is important in building insulation and related fields.
Air is mainly composed of nitrogen, oxygen and argon which
together constitute the major gases of the atmosphere. Nitrogen constitutes
78.084% of the total atmosphere where as oxygen is 20.946% and argon
0.9340%.The remaining gases are known as trace gases which are the greenhouse gases
such as carbon dioxide, water vapour, nitrous oxide, methane etc
Examples of Heat Transfer through Radiation:
Radiations coming out
from the burner of the electric stove or toaster coils. Transfer of heat energy
from Sun to Earth or to us takes place due to the electromagnetic rays emitted
by the sun. Another example is the heating of food inside the microwave.
Thermal Conductivity of Air
The property of the material that indicates its ability to
conduct heat is termed as thermal conductivity and is denoted by the letter
'k'. Thermal conductivity is measured in terms of Watt per Kelvin per metre (
W.K-1m-1 ). We multiply by a temperature difference in Kelvin and an area of
the material in square metres and divide it by the thickness of the material in
metres, the thermal conductivity will predict the heat loss in Watt through
that particular material.
what is Thermal conductivity? The reciprocal of thermal
conductivity is thermal resistivity usually measured in Kelvin metres per Watt
(K.m.W-1). The thermal conductance and the reciprocal property, thermal
resistivity can be described when we deal with a known amount of material.
Thermal conductance is quantity of heat that passes in unit time through a
plate of particular area and thickness when its opposite faces have a
temperature difference of one Kelvin. For a plate of thermal conductivity k,
area A and thickness L, thermal conductance is given by $\frac{kA}{L}$ measured
in W.K-1
Thermal conductivity depends on many properties of the
materials, mainly on it's structure and temperature for example pure
crystalline substances. Air and other gases are good insulators, in the absence
of convention.Therefore many insulation materials function simply by having a
large number of filled pockets which prevent large-scale convection. Thermal
conductivity is important in building insulation and related fields.
Air is mainly composed of nitrogen, oxygen and argon which
together constitute the major gases of the atmosphere. Nitrogen constitutes
78.084% of the total atmosphere where as oxygen is 20.946% and argon
0.9340%.The remaining gases are known as trace gases which are the greenhouse gases
such as carbon dioxide, water vapour, nitrous oxide, methane etc
.