We discussed different modes of heat transfer. Heat is always transferred from hot body to cold body therefore from the heating element to the body to be heated.
This transfer of heat due to different modes of heat transfer which is conduction, convection radiation or by combination of all of these methods.
Let’s we get explanation briefly,
Modes of heat transfer
Modes of heat transfer conduction
In this conduction heat transfer mode of transfer, the heat transfer from one atom to its neighbouring atom through molecular vibration.
In elements one molecule of substance gets heated and transfers the heat to attached molecules as shown in figure and therefore quickly heated.
This method rate at which heat is conducted by a substance depends on the temperature gradient.
The neighbour atom takes place of heat flow until there will be different in temperature and continuity.
Modes of heat transfer conduction method is very important in the event of heating insulating materials and refractory etc.
This conductivity is expressed in MJ per hour per square meter per meter or expressed in watt per square centimeter per centimeter.
Modes of heat transfer convection
In this convection heat transfer mode heat transfer occurs due to the transfer of energy through the bulk masses and it mostly takes place in liquid, for example as water.
Let’s take an example, we take a bowl full of water and heat it from the bottom. Water will be heated and present molecules will flow in upward direction because of this heated water goes in upward direction and during its motion.
The heat is also passed into the entire mass of the water, hence the entire mass of water is heated up.
For vertical heating surface in air, the heat dissipated due to convection is given by,
Where T1 = Absolute temperature of heating surface
T2 = Absolute temperature of air
And 3.875 = Constant and this value depend on heating surface arrangement
Modes of heat transfer Radiation
In this mode, heat transfer occurs as quantum packets of light energy. This is the mode by which we receive solar energy from the sun.
These waves do not heat the medium in between two bodies and heat the body which intercepts these waves.
Introduce rate of heat dissipation by Steffen’s law given by,
e = Emissivity
T1= Temperature of source and it is in kelvin
T2 = Temperature of substance to be heated in kelvin
The all heat rate by these conventional methods is as given in the table
|Conduction||20 watt / cm2 |
1000 watt /cm2 for intense flame
|Convection||0.5 watt /cm2|
|Radiation||8 watt / cm2|
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