## 08/06/2019

### Modes of heat transfer

The heat transfer from one medium to another medium by any of the following methods
( 1 ) Conduction
( 2 ) Convection
( 1 ) Conduction
• The heat transfers from one part of substance to another part without movement in the molecule of substance.
• The heat transfer takes places between object and direct contact.
• The rate of conduction of heat along the substance depends upon the temperature difference.
Example : e.g. heating of insulating materials,
• The amount of heat transfer through cubic body with two parallel plates during T hours is given by
Q = [ kA ( T1 – T2 ) / t ] T Mega Joules
Where
k = Co – efficient of thermal conductivity of the material ( MJ / m3 / oC / hour )
t = Thickness of cubic body
A = Cross section area of plate
T1 = Temperature of first plate in centigrade
T2 = Temperature of second plate in centigrade
( 2 ) Convection
• The heat transfers from one part to another part of substance due to actual motion of molecules.
• The rate of conduction of heat mainly depends upon difference in fluid density at different temperature.
Example : Boiling water, immersion heater, ice melting, hot air balloon, radiator
• The heat dissipation is given by H = a ( T1 – T2 )b Watt / meter2
• The radiation does not require any heat transfer medium.
• The heat transfer takes place through electromagnetic waves.
• The object is not necessary to be in direct contact with another object to transmit heat.
• The energy transmission through electromagnetic waves is called as radiant energy.
Exmaple : Sun energy , solar heaters
• The rate of heat dissipation through radiation is given by
H = 5.72 × 104 ke [ ( T1 / 1000 )4 - ( T2 / 1000 )4 ] watt / meter2
Where
T1 = Temperature of source in Kelvin
T2 = Temperature of substance to be heater in Kelvin
= 1 Single element
= 0.5 to 0.8 for several elements
e = Emissivity
= 1 for Black body
= 0.9 for resistance heating element
• The radiant heat is directly proportional to difference of the fourth power of the temperature.
Final Conclusion points
• The speed of conduction and convection is slower than radiation.
• The radiation obeys the law of reflection and refraction whereas the conduction and convection do not follow it.
• The conduction mainly occurs in solids through collision of molecules whereas the convection occurs in fluids by mass motion of molecules.
• The radiation takes places through the vaccum of space.
• The conduction takes place due to temperature difference whereas the convection takes place due to density difference.
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