8 June 2019

Heat Transfer Medium : Conduction, Convection and Radiation


Modes of heat transfer

The heat transfer from one medium to another medium by any of the following methods
( 1 ) Conduction
( 2 ) Convection
( 3 ) Radiation
( 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
( 3 ) Radiation
  • 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
k = Radiant efficiency
   = 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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