Introduction of heat transfer and coduction heat transfer.

Heat: Heat is a form of energy that can be transfer from a body at higher temperature to lower one because of temperature difference, and the energy transfer stop when the two medium reach the same temparature or equilibrium.

Heat transfer:- It is defined as the rate of heat transfer from one medium to another as a result of the temperature difference.  Heat always flow in the direction of decreasing temperature.

Look at below diagram you would understand ;

applications of heat transfer;
1. The human body,
2. Air conditioning system,
3. electronic equipments,
4.refrigeration system, etc.
Above all examples do work on heat transfer. you would notice.
  Now we understood that heat flow from one system to another now we will study about mechanism of heat transfer;
Heat can be transfer in different modes like
1. Conduction.
2. Convection.
3. Thermal radiation.
         All the modes of heat transfer require a existence of temperature difference.
1. Conduction; Conduction is the transfer of heat/energy from more energetic particles to less energetic particles of substance due to interplay between the particles.it can take place in solids, liquids and gases.
In solids , it occurs due to vibration of molecules a lattice and the energy transport by free electron.
  Rate of heat conduction through a solids/medium depends upon the thickness of material, geometry of medium, nature of medium and temperature difference.
Rate of heat conduction express by the J.B.J. Fourier's law or Fourier's law of heat conduction; It states that rate of heat conduction is directly proportional to the area and temoerature difference of material and inversely proportional to the thickness of layer. That is,
                 Qcond. is directly proportional to (Area)(Temperature difference)/thickness.

or,
     Qcond. =k(A)(T1-T2) =  - k(A)(∆T)/∆x.

                     k is proportionality constant or thermal conductivity or conductive heat transfer coefficient.
                    Q is rate of heat conduction in watts.
                    A is area of heat transfer in square meter.
                    ∆T is temperature difference in K.
            ∆x is thickness in meter.
           If we express above equation in limiting case we can write it,
     Qcond.=  -kAdT/dx.    (Watt)
      dT/dx is temperature difference or driving force for heat transfer.
   Thermal conductivity k, it is a property of material, it is defined as the rate of heat transfer through a unit thickness of material  per unit area and temoerature difference. it measure the ability of material to conduct heat. If the thermal conductivity of material is high then it is a good conductor of heat and vice versa. thermal conductivity of pure crystals and metal highest and and gases and insulating materials are lowest. such as silver, copper, Gold, aluminium is a good heat conductor and material like rubber, wood are low heat conductor.
  Unit of thermal conductivity is W/mK. in S.I. system.
   Thermal diffusivity, alpha ; it is the ratio of heat conduction to heat stored per unit volume.if the faster the thermal diffusivity then faster the heat enters into medium.
       Thermal diffusivity, alpha = k/p.cp
k is thermal conductivity.
p.cp is heat capacity which represent how much energy a material stores per unit volume.
Unit of thermal diffusivity is square of meter per second.

For more details you can go on YouTube channel of nptel chemical engineering videos and  and www.nptel.ac.in you can read from book of  heat and mass transfer by Yunus A. Cengel and Afshin J. Ghajar.