1. Introduction :

When two objects at different temperatures are kept into contact, heat flows from higher temperature object to lower temperature object. The heat flow is always in the direction of decrease in temperature. There are three mechanisms by which heat may flow: Conduction, Convection and radiation.

1.1 Conduction :

Conduction heat transfer is the transfer of heat by means of molecular excitement within a material without bulk motion of the matter. Conduction heat transfer in gases and liquids is due to the collisions and diffusion of the molecules during their random motion. On the other hand, heat transfer in solids is due to the combination of lattice vibrations of the molecules and the energy transport by free electrons.

1.2 Convection :

Convection heat transfer is the heat transfer from a solid to a fluid when the fluid is in bulk motion, which distinguishes it from conduction.

Natural convection is a method of heat transfer in which natural means influence the motion of the fluid. There is no influence from external facts. This movement of molecules in the fluid is due to the differences between densities of different regions of the same fluid. The density of a fluid decreases when it heats and vice versa. That is because of the thermal expansion of the fluid (the speed of molecules increases with the temperature increase, which results in the increase of the volume of the fluid. Although the volume increases, the mass remains constant. Therefore, the density decreases).

Forced convection is a method of heat transfer in which external means influence the motion of the fluid. There, external sources such as pumping, fans, suction devices, etc. are useful in generating the fluid motion. This method is very valuable because it can efficiently transfer heat from a heated object. Some common examples of this mechanism include air conditioning, steam turbines, etc.

1.3 Radiation :

Heat transfer from a body with a high temperature to a body with a lower temperature, when bodies are not in direct physical contact with each other or when they are separated in space, is called heat radiation.

Unlike conduction and convection, heat transfer by thermal radiation does not necessarily need a material medium for the energy transfer.