CONVECTION :

Convection requires a medium and is the process in which heat is transferred from one place to the other by the actual movement of heated substance (usually fluid). When a fluid in a container is heated, the fluid at the bottom gets heated due to conduction through the bottom of the container. Its density decreases and consequently it rises up. At the same time denser cool fluid will move down and thus convection currents are set up in the liquid and the whole fluid is heated by actual movement of mass. This type of convection which results from difference in densities is called natural convection. However, if a heated fluid is forced to move by a blower, fan or pump, the process is called

forced convection.