What is heat transfer?
Heat transfer, or heat, is energy in transition between communicating systems due to a temperature difference. Heat travels always from a hot body to a cold body. A temperature difference too low avoids the heat to flow. It means that it is impossible to heat a fluid up to exactly the same temperature as the heating fluid. Similarly, cool a fluid down to the same temperature as the cooling fluid is unachievable.
Three modes of heat transfer may be distinguished: conduction, convection and radiation.
In heat conduction, energy is transferred on a molecular scale with no movement of macroscopic portions of matter relative to one other. The conduction in solid is partly due to the impact of adjacent molecules vibrating around their average positions. In liquids and gases the conduction is due to the collisions and diffusion of molecules during their random movement. In these two states of matter the conduction is negligible compared to convection. Indeed fluids and especially gases are less conductive due to the large distance between atoms which implies fewer collisions between them.
Forced and natural convection
Heat convection refers to heat transfer that occurs between a surface and a moving fluid which are different temperatures. Convection heat transfer is subdivided into two different kinds according to the nature of the flow: forced and natural.
Forced convection occurs when the flow is caused by an external force such as a pump or a fan. When the flow is entirely due to density gradients, caused by temperature variations, the heat transfer is called natural convection.
The third mode of heat transfer, radiation, is the transport of energy between two contactless bodies without the assistance of an intervening medium. Unlike conduction and convection, radiation can occur in vacuum. All matter at temperatures above absolute zero emits electromagnetic waves of various wave-lengths. The energy of radiation fields is transported by these waves.