<P> Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems . Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes . Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer . While these mechanisms have distinct characteristics, they often occur simultaneously in the same system . </P> <P> Heat conduction, also called diffusion, is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems . When an object is at a different temperature from another body or its surroundings, heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium . Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as described in the second law of thermodynamics . </P> <P> Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid . The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer . The latter process is often called "natural convection". All convective processes also move heat partly by diffusion, as well . Another form of convection is forced convection . In this case the fluid is forced to flow by use of a pump, fan or other mechanical means . </P> <P> Thermal radiation occurs through a vacuum or any transparent medium (solid or fluid). It is the transfer of energy by means of photons in electromagnetic waves governed by the same laws . </P>

What kind of heat transfer is a fan