Editing Heat transfer (section)

===Convection===
{{main|Convective heat transfer}}
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 using a pump, fan, or other mechanical means.

[[Convection (heat transfer)|Convective heat transfer]], or simply, convection, is the transfer of heat from one place to another by the movement of [[fluids]], a process that is essentially the transfer of heat via [[mass transfer]]. The bulk motion of fluid enhances heat transfer in many physical situations, such as between a solid surface and the fluid.<ref>{{cite book |last=Çengel |first=Yunus |year=2003 |title=Heat Transfer: A practical approach |url=https://books.google.com/books?id=nrbfpSZTwskC |edition=2nd |publisher=McGraw-Hill |location=Boston |isbn=978-0-07-245893-0}}</ref> Convection is usually the dominant form of heat transfer in liquids and gases. Although sometimes discussed as a third method of heat transfer, convection is usually used to describe the combined effects of heat conduction within the fluid (diffusion) and heat transference by bulk fluid flow streaming.<ref>{{cite web |title=Convective heat transfer |url=https://www.thermalfluidscentral.org/encyclopedia/index.php/Convective_Heat_Transfer |work=Thermal-FluidsPedia |publisher=Thermal Fluids Central}}</ref> The process of transport by fluid streaming is known as advection, but pure advection is a term that is generally associated only with mass transport in fluids, such as advection of pebbles in a river. In the case of heat transfer in fluids, where transport by advection in a fluid is always also accompanied by transport via heat diffusion (also known as heat conduction) the process of heat convection is understood to refer to the sum of heat transport by advection and diffusion/conduction.

Free, or natural, convection occurs when bulk fluid motions (streams and currents) are caused by buoyancy forces that result from density variations due to variations of temperature in the fluid. ''Forced'' convection is a term used when the streams and currents in the fluid are induced by external means—such as fans, stirrers, and pumps—creating an artificially induced convection current.<ref>{{cite web |title=Convection — Heat Transfer |url=http://www.engineersedge.com/heat_transfer/convection.htm |publisher=Engineers Edge |access-date=2009-04-20}}</ref>

====Convection-cooling====
{{See also |Nusselt number}}
Convective cooling is sometimes described as [[Convective heat transfer#Newton's law of cooling|Newton's law of cooling]]:
{{Blockquote|''The rate of heat loss of a body is proportional to the temperature difference between the body and its surroundings''.}} However, by definition, the validity of Newton's law of cooling requires that the rate of heat loss from convection be a linear function of ("proportional to") the temperature difference that drives heat transfer, and in convective cooling this is sometimes not the case. In general, convection is not linearly dependent on [[temperature gradient]]s, and in some cases is strongly nonlinear. In these cases, Newton's law does not apply.