Editing Convection (section)

==Mechanisms==
Convection may happen in [[fluids]] at all scales larger than a few atoms. There are a variety of circumstances in which the forces required for convection arise, leading to different types of convection, described below. In broad terms, convection arises because of [[body force]]s acting within the fluid, such as gravity.

===Natural convection===
{{Unreferenced section|date=September 2023}}
[[Image:Bénard cells convection.ogv|thumb|300px|[[Rayleigh–Bénard convection|Rayleigh–Bénard cells]].]]
[[File:Thermal-plume-from-human-hand.jpg|thumb|This color [[schlieren]] image reveals [[thermal convection]] originating from heat conduction from a human hand (in silhouette) to the surrounding still atmosphere, initially by diffusion from the hand to the surrounding air, and subsequently also as advection as the heat causes the air to start to move upwards.]]
'''Natural convection''' is a flow whose motion is caused by some parts of a fluid being heavier than other parts. In most cases this leads to '''natural circulation''': the ability of a fluid in a system to circulate continuously under gravity, with transfer of heat energy.

The driving force for natural convection is gravity. In a column of fluid, pressure increases with depth from the weight of the overlying fluid. The pressure at the bottom of a submerged object then exceeds that at the top, resulting in a net upward [[buoyancy]] force equal to the weight of the displaced fluid. Objects of higher density than that of the displaced fluid then sink. For example, regions of warmer low-density air rise, while those of colder high-density air sink. This creates a circulating flow: convection.

Gravity drives natural convection. Without gravity, convection does not occur, so there is no convection in free-fall ([[inertial]]) environments, such as that of the orbiting International Space Station. Natural convection can occur when there are hot and cold regions of either air or water, because both water and air become less dense as they are heated. But, for example, in the world's oceans it also occurs due to salt water being heavier than fresh water, so a layer of salt water on top of a layer of fresher water will also cause convection.

Natural convection has attracted a great deal of attention from researchers because of its presence both in nature and engineering applications. In nature, convection cells formed from air raising above sunlight-warmed land or water are a major feature of all weather systems. Convection is also seen in the rising plume of hot air from [[fire]], [[plate tectonics]], oceanic currents ([[thermohaline circulation]]) and sea-wind formation (where upward convection is also modified by [[Coriolis force]]s). In engineering applications, convection is commonly visualized in the formation of microstructures during the cooling of molten metals, and fluid flows around shrouded heat-dissipation fins, and solar ponds. A very common industrial application of natural convection is free air cooling without the aid of fans: this can happen on small scales (computer chips) to large scale process equipment.

Natural convection will be more likely and more rapid with a greater variation in density between the two fluids, a larger acceleration due to gravity that drives the convection or a larger distance through the convecting medium. Natural convection will be less likely and less rapid with more rapid diffusion (thereby diffusing away the thermal gradient that is causing the convection) or a more viscous (sticky) fluid.

The onset of natural convection can be determined by the [[Rayleigh number]] ('''Ra''').

Differences in buoyancy within a fluid can arise for reasons other than temperature variations, in which case the fluid motion is called '''gravitational convection''' (see below). However, all types of buoyant convection, including natural convection, do not occur in [[microgravity]] environments. All require the presence of an environment which experiences [[g-force]] ([[proper acceleration]]).

The difference of [[density]] in the fluid is the key driving mechanism. If the differences of density are caused by heat, this force is called as "thermal head" or "thermal driving head." A fluid system designed for natural circulation will have a heat source and a [[heat sink]]. Each of these is in contact with some of the fluid in the system, but not all of it. The heat source is positioned lower than the heat sink.

Most fluids expand when heated, becoming less [[density|dense]], and contract when cooled, becoming denser. At the heat source of a system of natural circulation, the heated fluid becomes lighter than the fluid surrounding it, and thus rises. At the heat sink, the nearby fluid becomes denser as it cools, and is drawn downward by gravity. Together, these effects create a flow of fluid from the heat source to the heat sink and back again.

===Gravitational or buoyant convection===
'''Gravitational convection''' is a type of natural convection induced by buoyancy variations resulting from material properties other than temperature.  Typically this is caused by a variable composition of the fluid.  If the varying property is a concentration gradient, it is known as '''solutal convection'''.<ref>{{cite journal|citeseerx=10.1.1.15.8288 |title=Pattern Formation in Solutal Convection: Vermiculated Rolls and Isolated Cells |journal=Physica A: Statistical Mechanics and Its Applications |volume=314 |issue=1 |pages=291 |bibcode=2002PhyA..314..291C |last1=Cartwright |first1=Julyan H. E. |author1-link = Julyan Cartwright |last2=Piro |first2=Oreste |last3=Villacampa |first3=Ana I. |year=2002 |doi=10.1016/S0378-4371(02)01080-4 }}</ref> For example, gravitational convection can be seen in the diffusion of a source of dry salt downward into wet soil due to the buoyancy of fresh water in saline.<ref>{{cite journal|last=Raats|first= P. A. C.  |year=1969 |title=Steady Gravitational Convection Induced by a Line Source of Salt in a Soil|journal = Soil Science Society of America Proceedings |volume = 33 |pages = 483–487 | doi=10.2136/sssaj1969.03615995003300040005x |issue=4|bibcode=1969SSASJ..33..483R}}</ref>

Variable [[salinity]] in water and variable water content in air masses are frequent causes of convection in the oceans and atmosphere which do not involve heat, or else involve additional compositional density factors other than the density changes from thermal expansion (see ''[[thermohaline circulation]]''). Similarly, variable composition within the Earth's interior which has not yet achieved maximal stability and minimal energy (in other words, with densest parts deepest) continues to cause a fraction of the convection of fluid rock and molten metal within the Earth's interior (see below).

Gravitational convection, like natural thermal convection, also requires a [[g-force]] environment in order to occur.

===Solid-state convection in ice===
[[Sputnik Planitia#Convection cells|Ice convection on Pluto]] is believed to occur in a soft mixture of [[nitrogen ice]] and [[carbon monoxide]] ice.  It has also been proposed for [[Europa (moon)|Europa]],<ref name="On convection in ice I shells of ou">{{cite journal| doi=10.1016/j.icarus.2006.03.004 | bibcode=2006Icar..183..435M | volume=183 | issue=2 | title=On convection in ice I shells of outer Solar System bodies, with detailed application to Callisto | year=2006 | journal=Icarus | pages=435–450 | last1 = McKinnon | first1 = William B.}}</ref> and other bodies in the outer Solar System.<ref name="On convection in ice I shells of ou"/>

===Thermomagnetic convection===
{{main|Thermomagnetic convection}}
'''Thermomagnetic convection''' can occur when an external magnetic field is imposed on a [[ferrofluid]] with varying [[magnetic susceptibility]].  In the presence of a temperature gradient this results in a nonuniform magnetic body force, which leads to fluid movement. A ferrofluid is a liquid which becomes strongly magnetized in the presence of a [[magnetic field]].

===Combustion===
In a [[zero-gravity]] environment, there can be no buoyancy forces, and thus no convection possible, so flames in many circumstances without gravity smother in their own waste gases. Thermal expansion and chemical reactions resulting in expansion and contraction gases allows for ventilation of the flame, as waste gases are displaced by cool, fresh, oxygen-rich gas. moves in to take up the low pressure zones created when flame-exhaust water condenses.