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#REDIRECT [[Convection#Natural convection]]
[[Image:Bénard cells convection.ogv|thumb|300px|[[Rayleigh–Bénard convection|Rayleigh–Bénard cells]].]]
 
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'''Natural convection''' is a type of flow, of motion of a liquid such as water or a gas such as air, in which the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) but by some parts of the fluid being heavier than other parts. The driving force for natural convection is gravity. For example if there is a layer of cold dense air on top of hotter less dense air, gravity pulls more strongly on the denser layer on top, so it falls while the hotter less dense air rises to take its place. This creates circulating flow: convection. As it relies of gravity, 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.
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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.
 
==Parameters==
 
===Onset===
{{See also|Heat transfer}}
The onset of natural convection is determined by the [[Rayleigh number]] ('''Ra'''). This [[dimensionless number]] is given by
 
:<math>\textbf{Ra} = \frac{\Delta\rho g L^3}{D\mu}</math>
 
where
 
*<math>\Delta \rho</math> is the difference in density between the two parcels of material that are mixing
*<math>g</math> is the local [[gravitational acceleration]]
*<math>L</math> is the characteristic length-scale of convection: the depth of the boiling pot, for example
*<math>D</math> is the [[diffusivity]] of the characteristic that is causing the convection, and
*<math>\mu</math> is the [[dynamic viscosity]].
 
Natural convection will be more likely and/or more rapid with a greater variation in density between the two fluids, a larger acceleration due to gravity that drives the convection, and/or a larger distance through the convecting medium. Convection will be less likely and/or less rapid with more rapid diffusion (thereby diffusing away the gradient that is causing the convection) and/or a more viscous (sticky) fluid.
 
For thermal convection due to heating from below, as described in the boiling pot above, the equation is modified for thermal expansion and thermal diffusivity. Density variations due to thermal expansion are given by:
 
:<math>\Delta\rho=\rho_0 \beta \Delta T</math>
 
where
 
*<math>\rho_0</math> is the reference density, typically picked to be the average density of the medium,
*<math>\beta</math> is the [[coefficient of thermal expansion]], and
*<math>\Delta T</math> is the temperature difference across the medium.
 
The general diffusivity, <math>D</math>, is redefined as a [[thermal diffusivity]], <math>\alpha</math>.
 
:<math>D=\alpha</math>
 
Inserting these substitutions produces a Rayleigh number that can be used to predict thermal convection.<ref>{{cite book|isbn=978-0-521-66624-4|author1=Donald L. Turcotte |author2=Gerald Schubert. |year=2002|publisher=Cambridge University Press|location=Cambridge|title=Geodynamics}}</ref>
 
:<math>\textbf{Ra} = \frac{\rho_0 g \beta \Delta T L^3}{\alpha \mu}</math>
 
===Turbulence===
The tendency of a particular naturally convective system towards turbulence relies on the [[Grashof number]] (Gr).<ref>{{cite book |author1=Kays, William |author2=Crawford, Michael |author3=Weigand, Bernhard | title=Convective Heat and Mass Transfer, 4E | publisher=McGraw-Hill Professional | year=2004 | isbn=978-0072990737}}</ref>
 
:<math> Gr= \frac{g \beta \Delta T L^3}{\nu^2} </math>
 
In very sticky, viscous fluids (large ''&nu;''), fluid movement is restricted, and natural convection will be non-turbulent.
 
Following the treatment of the previous subsection, the typical fluid velocity is of the order of <math>g \Delta \rho L^2 / \mu</math>, up to a numerical factor depending on the geometry of the system. Therefore, Grashof number can be thought of as [[Reynolds number]] with the velocity of natural convection replacing the velocity in Reynolds number's formula. However In practice, when referring to the Reynolds number, it is understood that one is considering forced convection, and the velocity is taken as the velocity dictated by external constraints (see below).
 
===Behavior===
The [[Grashof number]] can be formulated for natural convection occurring due to a [[concentration gradient]], sometimes termed thermo-solutal convection. In this case, a concentration of hot fluid diffuses into a cold fluid, in much the same way that ink poured into a container of water diffuses to dye the entire space. Then:
 
:<math> Gr= \frac{g \beta \Delta C L^3}{\nu^2} </math>
 
Natural convection is highly dependent on the geometry of the hot surface, various correlations exist in order to determine the heat transfer coefficient.
A general correlation that applies for a variety of geometries is
 
: <math>Nu = \left[Nu_0^\frac{1}{2} + Ra^ \frac{1}{6} \left(\frac {f_4\left(Pr\right)}{300}\right)^\frac{1}{6} \right]^2 </math>
 
The value of f<sub>4</sub>(Pr) is calculated using the following formula
 
: <math>f_4(Pr)= \left[1+ \left ( \frac {0.5}{Pr} \right )^\frac{9}{16} \right]^\frac{-16}{9}</math>
 
Nu is the [[Nusselt number]] and the values of Nu<sub>0</sub> and the characteristic length used to calculate Ra are listed below (see also Discussion):
 
{| class="wikitable"
|-
! '''Geometry'''
! '''Characteristic length'''
! '''Nu<sub>0</sub>'''
|-
| Inclined plane
| x (Distance along plane)
| 0.68
|-
| Inclined disk
| 9D/11 (D = diameter)
| 0.56
|-
| Vertical cylinder
| x (height of cylinder)
| 0.68
|-
| Cone
| 4x/5 (x = distance along sloping surface)
| 0.54
|-
| Horizontal cylinder
| <math>\pi D/2</math> (D = diameter of cylinder)
| 0.36<math>\pi</math>
|}
'''Warning''': The values indicated for the '''Horizontal cylinder''' are '''wrong'''; see discussion.
 
==Natural convection from a vertical plate==
In this system heat is transferred from a vertical plate to a fluid moving parallel to it by natural convection. This will occur in any system wherein the density of the moving fluid varies with position. These phenomena will only be of significance when the moving fluid is minimally affected by forced convection.<ref name=unitop>{{cite book | author= W. McCabe J. Smith | title=Unit Operations of Chemical Engineering | publisher=McGraw-Hill | year=1956 | isbn= 978-0-07-044825-4}}</ref>
 
When considering the flow of fluid is a result of heating, the following correlations can be used, assuming the fluid is an ideal diatomic, has adjacent to a vertical plate at constant temperature and the flow of the fluid is completely laminar.<ref name=bennett>{{cite book | author=Bennett | title=Momentum, Heat and Mass Transfer | url=https://archive.org/details/momentumheatmass00benn | url-access=registration | publisher=McGraw-Hill | year=1962 | isbn = 978-0-07-004667-2 }}</ref>
 
Nu<sub>m</sub> = 0.478(Gr<sup>0.25</sup>)<ref name=bennett />
 
Mean [[Nusselt number]] = Nu<sub>m</sub> = h<sub>m</sub>L/k<ref name=bennett />
 
where
 
*h<sub>m</sub> = mean coefficient applicable between the lower edge of the plate and any point in a distance L (W/m<sup>2</sup>. K)
*L = height of the vertical surface (m)
*k = thermal conductivity (W/m. K)
 
[[Grashof number]] = Gr = <math>[gL^3(t_s-t_\infty)]/v^2T</math> <ref name=unitop /><ref name=bennett />
 
where
 
*g = gravitational acceleration (m/s<sup>2</sup>)
*L = distance above the lower edge (m)
*t<sub>s</sub> = temperature of the wall (K)
*t∞ = fluid temperature outside the thermal boundary layer (K)
*v = kinematic viscosity of the fluid (m²/s)
*T = absolute temperature (K)
 
When the flow is turbulent different correlations involving the Rayleigh Number (a function of both the [[Grashof number]] and the [[Prandtl number]]) must be used.<ref name=bennett />
 
Note that the above equation differs from the usual expression for [[Grashof number]] because the value <math>\beta</math> has been replaced by its approximation <math>1/T</math>, which applies for ideal gases only (a reasonable approximation for air at ambient pressure).
 
==Pattern formation==
 
[[Image:Convection1.png|thumb|right|A fluid under [[Rayleigh–Bénard convection]]: the left picture represents the thermal field and the right picture its two-dimensional [[Fourier transform]].]]
 
Convection, especially Rayleigh–Bénard convection, where the convecting fluid is contained by two rigid horizontal plates, is a convenient example of a [[Pattern formation|pattern-forming system]].
 
When heat is fed into the system from one direction (usually below), at small values it merely diffuses (''conducts'') from below upward, without causing fluid flow. As the heat flow is increased, above a critical value of the [[Rayleigh number]], the system undergoes a [[Bifurcation theory|bifurcation]] from the stable ''conducting'' state to the ''convecting'' state, where bulk motion of the fluid due to heat begins. If fluid parameters other than density do not depend significantly on temperature, the flow profile is symmetric, with the same volume of fluid rising as falling. This is known as [[Boussinesq approximation (buoyancy)|Boussinesq]] convection.
 
As the temperature difference between the top and bottom of the fluid becomes higher, significant differences in fluid parameters other than density may develop in the fluid due to temperature. An example of such a parameter is [[viscosity]], which may begin to significantly vary horizontally across layers of fluid. This breaks the symmetry of the system, and generally changes the pattern of up- and down-moving fluid from stripes to hexagons, as seen at right. Such hexagons are one example of a [[convection cell]].
 
As the [[Rayleigh number]] is increased even further above the value where convection cells first appear, the system may undergo other bifurcations, and other more complex patterns, such as [[spiral]]s, may begin to appear.
 
== Water convection at freezing temperatures ==
[[Water]] is a fluid that does not obey the Boussinesq approximation.<ref name=":0">{{Cite journal|last=Banaszek|first=J.|last2=Jaluria|first2=Y.|last3=Kowalewski|first3=T. A.|last4=Rebow|first4=M.|date=1999-10-01|journal=Numerical Heat Transfer, Part A: Applications|language=en|volume=36|issue=5|pages=449–472|doi=10.1080/104077899274624|issn=1040-7782|title=Semi-Implicit Fem Analysis of Natural Convection in Freezing Water|url=https://semanticscholar.org/paper/bf8dc37b3e24771e3aa77fcac0741ca9c1786034}}</ref> This is because its density varies nonlinearly with temperature, which causes its thermal expansion coefficient to be inconsistent near freezing temperatures.<ref>{{Cite web|url=https://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html|title=Water - Density, Specific Weight and Thermal Expansion Coefficient|website=www.engineeringtoolbox.com|language=en|access-date=2018-12-01}}</ref><ref name=":1">{{Cite news|url=http://polymer.bu.edu/hes/articles/ds03.pdf|title=Supercooled and Glassy Water|last=Debenedetti|first=Pablo G.|date=June 2003|work=Physics Today|access-date=1 December 2018|last2=Stanley|first2=H. Eugene}}</ref> The [[density of water]] reaches a maximum at 4&nbsp;°C and decreases as the temperature deviates. This phenomenon is investigated by experiment and numerical methods.<ref name=":0" /> Water is initially stagnant at 10&nbsp;°C within a square cavity. It is differentially heated between the two vertical walls, where the left and right walls are held at 10&nbsp;°C and 0&nbsp;°C, respectively.  The density anomaly manifests in its flow pattern.<ref name=":0" /><ref>{{Cite journal|last=Giangi|first=Marilena|last2=Stella|first2=Fulvio|last3=Kowalewski|first3=Tomasz A.|date=December 1999|title=Phase change problems with free convection: fixed grid numerical simulation|journal=Computing and Visualization in Science|language=en|volume=2|issue=2–3|pages=123–130|doi=10.1007/s007910050034|issn=1432-9360|citeseerx=10.1.1.31.9300}}</ref><ref>{{Cite journal|last=Tong|first=Wei|last2=Koster|first2=Jean N.|date=December 1993|title=Natural convection of water in a rectangular cavity including density inversion|url=|journal=International Journal of Heat and Fluid Flow|volume=14|issue=4|pages=366–375|doi=10.1016/0142-727x(93)90010-k|issn=0142-727X}}</ref><ref>{{Cite journal|last=Ezan|first=Mehmet Akif|last2=Kalfa|first2=Mustafa|date=October 2016|title=Numerical investigation of transient natural convection heat transfer of freezing water in a square cavity|journal=International Journal of Heat and Fluid Flow|volume=61|pages=438–448|doi=10.1016/j.ijheatfluidflow.2016.06.004|issn=0142-727X}}</ref> As the water is cooled at the right wall, the density increases, which accelerates the flow downward. As the flow develops and the water cools further, the decrease in density causes a recirculation current at the bottom right corner of the cavity.
 
Another case of this phenomenon is the event of [[Supercooling|super-cooling]], where the water is cooled to below freezing temperatures but does not immediately begin to freeze.<ref name=":1" /><ref name=":2">{{Cite journal|last=Moore|first=Emily B.|last2=Molinero|first2=Valeria|date=November 2011|title=Structural transformation in supercooled water controls the crystallization rate of ice|journal=Nature|language=En|volume=479|issue=7374|pages=506–508|doi=10.1038/nature10586|pmid=22113691|issn=0028-0836|arxiv=1107.1622}}</ref> Under the same conditions as before, the flow is developed. Afterward, the temperature of the right wall is decreased to −10&nbsp;°C. This causes the water at that wall to become supercooled, create a counter-clockwise flow, and initially overpower the warm current.<ref name=":0" /> This plume is caused by a delay in the [[Nucleation of ice|nucleation of the ice]].<ref name=":0" /><ref name=":1" /><ref name=":2" /> Once ice begins to form, the flow returns to a similar pattern as before and the solidification propagates gradually until the flow is redeveloped.<ref name=":0" />
 
==Mantle convection==
{{Main|Mantle convection}}
Convection within [[Earth's mantle]] is the driving force for [[plate tectonics]]. Mantle convection is the result of a thermal gradient: the lower mantle is hotter than the [[upper mantle (Earth)|upper mantle]], and is therefore less dense. This sets up two primary types of instabilities. In the first type, plumes rise from the lower mantle, and corresponding unstable regions of [[lithosphere]] drip back into the mantle. In the second type, subducting oceanic plates (which largely constitute the upper thermal boundary layer of the mantle) plunge back into the mantle and move downwards towards the [[core-mantle boundary]]. Mantle convection occurs at rates of centimeters per year, and it takes on the order of hundreds of millions of years to complete a cycle of convection.
 
Neutrino flux measurements from the Earth's core (see [[kamLAND]]) show the source of about two-thirds of the heat in the inner core is the [[radioactive decay]] of [[potassium|<sup>40</sup>K]], uranium and thorium. This has allowed plate tectonics on Earth to continue far longer than it would have if it were simply driven by heat left over from Earth's formation; or with heat produced from gravitational [[potential energy]], as a result of physical rearrangement of denser portions of the Earth's interior toward the center of the planet (i.e., a type of prolonged falling and settling).
 
== See also ==
* [[Combined forced and natural convection]]
* [[Forced convection]]
* [[Convection]]
* [[Heat transfer]]
* [[Heat exchanger]]
 
== References ==
 
{{Reflist}}
 
[[Category:Heat transfer]]