Editing Heat transfer (section)

===Radiation===
[[File:Hot metalwork.jpg|thumb|left|Red-hot iron object, transferring heat to the surrounding environment through thermal radiation]]
Radiative heat transfer is the transfer of energy via [[thermal radiation]], i.e., [[Electromagnetic radiation|electromagnetic waves]].<ref name=Geankoplis /> It occurs across [[vacuum]] or any [[transparency (optics)|transparent]] [[Optical medium|medium]] ([[solid]] or [[fluid]] or [[gas]]).<ref>{{cite web |title=Radiation |url=https://www.thermalfluidscentral.org/encyclopedia/index.php/Radiation |work=Thermal-FluidsPedia |publisher=Thermal Fluids Central}}</ref> Thermal radiation is emitted by all objects at temperatures above [[absolute zero]], due to random movements of atoms and molecules in matter. Since these atoms and molecules are composed of charged particles ([[proton]]s and [[electron]]s), their movement results in the emission of [[electromagnetic radiation]] which carries away energy. Radiation is typically only important in engineering applications for very hot objects, or for objects with a large temperature difference.

When the objects and distances separating them are large in size and compared to the wavelength of thermal radiation, the rate of transfer of [[radiant energy]] is best described by the [[Stefan-Boltzmann equation]]. For an object in vacuum, the equation is:
<math display="block"> \phi_q=\epsilon \sigma T^4. </math>

For [[radiative transfer]] between two objects, the equation is as follows:
<math display="block"> \phi_q=\epsilon \sigma F (T_a^4 - T_b^4), </math>
where
* <math>\phi_q</math> is the [[heat flux]],
* <math>\epsilon </math> is the [[emissivity]] (unity for a [[black body]]),
* <math>\sigma </math> is the [[Stefan–Boltzmann constant]],
* <math>F</math> is the [[view factor]] between two surfaces a and b,<ref>{{cite book |last1=Howell |first1=John R. |last2=Menguc |first2=M.P. |last3=Siegel |first3=Robert |title=Thermal Radiation Heat Transfer |year= 2015 |publisher=Taylor and Francis}}</ref> and
* <math> T_a</math> and <math> T_b</math> are the absolute temperatures (in [[kelvin]]s or [[degrees Rankine]]) for the two objects.

The blackbody limit established by the [[Stefan-Boltzmann equation]] can be exceeded when the objects exchanging thermal radiation or the distances separating them are comparable in scale or smaller than the [[Wien's displacement law|dominant thermal wavelength]]. The study of these cases is called [[near-field radiative heat transfer]].

Radiation from the sun, or solar radiation, can be harvested for heat and power.<ref>{{cite journal |last1=Mojiri |first1=A |year=2013 |title=Spectral beam splitting for efficient conversion of solar energy—A review |journal=Renewable and Sustainable Energy Reviews |volume=28 |pages=654–663 |doi=10.1016/j.rser.2013.08.026|bibcode=2013RSERv..28..654M }}</ref> Unlike conductive and convective forms of heat transfer, thermal radiation – arriving within a narrow-angle i.e. coming from a source much smaller than its distance – can be concentrated in a small spot by using reflecting mirrors, which is exploited in [[concentrating solar power]] generation or a [[burning glass]].<ref>{{cite journal |last1=Taylor |first1=Robert A. |last2=Phelan |first2=Patrick E. |last3=Otanicar |first3=Todd P. |last4=Walker |first4=Chad A. |last5=Nguyen |first5=Monica |last6=Trimble |first6=Steven |last7=Prasher |first7=Ravi |title=Applicability of nanofluids in high flux solar collectors |url=http://digitalcommons.lmu.edu/cgi/viewcontent.cgi?article=1019&context=mech_fac |journal=Journal of Renewable and Sustainable Energy |date=March 2011 |volume=3 |issue=2 |pages=023104 |doi=10.1063/1.3571565|url-access=subscription }}</ref> For example, the sunlight reflected from mirrors heats the [[PS10 solar power tower]] and during the day it can heat water to {{convert|285|°C|°F}}.<ref>{{Cite web|title=Solar thermal power plants - U.S. Energy Information Administration (EIA)|url=https://www.eia.gov/energyexplained/solar/solar-thermal-power-plants.php|access-date=2022-01-28|website=www.eia.gov}}</ref>

The reachable temperature at the target is limited by the temperature of the hot source of radiation. (T<sup>4</sup>-law lets the reverse flow of radiation back to the source rise.) The (on its surface) somewhat 4000 K hot [[sun]] allows to reach coarsely 3000 K (or 3000&nbsp;°C, which is about 3273 K) at a small probe in the focus spot of a big concave, concentrating mirror of the [[Mont-Louis Solar Furnace]] in France.<ref>Megan Crouse: [https://www.manufacturing.net/news/2016/07/gigantic-solar-furnace-can-melt-steel This Gigantic Solar Furnace Can Melt Steel] manufacturing.net, 28 July 2016, retrieved 14 April 2019.</ref>

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