Editing Heat transfer (section)

==Phase transition==
[[Image:Lightning in Arlington.jpg|thumb|[[Lightning]] is a highly visible form of [[energy]] transfer and is an example of plasma present at Earth's surface. Typically, lightning discharges 30,000 amperes at up to 100 million volts, and emits light, radio waves, X-rays and even gamma rays.<ref>See [http://www.nasa.gov/vision/universe/solarsystem/rhessi_tgf.html Flashes in the Sky: Earth's Gamma-Ray Bursts Triggered by Lightning]</ref> Plasma temperatures in lightning can approach 28,000 kelvins (27,726.85&nbsp;°C) (49,940.33&nbsp;°F) and electron densities may exceed 10<sup>24</sup> m<sup>−3</sup>.]]
[[Phase transition]] or phase change, takes place in a [[thermodynamic system]] from one phase or [[state of matter]] to another one by heat transfer. Phase change examples are the melting of ice or the boiling of water.
The [[Mason equation]] explains the growth of a water droplet based on the effects of heat transport on [[evaporation]] and condensation.

Phase transitions involve the [[State of matter#Four fundamental states|four fundamental states of matter]]:
* [[Solid]] – Deposition, freezing, and solid-to-solid transformation.
* [[Liquid]] – Condensation and [[Melting|melting / fusion]].
* [[Gas]] – Boiling / evaporation, [[Plasma recombination|recombination]]/ [[deionization]], and [[Sublimation (phase transition)|sublimation]].
* [[Plasma (physics)|Plasma]] – [[Ionization]].

===Boiling===
[[Image:Kochendes wasser02.jpg|thumb|left|Nucleate boiling of water.]]
The [[boiling point]] of a substance is the temperature at which the [[vapor pressure]] of the liquid equals the pressure surrounding the liquid<ref>{{cite book |author=David.E. Goldberg |title=3,000 Solved Problems in Chemistry |edition=1st |publisher=McGraw-Hill |year=1988 |isbn=0-07-023684-4 |at=Section 17.43, page 321}}</ref><ref>{{cite book |author=Dupont |first1=R. Ryan |title=Pollution Prevention: The Waste Management Approach to the 21st Century |last2=Theodore |first2=Louis |last3=Ganesan |first3=Kumar |publisher=CRC Press |year=1999 |isbn=1-56670-495-2 |at=Section 27, page 15}}</ref> and the liquid [[Evaporation|evaporates]] resulting in an abrupt change in vapor volume.

In a [[closed system]], ''saturation temperature'' and ''boiling point'' mean the same thing. The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase. The liquid can be said to be saturated with thermal energy. Any addition of thermal energy results in a phase transition.

At standard atmospheric pressure and '''low temperatures''', no boiling occurs and the heat transfer rate is controlled by the usual single-phase mechanisms. As the surface temperature is increased, local boiling occurs and vapor bubbles nucleate, grow into the surrounding cooler fluid, and collapse. This is ''sub-cooled nucleate boiling'', and is a very efficient heat transfer mechanism. At high bubble generation rates, the bubbles begin to interfere and the heat flux no longer increases rapidly with surface temperature (this is the [[Nucleate boiling#Departure from nucleate boiling|departure from nucleate boiling]], or DNB).

At similar standard atmospheric pressure and '''high temperatures''', the hydrodynamically quieter regime of [[Boiling#Film|film boiling]] is reached. Heat fluxes across the stable vapor layers are low but rise slowly with temperature. Any contact between the fluid and the surface that may be seen probably leads to the extremely rapid nucleation of a fresh vapor layer ("spontaneous [[nucleation]]"). At higher temperatures still, a maximum in the heat flux is reached (the [[critical heat flux]], or CHF).

The [[Leidenfrost Effect]] demonstrates how nucleate boiling slows heat transfer due to gas bubbles on the heater's surface. As mentioned, gas-phase thermal conductivity is much lower than liquid-phase thermal conductivity, so the outcome is a kind of "gas [[thermal barrier]]".

===Condensation===
[[Condensation]] occurs when a vapor is cooled and changes its phase to a liquid. During condensation, the [[latent heat of vaporization]] must be released. The amount of heat is the same as that absorbed during vaporization at the same fluid pressure.<ref>{{Cite book |last=Tro |first=Nivaldo |year=2008 |title=Chemistry: A Molecular Approach |publisher=Prentice Hall |pages=479 |location=Upper Saddle River, New Jersey |quote=When a substance condenses from a gas to a liquid, the same amount of heat is involved, but the heat is emitted rather than absorbed.}}</ref>

There are several types of condensation:
* Homogeneous condensation, as during the formation of fog.
* Condensation in direct contact with subcooled liquid.
* Condensation on direct contact with a cooling wall of a heat exchanger: This is the most common mode used in industry: {{unordered list
| Filmwise condensation is when a liquid film is formed on the subcooled surface, and usually occurs when the liquid wets the surface.
| Dropwise condensation is when liquid drops are formed on the subcooled surface, and usually occurs when the liquid does not wet the surface.
}} Dropwise condensation is difficult to sustain reliably; therefore, industrial equipment is normally designed to operate in filmwise condensation mode.

===Melting===
[[File:Melting icecubes.gif|thumb|upright|Ice melting]]
[[Melting]] is a thermal process that results in the phase transition of a substance from a [[solid]] to a [[liquid]]. The [[internal energy]] of a substance is increased, typically through heat or pressure, resulting in a rise of its temperature to the [[melting point]], at which the ordering of ionic or molecular entities in the solid breaks down to a less ordered state and the solid liquefies. Molten substances generally have reduced viscosity with elevated temperature; an exception to this maxim is the element [[sulfur]], whose viscosity increases to a point due to [[polymerization]] and then decreases with higher temperatures in its molten state.<ref>C. Michael Hogan (2011) ''Sulfur'', Encyclopedia of Earth, eds. A. Jorgensen and C. J. Cleveland, National Council for Science and the environment, Washington DC</ref>