Editing Heat transfer (section)

===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]]".