Editing Boiling (section)

== Types ==

=== Free convection ===

The lowest heat flux seen in boiling is only sufficient to cause [[Convection|natural convection]], where the warmer fluid rises due to its slightly lower [[density]]. This condition occurs only when the superheat is very low, meaning that the hot surface near the fluid is nearly the same temperature as the boiling point.

=== Nucleate ===
[[File:Boiling water - close up (short shutter speed).ogv|thumb|A video showing water being boiled. As boiling proceeds, more nucleation sites (where bubbles are formed) can be seen.]]
[[Nucleate boiling]] is characterised by the growth of bubbles or pops on a heated surface (heterogeneous nucleation), which rises from discrete points on a surface, whose temperature is only slightly above the temperature of the liquid. In general, the number of [[nucleation]] sites is increased by an increasing surface temperature.

An irregular surface of the boiling vessel (i.e., increased surface roughness) or additives to the fluid (i.e., surfactants and/or [[nanoparticle]]s) facilitate nucleate boiling over a broader temperature range,<ref>{{Cite journal|last1=Doretti|first1=L.|last2=Longo|first2=G. A.|last3=Mancin|first3=S.|last4=Righetti|first4=G.|last5=Weibel|first5=J. A.|date=2017|title=Nanoparticle Deposition During Cu-Water Nanofluid Pool Boiling|journal=Journal of Physics: Conference Series|language=en|volume=923|issue=1|pages=012004|doi=10.1088/1742-6596/923/1/012004|issn=1742-6596|bibcode=2017JPhCS.923a2004D|doi-access=free}}</ref><ref name="sciencedirect.com">{{cite journal|title= Pool boiling of nanofluids: Comprehensive review of existing data and limited new data|journal= International Journal of Heat and Mass Transfer|volume= 52|issue= 23–24|pages= 5339–5347|doi= 10.1016/j.ijheatmasstransfer.2009.06.040|year= 2009|last1= Taylor|first1= Robert A.|last2= Phelan|first2= Patrick E.|bibcode= 2009IJHMT..52.5339T}}</ref><ref>Robert A Taylor, Patrick E Phelan, Todd Otanicar, Ronald J Adrian, Ravi S Prasher, ''[http://digitalcommons.lmu.edu/cgi/viewcontent.cgi?article=1017&context=mech_fac Vapor generation in a nanoparticle liquid suspension using a focused, continuous laser]'', Applied Physics Letters, Volume 95,  Issue 16, 2009</ref> while an exceptionally smooth surface, such as plastic, lends itself to [[superheating]]. Under these conditions, a heated liquid may show [[boiling delay]] and the temperature may go somewhat above the boiling point without boiling.

Homogeneous nucleation, where the bubbles form from the surrounding liquid instead of on a surface, can occur if the liquid is warmer in its center, and cooler at the surfaces of the container. This can be done, for instance, in a microwave oven, which heats the water and not the container.

===Critical heat flux===
[[Critical heat flux]] (CHF) describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat [[water]]), which suddenly decreases the efficiency of [[heat transfer]], thus causing localised overheating of the heating surface. As the boiling surface is heated above a critical temperature, a film of vapour forms on the surface. Since this vapour film is much less capable of carrying heat away from the surface, the temperature rises very rapidly beyond this point into the [[transition boiling]] regime. The point at which this occurs is dependent on the characteristics of boiling fluid and the heating surface in question.<ref name="sciencedirect.com"/>

===Transition===
[[Transition boiling]] may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling.

The formation of [[Liquid bubble|bubbles]] in a heated liquid is a complex physical process which often involves [[cavitation]] and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface.

===Film===
{{main|Leidenfrost effect}}

If a surface heating the liquid is significantly hotter than the liquid then film boiling will occur, where a thin layer of vapour, which has low [[thermal conductivity]], insulates the surface. This condition of a vapour film insulating the surface from the liquid characterises ''film boiling''.