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Tag: nucleate boiling

  • Different Kinds of Boiling

    Different Kinds of Boiling

    When you put a pot of water on to boil, you probably don’t give much thought to the process. In our daily lives, we pretty much only see one kind of boiling: the sort where lots of small bubbles form on a hot surface and then rise. That’s nucleate boiling (top image), and it’s typical when you have a surface close to the boiling point of a liquid. 

    But when you continue raising the temperature of the surface, you get a transition to a different boiling regime (middle image). In this final regime (bottom image), a film of vapor envelopes the heated surface; hence its name: film boiling. Because vapor is less efficient for heat transfer than a liquid, a surface undergoing film boiling can become much, much hotter because it cannot transfer its heat away efficiently. In this experiment, the tube starts at 375K during nucleate boiling and rises to a temperature nearly three times higher during film boiling. (Image credit: TSL, source)

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    Peering Inside the Kettle

    Here natural convection is explored experimentally in a quasi-2D environment. The researchers demonstrate how this phenomenon, which is much like that seen in a boiling pot, can be investigated by measuring the refractive distortions caused by the thin heated fluid layer. They also demonstrate types of boiling that can occur.  Typically, bubbles nucleate at the heated surface and then rise to pull hot fluid with them.  At high enough temperatures above the liquid’s boiling point, however, an unstable layer of vapor can form over the heated surface.  This “boiling crisis” or critical heat flux produces a marked reduction in heat transfer due to the insulation provided by the vapor layer. (Video credit: S. Wildeman et al.)