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I Temperature of boiling surface

  1. May 1, 2016 #1
    What determines the surface temperature inside a pot of boiling water, right over the heat source? Can it go much over boiling temp if the water's just gently boiling?
     
  2. jcsd
  3. May 2, 2016 #2
    What is your assessment of this?
     
  4. May 2, 2016 #3
    From intuition: As long as the surface is covered with water (no lingering/giant bubbles) then the surface should remain right at boiling temp.
    From other sources: Heat Transfer Coefficient is involved somehow.

    However, I'm interested in any phenomena that could influence the surface temperature, not just the ones I'm already considering.
     
  5. May 2, 2016 #4

    russ_watters

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    You are correct: the temperature at the surface of boiling water must be nearly exactly the boiling temp.
     
  6. May 2, 2016 #5
    What could I do to get my pot surface hotter than boiling temp? I've seen correlations for boiling heat transfer coefficient; do those only account for bubbles insulating boiling surfaces or other phenomena as well?
     
  7. May 3, 2016 #6

    russ_watters

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    You mean the surface of the water? Nothing. Water cannot be liquid above its boiling temp.
    I'm not sure what you are referring to. Could you explain?
     
  8. May 3, 2016 #7

    jbriggs444

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    Use pure water and a clean surface. In the absence of nucleation sites, water can be raised above its boiling temperature without boiling. Most commonly, this is seen when heating water in a microwave.

    However, this goes counter to the description of the situation in post #1: "gently boiling".
     
  9. May 3, 2016 #8

    Nidum

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    Yarbles may be referring to the temperature of the shell of the heating vessel rather than the temperature of the water .

    In that case answer to question is that the shell temperature could be much higher than the water temperature . It has to be higher to some degree anyway otherwise there would be no heat transfer .
     
  10. May 3, 2016 #9

    russ_watters

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    Oops, forgot about superheating. It's a good caveatime, though yes, I agree is outside the scope of the OP.
     
  11. May 3, 2016 #10
    Yes I'm interested in the temperature of the pot's surface and how much it can differ from boil temp.
    Sorry to be unclear.
     
  12. May 3, 2016 #11

    CWatters

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    Change the boiling temperature by increasing the pressure?

    Allow the surface to become coated with an insulator such as scale?
     
  13. May 3, 2016 #12
    I was referring to correlations that give a delta between boiling surface temp (the solid surface) and boil temp.
     
  14. May 3, 2016 #13
    If I change the boiling temp will the boiling surface (is 'hot wall' a better term?) change by the same amount?

    I think I'd rather stay away from adding surface material (scale) to the mix for now.

    Thanks!
     
  15. May 3, 2016 #14

    jim mcnamara

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    It is possible to deglaze a cooking pan - SOP in the kitchen to make au jus gravy for example, after cooking meat in fat.

    The pan has to be hot - say 100C more than boiling point. Add a cup of cold water, wine, beer, fruit juice...whatever. For a very short time, until the system comes to equilibrium, the pot is so hot that there is a layer of steam on the boiling surface. The metal is above boiling point. This is also a great way to remove burned spots in a pan. After a short period either water evaporates completely or the pan surface drops to boiling point. You want the latter condition for making sauces. So, for a few seconds you get what you asked for. And a nice sauce. Bon appetit.
     
  16. May 3, 2016 #15

    Merlin3189

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    If the water is "boiling gently" that suggests to me that you want the water to be at the same temperature as the pot surface. You are minimising the heat flow, so that hotspots won't occur. If you did want to get hotspots, then a very high rate of heat flow would seem to be what you need. If you can supply heat to a piece of metal faster than it can lose that heat, its temperature must increase.

    What do you mean by your "pot surface"? If you have a piece of heated metal in contact with the water, say the base of a kettle, and you want that metal to get hotter, then you could insulate it - cover the inside of the base of your kettle with a layer of less conductive material, such as (in my home) a layer of calcium sulphate. The top surface of the insulation is then at say 100oC in contact with the water and the lower surface is necessarily at a higher temperature to support the heat flow through it.

    I hope that doesn't sound facetious: I am just trying to examine the possibilities. You have mentioned yourself the possibility of an insulating layer of water bubbles, which would probably be one of the results of high rate of heat supply.

    Since you say, "get my pot surface hotter than boiling temp" I wonder whether increasing the pressure is an answer. It raises the boiling temperature and the pot will therefore be hotter, but it is still at the (new) boiling temperature. If the pot were very tall so that the top of the water boiled at a lower temperature than the bottom, I don't know which temperature qualifies as "the" boiling temp.

    Finally, why? As usual with PF questions, I wonder, what is it that is really wanted here?
     
  17. May 3, 2016 #16

    CWatters

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    That's a badly worded question. Simply changing the boiling point won't magically change the temperature of the pan.

    However increasing the boiling temperature allows you to turn up the power of the burner raising the temperature of the pan while still meeting your condition that "the water's just gently boiling".
     
  18. May 3, 2016 #17

    jbriggs444

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    Which surface of the pot? It has two. One in contact with the water and one in contact with the flames/heating element.
     
  19. May 3, 2016 #18

    russ_watters

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    Inside surface or outside surface? The inside surface won't be much above boiling temp, but the outside surface can be.
     
  20. May 3, 2016 #19
    To be clear from here on out:

    I wish to know how much the temperature of the inside surface (solid surface that's in contact with the fluid) can differ from the current boiling temp of the fluid.
    More importantly, I'm trying to understand the mechanisms that can cause the temperature difference.
     
  21. May 3, 2016 #20

    jbriggs444

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    See post #6. Ignoring the possibility of superheating, the inner pot surface cannot be hotter than boiling.
     
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