Temperature of boiling surface

[Yarbles]

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?

 

[Chestermiller]

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?

What is your assessment of this?

 

[Yarbles]

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.

 

[russ_watters]

You are correct: the temperature at the surface of boiling water must be nearly exactly the boiling temp.

 

[Yarbles]

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?

 

[russ_watters]

What could I do to get my pot surface hotter than boiling temp?

You mean the surface of the water? Nothing. Water cannot be liquid above its 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?

I'm not sure what you are referring to. Could you explain?

 

[jbriggs444]

What could I do to get my pot surface hotter than boiling temp?

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".

 

[Nidum]

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 .

 

[russ_watters]

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".

Oops, forgot about superheating. It's a good caveatime, though yes, I agree is outside the scope of the OP.

 

[Yarbles]

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 .

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.

 

[CWatters]

What could I do to get my pot surface hotter than boiling temp?

Change the boiling temperature by increasing the pressure?

Allow the surface to become coated with an insulator such as scale?

 

[Yarbles]

...I'm not sure what you are referring to. Could you explain?

I was referring to correlations that give a delta between boiling surface temp (the solid surface) and boil temp.

 

[Yarbles]

Change the boiling temperature by increasing the pressure?

Allow the surface to become coated with an insulator such as scale?

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!

 

[jim mcnamara]

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.

 

[Merlin3189]

What could I do to get my pot surface hotter than boiling temp?...
...I'm interested in the temperature of the pot's surface and how much it can differ from boil temp.

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 100^oC 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?

 

[CWatters]

If I change the boiling temp will the boiling surface (is 'hot wall' a better term?) change by the same amount?

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".

 

[jbriggs444]

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.

Which surface of the pot? It has two. One in contact with the water and one in contact with the flames/heating element.

 

[russ_watters]

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.

Inside surface or outside surface? The inside surface won't be much above boiling temp, but the outside surface can be.

 

[Yarbles]

Inside surface or outside surface? The inside surface won't be much above boiling temp, but the outside surface can be.

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.

 

[jbriggs444]

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.

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

 

[Yarbles]

If I change the boiling temp will the boiling surface (is 'hot wall' a better term?) change by the same amount?

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".

If I increase boiling point then won't the boiling surface increase in temperature until boiling begins again? At that point, if heat in (power of the burner) is unchanged then the rate of boiling (mass of steam created per second) should remain unchanged?

 

[jbriggs444]

If I increase boiling point then won't the boiling surface increase in temperature until boiling begins again? At that point, if heat in (power of the burner) is unchanged then the rate of boiling (mass of steam created per second) should remain unchanged?

If you increase the temperature of the inner surface of the pan by (for instance, 1 degree) then the temperature of the outer surface of the pan will change as well. Possibly not by 1 degree but by some amount.

If you increase the temperature of the outer surface of the pan by some amount then the rate of heat transfer from the flames may change. Not by much, probably, but by some amount.

If you increase the boiling temperature by some mechanism, the latent heat involved in the transition from liquid to gas may also change.

Where is Chester Miller when you need him :-)

 

[Yarbles]

Not facetious at all Merlin3189. I mentioned bubbles causing insulation just as a possible cause for the delta between the heating surface and boiling point. I don;t want to consider solids added to the surface tho, just fluid mechanisms.

...

Finally, why? As usual with PF questions, I wonder, what is it that is really wanted here?

Not sure what a 'PF' question is.
The question/thread is just intended to increase my understanding and point me towards mechanisms that hadn't occurred to me yet. I'm not trying to solve a particular problem. When self-educating it's very hard to know what to learn. Threads like this often send me down interesting paths that I wouldn't come across on my own but that often prove very useful.

 

[Merlin3189]

Not sure what a 'PF' question is.
The question/thread is just intended to increase my understanding and point me towards mechanisms that hadn't occurred to me yet. I'm not trying to solve a particular problem. When self-educating it's very hard to know what to learn. Threads like this often send me down interesting paths that I wouldn't come across on my own but that often prove very useful.

Quite agree about the interesting paths. And strongly approve of self education. Both reasons why I participate PF.
What I mean by PF questions, is that often it takes many posts to find out what people are really asking. Often with the best of intentions people simplify their question or take some particular aspect to ask about, rather than telling the full story.
Here, for example, I still can't see quite why you want to make the boiling surface hotter than the boiling temperature of the liquid? IMO there must be micro-variations in temperature where bubbles form, where eddies form in the liquid. I think JBriggs has also got into the question of temperature differences through the thickness of the plate: these must exist as no substance is a perfect conductor at the sort of temperatures in question. Should we focus on any of these, or are they just negligible?
I see now you are interested in the mechanisms of heat transfer. That is (to me) a much more understandable question, even though I have no knowledge of the answer! I'm sure this is a well researched area and I would not attempt to post on such a complex question in the way that I would try to help someone wondering how he managed to melt the bottom of his kettle.

 

[Yarbles]

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

Post 6 states 'Water cannot be liquid above its boiling temp'. No mention of inner pot surface.

Why would we ignore the possibility of superheating?

 

[jbriggs444]

Post 6 states 'Water cannot be liquid above its boiling temp'. No mention of inner pot surface.

The water and the inner pot surface are in contact. They must be at the same temperature or have an infinite rate of heat flow.

Why would we ignore the possibility of superheating?

Because the problem becomes much more difficult if we have to account for the number of nucleation sites that may or may not exist.

 

[Nidum]

There is a boundary layer with a very steep temperature gradient between the pot inner surface and the bulk of the water .

 

[Yarbles]

Quite agree about the interesting paths. And strongly approve of self education. Both reasons why I participate PF.
What I mean by PF questions, is that often it takes many posts to find out what people are really asking. Often with the best of intentions people simplify their question or take some particular aspect to ask about, rather than telling the full story.
Here, for example, I still can't see quite why you want to make the boiling surface hotter than the boiling temperature of the liquid? IMO there must be micro-variations in temperature where bubbles form, where eddies form in the liquid. I think JBriggs has also got into the question of temperature differences through the thickness of the plate: these must exist as no substance is a perfect conductor at the sort of temperatures in question. Should we focus on any of these, or are they just negligible?
I see now you are interested in the mechanisms of heat transfer. That is (to me) a much more understandable question, even though I have no knowledge of the answer! I'm sure this is a well researched area and I would not attempt to post on such a complex question in the way that I would try to help someone wondering how he managed to melt the bottom of his kettle.

Ah, I don't actually want to make the solid surface hotter but I'm interested in what could/would make it hotter than boiling temp. Temperature gradient through the thickness is not something I need to consider.

I appreciate your patience with my PF question (I'm assuming it doesn't stand for Perfectly Formed?). At the very least, trying to communicate what I'm trying to understand is forcing me to get things straight in my head.

 

[Yarbles]

There is a boundary layer with a very steep temperature gradient between the pot inner surface and the bulk of the water .

Does the pot side of the boundary layer go above boiling temp? The microwaved super-heated water example above suggests this might be possible, maybe while the water waits to nucleate? (note I know next to nothing about nucleation)

 

[Nidum]

Does the pot side of the boundary layer go above boiling temp?

Yes .

 

[russ_watters]

Because the problem becomes much more difficult if we have to account for the number of nucleation sites that may or may not exist.

I liked the reason you gave before: the OP specifies that the water is boiling and the water can't be boiling and superheated at the same time. Boiling is what happens when the superheat gets "broken".

I'll also add that what happens on the inner surface of the pot is highly dynamic/unsteady/not uniform. Yes, when water is touching the pot it is at boiling, but then it locally forms tiny bubbles of steam that can get hotter and grow until they break away from the surface of the pot and rise in the water.so the surface of the pot touching steam can (will) be hotter than water's boiling point.

 

[256bits]

Ah, I don't actually want to make the solid surface hotter but I'm interested in what could/would make it hotter than boiling temp. Temperature gradient through the thickness is not something I need to consider.

I appreciate your patience with my PF question (I'm assuming it doesn't stand for Perfectly Formed?). At the very least, trying to communicate what I'm trying to understand is forcing me to get things straight in my head.

Hello Yarbles,

I just have to write something for you about boiling.

Take as an example an electric kettle, freshly filled from the tap.

When you first plug it in the liquid water is at a temperature below its saturation temperature, or subcooled. Basically all that means is that the water is below its normal boiling temperature of 100C. The hissing that you begin to notice is the element beginning to heat up ABOVE the saturation temperature of the water, as micro bubbles begin to form on the surface, grow, and detach into the liquid, and collapse before making it to the surface. The movement of the bubbles churns up the liquid water and this allows for a greater heat transfer coeficient, and subsequentially more heat flux transfer from the element to the water. The water increases in temperature, and so does the element surface temperature. The element temperature increases more and more above the saturation temperature of the water as time progresses.
This is the boiling regime for water with the difference in the temperature of the heating element being above the temperature of the bulk water from 4C to 10C.

From 10C to 30C temperature difference, more bubbles form more quickly on the element surface. As a result, the element is being interferred with in transferring heat, and while the heat flux still is increasing, it does so at a lessening rate. There is a maximum rate of heat transfer at 30C temperature difference, and up to this point, you should be observing a rolling boil.
Most cases you do not want to go above this temperature difference.

Above 30C difference, a film of vapour surrounds the element, and heat transfer beigins to transform from conduction to radiation.
Same effect when you put a drop of water on a hot skillet and see it bouncing around on a film of vapour.

Further increase in temperature difference and the element will eventually melt.

Here is your equation, which is for the basic engineering initiation to boiling.

How exact that equation is, well, more study is always being done on boiling.

You can read more at
https://en.wikipedia.org/wiki/Nucleate_boiling

More at,
http://thermopedia.com/content/990/

 

[Yarbles]

Thanks 256bits that's a lot of stuff to explore. I really appreciate you spending the time. And everyone else of course.