Temperature of boiling surface

[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.