Prentice Bisbal wrote:
Lux, Jim (337C) wrote:
If you put something hot into a liquid, you have to worry about forming
a film of vapor that keeps the liquid from touching the hot thing, and
radically reduces the heat transfer. It’s all tied up with the
turbulence in the liquid, the surface tension of the liquid, etc.
I'm having flashbacks of my Transport Phenomena class from college.
Thanks, Jim!
There is treatment available...
Boiling is a really good way to move heat: the heat of vaporization is
huge, for a small temperature change,
Technically, the heat of vaporization occurs at zero temperature change. ;)
compared to just the liquid’s
specific heat. But, it’s more complex to design. It’s used in very
high power solid state electronics and in high power vacuum tubes, as
well. The key is that the boiling point of the liquid has to be close
to the desired operating temperature of the parts being cooled. Various
Freons work well.
Look up Leidenfrost effect (why LN2 droplets skitter around, or water on
a hot pancake griddle)..
It’s also related to why you can walk across burning coals in bare feet.
(the true test of belief in Physics)
Here's another party trick based on this: Fill a cup (preferably a
Styrofoam cup for insulation purposes) with liquid nitrogen (LN2) . Then
stick your finger in it and pull it out real quick. Even though LN2 is
very cold, you won't fell a thing - the heat from your finger causes the
LN2 vaporize before you even contact it, creating an insulating layer
(film) of nitrogen gas. It's not stable, so if your keep your finger in
it for longer than a split second, you WILL get freeze your finger!
Of course, this requires you bringing our own tank of LN2 to the party
in the first place.
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