Rex skrev:
Now, back to the subject of heat, I have a strange observation that I
posted on the web a few years ago. A few people thought they had seen
the same thing, but most thought what I noticed was not real. I posted
because, if it was true, it seemed unexpected and I had never heard
anything that could explain it.
I was welding or heat treating steel. Imagine a steel bar about 1 inch
(2.54 cm) in diameter and a foot to 18 " (30-40 cm) long. The bar is
clamped in a vise and with a torch one end is quickly brought up to red
heat. The other end is still cool enough that with my bare hand I can
hold the bar by the cool end and carry it into the next room. I carry it
there to cool it in the sink. A stream of cold water turned on, I
quickly cool the hot end in the water. My observation, from doing this
several times, is that the cold water quickly absorbes heat from the red
end, but also seems to chase a lot of the heat quickly up toward the
cold end, making the bar rapidly uncomfortable to hold. So that's my
observation. I think the sudden cooling of the very hot end has somehow
chased a glob of heat toward the cool end. If true, I have no
explanation. I don't think it is related to steam; it seems to me to be
something happening inside the bar.
The propagation speed for the heat comes into play. It takes time for
the heat from the hot end to reach the cold end.
As we are fairly off track here, let me relay a similar story. My mother
has been working with food all her professional life. A christmas
tradition here in Sweden is to have big lumps of ham from which you
carve slices. However, the damn thing needs to be cooked. If you do it
in the oven it dries out, if you only boil it you do not get that crisp
surface people want. You can do a bit of both. However, one year she
thought about cooking it in the microwave oven. She has no formal
training in thermodynamics and didn't really involve me in the thought
process, but she figured that if she ran the microwave for half an hour,
after wrapping the ham in microwave-grade plastic, just to avoid it to
dry out, and then just let it sit on the bench, then it would hit those
70 degrees in the core after a while anyway. Sure thing, it did. Worked
like a charm. Perfectly cooked, juicy. What happends is that it takes
time for the heat-wave to reach the core, so even if she stopped
providing more heat the heat-wave was still in progress and just could
not be stopped.
Most people thought it was coincidence of heat propagating up the bar
just at that time, or steam. Could be, but I still think it is real. The
cold end of the bar was slowly getting warmer as I carried it, but after
the sudden cooling of the hot end, the cold end seemed to get hot fast.
Yes... it would have got hot regardless of cooling or not at the hot end.
Also, the heat-wave wavefront isn't a flat surface...
I meant to try an experiment with two bars and dual thermocouples, but I
never got around to it. The main problem is getting things close enough
to compare without questioning the heated states. My plan would have
been: attach two themocouples to the cold end of two identical bars.
Heat the two other ends rapidly to red heat (that is the very hard part
to get right and balanced) and then just cool one bar rapidly while
recording both temp profiles of the cold ends. If I figure out how to
do the heating quick and balanced, I may still try the experiment.
That would be a neat exercise to demonstrate things...
So I started with a bit of complaining about the rambling of the thread,
and now I've rambled it in a whole nother direction. Sorry, I guess.
I guess it is an interesting side topic, the food aside.
Cheers,
Magnus
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