Hal Murray wrote:
[email protected] said:
Can I get reflections without some inductance?
Is there any inductance in a system of alternating
layers of insulation/storage?
I think you are overstretching the badly chosen nomenclatures
parallels to electricity.
It was actually a (somewhat?) serious question on several grounds.
Can I get reflections from a lumped circuit model of a transmission line made
out of just Rs and Cs? If so, I can probably do the same in the thermal
world.
Can I get reflections in a thermal context? Bruce's URLs say yes, but my
math is rusty enough that I can't quickly understand what's going on.
If a thermal problem can generate reflections, does that mean it also has
something corresponding to inductance? If so, what is it?
It's possible that the key idea is time-delay. In the electrical world, a
delay is a transmission line which has both C and L. I'm not sure what the
one-dimensional equivalent in the thermal world is.
What's the speed-of-light equivalent in the thermal world?
Why were you somewhat serious about this?
If you want to extropolate heat into electromagnestic waves, what would
be the analog of frequency? There are a few parallels in the two realms
by analogy but that doesn't mean they map in all aspects. Sometimes, to
help learning ohms law, the analogy of water is used with pressure =
voltage, flow = current, resistance = narrow pipes. It sort of makes the
concepts easier to grasp, but when you get to AC and wave reflections I
think one has to struggle to make the water analogy useful. For heat, I
think the water analog might be more useful than trying to map the EM
waves to heat.
The reflection idea did remind me of something that occurred to me, a
gallows-humor joke from years back. I'm sure most of you remember
hearing about the 1989 San Francisco earthquake. The earthquake
epicenter was between Santa Cruz and San Jose, about 40 miles south of
San Francisco, but a lot of the serious damage and fires occurred in San
Francisco near the tip of the penninsula at the bay shore. There was a
lot of discussion about this localized damage so far away, and how that
could happen. San Francisco is at the tip of a peninsula that forms the
Bay. I immediately thought that the problem was obvious. The penninsula
was excited at its bottom end and was left improperly terminated at San
Francisco. I couldn't tell this joke for two reasons, one: it was in bad
taste, but two: I only knew a few people who would get it -- the
mismatch/termination joke.
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.
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.
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.
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.
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