Well, you never know. There are some interesting phenomena out there
sometimes, though they are eventually explained by science.
I used to think the old adage "hot water freezes faster than cold water" was
an idiotic wives' tale. But then I saw it addressed in a magazine Q&A where
they treated it seriously. They theorized that a warm ice cube tray might
melt the frost under it in the freezer and surround the tray with liquid
water that would enhance its heat transfer. I'm still not buying it though.
Don
Kansas City
Yes there is - simply heat the rod in the middle. Your model is way to
simple. Its not simply a matter of inputting heat at one end and and
outputting it at the other.
This is not exactly the same as Henry, but I've done this: Take a 20"
rod and heat it with an acetylene torch near the end while holding the
other end. After a couple minutes its cherry red . Then clamp the end in
a vise and bend it in an L. Then use a hammer and anvil to complete the
bend into a J. When finished toss it in a bucket with about 15" of
water. When you toss it in the water the end your holding is still stone
cold. after 30-40 seconds lift it out of the water now the end your
holding is quite warm. I'm pretty sure had I beat on it for an
additional 30 seconds the temperature of that end would not have changed
noticeably.
-jim
To get a complete picture of what exactly happened, we'd have to know
more about the elapsed time for each case (quench vs. hammer), as well
as the moments preceding the rod heating. For instance, if you had
heated the rod up from a cold state, hammered it, then let it cool, you
might not notice a huge temperature change in the held end. However,
if you heated it, went to ask a question about something, came back,
then quenched it, the heat might have a lot longer to travel up the
rod. If something like that happened, it would actually be hotter than
if you'd hammered it outright. There's many other factors that could
affect it also. Ex: Was it heated 5 minutes ago, then cooled, then
heated again prior to quenching?
If you can give us a minute-by-minute blow of exactly what happened and
how much time events took, that'd help a lot. Although given the time
that's passed, that might be a tall order. I can't even remember what
I watched on TV last night...
Dave
What an interesting and timely account given by Mr. Kolesnik, as I have
experienced the same phenomenon. I recently posted in several physics
newsgroups looking for an explanation after ~20 years of wondering about it.
Responses have been sparse and mostly theoretical guesswork or conjectures
(which I appreciate and consider), but without any experience of the
phenomenon or knowledge of cases (my actual question). One respondent in
sci.physics.research ( snipped-for-privacy@imperial.ac.uk) suggested that I try asking
in an engineering newsgroup (he also mentioned hearing about "thermal
inertia" being used to describe this phenomenon). I started doing that
today and came across Mr. Kolesnik's account. So I will my post
below after a few comments.
In response to a sci.physics respondent (tadchem, Tom Davidson), I would
like to point out in the account I give below that the temperature
measurement was differential with both hands starting at room temperature.
He is generally right in saying "The human nervous system is a notoriously
unreliable and impossible to calibrate sensor", especially with single-ended
measurements (one hand). But the statement also implies an exaggeration to
the exclusion of trusting what we sense as humans and trusting only modern
test data. I see that Mr. Kolesnik experienced some of the same in his
respondents, but in his case he repeatedly experienced the same phenomenon
(I experienced it once), and neither of us fell of the potato truck
yesterday. True, in modern life it is possible to measure the phenomenon
with electronic instrumentation, and wouldn't that be nice to do. But such
a thing requires some effort and costs, which also in modern times is
generally not done without some chance of a payback on the investment. Such
an experiment might be done when someone with authority over efforts and
costs decides that maybe Mr. Kolesnik and I actually experienced something
interesting, interesting enough to imagine a payback of some kind for their
operation. Until then, the rest of us are stuck with our nervous systems to
observe with our hand (differential is better) the heat spike produced when
a steel rod with a red hot end is quenched. Like Mr. Kolesnik, it isn't the
steam produced.
As an example, from page 34 of
formatting link
(other references easy
to find)
"Thermal Inductance: No significant physical phenomenon has been observed
which corresponds to energy storage due to heat flow in a "thermal
inductor". Thus only one thermal energy storage element, the thermal
capacitance, is defined."
and of course, we have thermal resistance.
So it appears that thermal resistance and thermal capacitance but no thermal
inductance phenomena have been observed. Does anyone know of any cases
where thermal inductance has been observed?
Thanks for your response (and Mikko Kiviranta's), as there have been very
few in this (and other newsgroups). I am asking others about their
knowledge of any cases of thermal inductance being observed, because it is
one explanation for an event that I experienced about 20 years ago. I'll
briefly describe the event.
I was holding a 6 foot long piece of rebar (about 1/2" diameter steel rod,
commonly used to reinforce concrete) at arms length while a friend I was
helping was cutting it in half with an oxy-acetylene torch (I think steel
melts at ~1400 C). It was cut in half and both ends were brightly red hot.
After cutting, I moved my hands towards the ends to hold one 3 foot piece in
each hand at the cool end. There was a D-shaped tank (about 12" wide, 3
feet long, 2 feet high, flat of D facing up) filled with water nearby, about
2 inches from the top. So I dipped one of the hot ends in the water
gingerly a few times to cool it at about a 30 degree angle from the floor.
I didn't want to make a lot of steam quickly, as I thought that rapid
cooling might alter the metal somehow and my friend was going to weld the
two pieces to a piece of angle iron afterwards. The steam ball made was
about 4-6 inches in diameter at most as I recall. Within 15 seconds or so,
the 3 foot rod I was cooling became so hot that I couldn't hold it anymore,
so I dropped it into the tank. I was still holding the other rod in my
other hand (still glowing red hot) and it was only slightly warm where I
held it. I cooled it off too and we went about the next step of welding the
two pieces to the angle iron.
I do not have access to a torch or rebar to recreate the event since then,
but I've often thought about what I experienced ever since. I wonder if I
experienced thermal inductance, analogous to an inductive kick in
electronics. I still wonder about it and would appreciate any considered
explanations you may have (or others in this group).
I looked into the case you state regarding the violation of the second law
and found an interesting statement on page 144 of
formatting link
that leads towards "extended irreversible thermodynamics" (I'm still reading
on this). It notes the same thing you stated, and goes on to say:
"Now we can only draw the conclusion that this effect is usually of no
importance for us, but in no way we can conclude that there is no such thing
like a thermal inductance. An apparent contradiction results between
thermal inductance and the second law of the thermodynamics, which states
the well known fact that entropy is always increasing. By this it is
usually concluded, that a heat flow is directed along the gradient of
temperature. On the other hand, due to the relation (see text) a constant
heat flow can appear at a thermal inductance also if the temperature
difference disappears, which apparently contradicts the statement of the
second law of thermodynamics. The above consideration shows that the
phenomenon of thermal inductance is of importance, if we consider a
high-speed process, for example the heating by short-time laser pulses.
Classical thermodynamics assumes slow changes, so that the system is in a
quasi-stationary status. By inclusion of an additional state variable (i.e.
the flux of energy), it is possible to introduce a definition of entropy,
which is in accordance to the second law of thermodynamic, whereby the
contradiction is eliminated. This theory is call extended irreversible
thermodynamics."
This has been going on for several years at least. Besides posting
someone else's e-mail address on a public forum without permission
is a bad idea in general.
Point taken. I hate spam too, but it still comes. So I sort it with ~20
filters, go through it (in case a legitimate email gets sent), and delete
it. The ISPs seem to be catching on, because the amount of spam has dropped
over an order of magnitude in the last year (likely some law was enacted).
Regarding my post, do you know of any cases or have heard of any stories
regarding the "thermal inductance" being observed from quenching a red hot
steel rod in water?
Dear s.morra:
...
groups.google.com
search for:
rod anvil "sci.engr.mech"
posting by:Henry Kolesnik
dated: 2005jun25
title: heat transfer in a steel rod ??
... and he got the same advice you did.
David A. Smith
The assumption that this phenomena is an illusion or is due to "thermal
inductance" is incorrect. The cause is simply due to the enormous heat
transfer and expansion capabilities of steam. That's right it *is*
caused by the steam produced.
You state:
"The steam ball made was about 4-6 inches in diameter at most as I
recall. Within 15 seconds or so, the 3 foot rod I was cooling became so
hot that I couldn't hold it anymore, so I dropped it into the tank."
The steam ball as you describe it may well be 4"-6" in size under
water, but had you held your hand a foot or so from where it was
emerging from the water your hand would have gotten very hot (perhaps
scalded) as the steam expands greatly when it is not confined by water
pressure (note: steam is invisible). The metal rod that is emerging from
the water is exposed to this same heat that your hand would feel if you
were brave enough to place your hand where the rod emerges from the
water. Even in the under water part steam is very rapidly removing heat
from the red hot end and rapidly transferring that heat outwards (which
includes the rest of the rod). Steam is the mechanism for the rod
becoming heated at a distance from the red hot end.
-jim
I'm sure convection from steam plays a part in accelerating the heat
transfer up the rod, but I wouldn't give that theory the green light
without some numbers. You basically have 2 situations:
1. A large temperature gradient in the steel rod in ambient air
2. A lesser thermal gradient in the steel at the hot end, but
convection adding additional heat a couple inches up from the hot end.
You contend that situation #2 produces a higher heat flux further up
the rod, but remember you're also taking a HUGE amount of thermal
energy away from the steel when you vaporize water. That drops the
heat flux from conduction significantly. So I wouldn't bite on that
theory unless you could apply some numbers to it.
Dave
Your sure but you doubt it?
I contend while quenching the ambient temperature further up the rod is
hotter than the rod thus heating it.
but remember you're also taking a HUGE amount of thermal
When the rod is not quenched the heat flow from conduction is
insignificant. The rod not quenched shows no noticeble change in
temperature at the held end while in the same period of time its
quenched counterpart will show a large change in temperature at the held
end.
Don't need numbers - any fool would know not to put their hand near
where the steam is being generated. If it's so easy to understand the
concept of your hand getting hot, why is it difficult to understand that
the part of the rod that is in the same location where your hand would
be also gets hot? This is not a difficult experiment. Just try it.
-jim
Yes. I'm sure it occurs, but I doubt it's significant.
I wouldn't say that heat flow from conduction is insignificant. It's
possible that this phenomenon occurs from other mechanisms, or it's
incorrectly percieved by the rod holder.
That doesn't mean that steam convection is significant enough to induce
a heat flux high enough to cause a noticable temperature change further
up the rod.
The rod is hot so your hand gets hot? I don't think that leads any
support to your argument.
It's even easier to solve the heat equation for this situation. I'll
solve it if someone can estimate a heat transfer coefficient for steam
under these conditions.
Dave
Your wrong.
maybe its voodoo magic?
"induce a heat flux" what does this mean and who has ever said this is
what is happening.
Please try to read and comprehend. Your hand would get hot from the
steam if you were dumb enough to place your hand at the same distance
(in any direction) from the red hot end as the part of the rod that is
being heated is. If you were to actually try this you would have no
trouble understanding where the heat was coming from. The steam as it
emerges from the water will expand outwards very rapidly.
That would be like trying to determine how much of the heat transfer in
steam heating system comes from the steam and how is conducted through
the metal piping. The magnitude of the 2 are so disparate as to make any
calculation meaningless.
-jim
Ok, if you're not familiar with what 'inducing heat flux' means, then I
don't think you should have attempted to answer this question. Anytime
heat transfer occurs, there is "heat flux" involved, so this is
definately happening in this situation. It's the amount of heat going
through a surface divided by the cross-section of that surface.
So you're saying it was the steam directly on the holder's hand that
caused it to seem hotter? Henry Kolesnik, the originator of this
question, stated very early in this thread:
Note the "my palm felt the heat so it wasn't steam".
That's an incorrect analogy. Steam in a heating system is surrounded
by pipe, so heat has to leave through the pipe. In the rod scenario,
only a fraction of the steam's heat is tranfsered to your hand OR back
to the rod. Most of it escapes to the air.
Secondly, steam moving through a thin-walled pipe vs. steam convection
around a 1" thick rod produce drastically different heat transfer
coefficients (i.e. one is much more severe than the other).
Dave
I know what the words mean separately. I don't know what you think they
mean strung together in the context of interpreting how the heat gets
from the hot end of the rod to the cold end. The condensation of steam
transfers heat to the rod.
No I did not say that. I've said it enough times already there's little
hope for you comprehending if I say it again.
Yes but he held his hand just outside the envelope where the steam would
heat it. He knew if he held his hand closer he would have felt the heat
that was heating the rod. Why is that statement so hard to understand?
Steam when it condenses transfers heat at a rate much greater than
simply transferring heat from a gas to a solid. Not only that the
condensation causes a rapid pressure drop that draws more steam. A good
bit of the heat contained in the steam is transferred to the water
surface and any other surface that it contacts and condenses on. Due to
condensation the steam expands outward on the waters surface much more
than an ordinary hot gas would. Condensation is responsible for most of
the heat transfer to the rod and to the water.
Hmmm, I would think the condensation of a gram of water would release
the same amount of heat no matter where it occurs.
-jim
Earlier you stated (your exact words): "Your hand would get hot from
the steam". If you really meant condensation (i.e. phase change) from
the start, you should have said that. Not one of your previous posts
even mentioned the words "condensation" or "phase change".
Well, this statement certainly sounds like you thought steam on the
holder's hand was the main heating mechanism: "Your hand would get hot
from the
steam." Please be more clear. You didn't say steam would condense on
the rod, heat the rod, and then conduct towards your hand.
That's a general statement that's only correct under the right
circumstances. It depends on the MANY factors (i.e. temperature
difference, reynolds number, geometry, gas properties, etc).
Wrong. It is occuring at ambient pressure (about 14.7psi). There's
practically no pressure drop. The steam is not really "contained" in
anything, and the pressure due to being under water is only .43 PSI
more under 1ft of water. The steam has very little resistance to
expand or contract.
Yes, I agree with that, depending on what you mean by "good bit".
1. Why do you think steam would act differently than any other gas,
other than some of it condensing? Steam is lighter than air, and
should rise away from the water.
2. Don't you mean contract? Condesation would reduce the volume of the
steam.
Yeah, it would release the same amount of heat that it took to vaporize
it, which was taken from the rod to begin with. So you're saying you
want to vaporize X grams of water and have X% of it recondense further
up the rod. Only a FRACTION of the total steam will recondense on the
rod. Most will escape into the air, and the heat transfered up the rod
would be pretty small. How about some numbers... what percentage of it
do you think will recondense on the rod? And where do you think it
will recondense on the rod?
Dave
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