heat transfer in a steel rod ??



Heat can only spontaneously conduct from an area of higher temperature to an area of lower temperature. You cannot have a "pulse" of heat that travels in one direction. There will be a flow of heat from the hot end to the cold end.
The only way for the cold end to get hotter quicker is if the heat transfer is hastened. For a rod of a given length and a given material, the only way to speed up the heat transfer in this case is to *increase* the temperature difference. There is no other way.
Don Kansas City
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I agree but I thought there might be some other phenomena that I wasn't aware of. Hank

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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
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"Don A. Gilmore" wrote:

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
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Henry Kolesnik wrote:

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
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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 <repeat> 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.
<repeat post 1 on> As an example, from page 34 of http://web.mit.edu/2.151/www/Handouts/EnPwrFlow.pdf (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? <repeat post 1 off>
<repeat post 2 on> 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 http://www.tu-harburg.de/mst/deutsch/lehre/mikrosystementwurf/pdf/kap6_eng.pdf 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." <repeat post 2 off>
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s.morra wrote:

Posting e-mail addresses in usenet is a good way to get spam. I hope he has spam blocking..
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Oops! Sorry about that and thanks for pointing it out. I forgot about that new feature of the internet, as I haven't posted in many years until now.


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s.morra wrote:

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.
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Jeff Finlayson wrote:

Sorry to go on. I hate spam. Good Day to you.
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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?
suggested

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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
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"s.morra" wrote:

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
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jim wrote:

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
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"dave.harper" wrote:

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
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jim wrote:

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
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"dave.harper" wrote:

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
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jim wrote:

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
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"dave.harper" wrote:

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
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jim wrote:

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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