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---Wonder how bad it is for graphite rods used in electric furnaces? Of course
graphite has a much higher melting temperature so it can withstand a strong
gradient. But graphite, with its lower thermal conductivity and higher
resistivity, probably develops a very strong gradient. Coupled with the
temperature coefficient of resistivity, it might make for an interesting
current distribution. Even for DC applications.
---On Sat, 06 Aug 2005 15:23:30 -0500, John Fields
jfields@austininstruments.com> Gave us:
Maybe, but that's sure as hell not you! You want everybody to think
you're right up there with all the facts at your fingertips, so when
someone explains something that you _didn't_ understand or know
about, you say, "Oh yeah, I knew about that",
You're full of shit, boy. If you had read the thread, you would
have noted where I said that I have had Nasa's Laser Disc on the
subject for over 15 years.
---Did your lame ass even know what decent video was back then? Did
you get into laser discs? I doubt it. You're a twit. Nothing more.
---and if you get caught
you yell and scream and stamp your feet after you try the "Oh,
wait, that's not what I meant..."
You're an idiot.
dance and you get busted on that
one too, ya slobbering, lightweight goon.
You still like mouthing off when you're more than an arm's length
away. You're still also full of shit, Stinky.
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:6d08f1d39hgu458cpg4cjchqgr91ln1l12@4ax.com...
On Fri, 05 Aug 2005 21:17:47 GMT, "daestrom"
daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:
snip
Well I *almost* agree with you. To get a severe gradient, you do need to
run a lot of current. But it still does *NOT* matter what is in contact
with the outside surface.
Sure it does. If you run a copper wire in air, and dump in enough
current to produce a decent radial gradient, it will vaporize. You'd
have to water cool it (boiling water is ideal) to sustain the power
levels necessary for a non-trivial gradient.
Copper conducts heat about 12,000 times as well as air, and there's a
lot more air available than copper in most situations. So, very
roughly, a 1mm copper wire surrounded by a 10 mm air gap, with enough
current flowing to create a 1 deg C internal gradient, will have a
surface temp of 120,000 C.
Nah... The thermal conductivity of a film coefficient for air is not the
same as the thermal conductivity of air. The thermal conductivity is only
relavent in a very thin layer against the surface (much less than 10 mm).
Moving outward, the viscosity and velocity of the air become dominant.
Given the film coefficient of air against a vertical surface of about 25
W/(m^2-K), I make it out to only be about 8,000 C. ;-) Having forced air
convection (or a good 'stiff' wind) can improve the film coefficient to
almost 200 W/(m^2-K) (down to 1,000 C ;-). Water cooling can be as high as
5000 to 10000 W/(m^2-K) (as low as 20 C).
A spiral of #10 bare copper wire in a plastic garbage can full ofBut larger wires, and those of Al can develop such a gradient more easily.
And true, boiling heat transfer can be several orders of magnitude better,
but one then has to worry about exceeding the critical heat flux (also known
as 'departure from nucleat boiling', 'boiling transition', or 'dryout').
Whether the water is circulating or not, and how far the bulk water
temperature is from saturation also become important (i.e. becomes a real
engineering nightmare).
The industry has a long history of success using pressurized hydrogen. Most
large generators and their connections to step-up transformers are cooled
this way. Much better cooling than plain air, allowing much higher current
densities. And with the same material properties, stronger temperature
gradients.
Except all of the H2-cooled gen-xfmr leads that I've seen use hollow
conductors with H2 forced through the center as well as surrounding the
outside. Similarly, the water-cooled conductors that I've seen are those
found in generators and the water flows down the center of the hollow
conductor. Not much of a temperature profile when the cross-section is
mostly cooling water ;-)
The internal gradient is a function of the heat
generated per unit mass and the thermal conductivity of the material.
Period. Nothing else.
Not once it's gaseous.
True, but one usually designs to avoid melting, much less boiling.
Fact is, in 60hz applications, the usual design restrictions regarding
skin-effect overshadow any problems with centerline temperature concerns.
Perhaps engineers working with high-current DC applications are more
concerned with the temperature gradient issues. But I suspect it is still
small for good thermal conductors like copper.
I jumped into this fray when 'TokaMundo' said, "In a wire,....would show the
wire at the same temp from center to outer surface". I think we agree this
is wrong. And I agree that the temperature gradient is not severe for
conductors made of Cu or Al under normal circumstance such as air cooling.
But *some* gradient *must* exist, otherwise the centerline temperature must
increase (due to heat generated and not conducted away) until a gradient
begins to conduct heat away as fast as it's created by the electric current.
Wonder how bad it is for graphite rods used in electric furnaces? Of course
graphite has a much higher melting temperature so it can withstand a strong
gradient. But graphite, with its lower thermal conductivity and higher
resistivity, probably develops a very strong gradient. Coupled with the
temperature coefficient of resistivity, it might make for an interesting
current distribution. Even for DC applications.
The external medium will determine the exact temperature of the outer
surface, and by virtue of the gradient for the specific material/power,
the
centerline temperature. But the shape and relative height of the gradient
is irrespective of the external surface (as long as the thermal
conductivity
and heat produced are assumed constant).
Thermal conductivity is itself a function of temperature, so the
gradient does depend mildly on the absolute temperature of the whole
rig. Especially after the copper melts.
True. But below the melting point, it isn't hard to approximate the
variance with a low-order polynomial using temperature alone as the
independent variable. I would think this would make it relatively easy to
incorporate into the integration. Haven't tried it though, so who knows???
daestrom
Yeah. The truck pulling up outside brings no joy if it can't unload"NunYa Bidness" <nunyabidness@nunyabidness.org> wrote in message
news:0539f1934hof6069l96hma7s749gvjo6v9@4ax.com...
On Fri, 05 Aug 2005 21:24:21 GMT, "daestrom"
daestrom@NO_SPAM_HEREtwcny.rr.com> Gave us:
Often called the 'zeroth law of thermodynamics', no net heat is
transferred
without a temperature difference.
It is in motion. Always. Whether in local thermal equilibrium or
not.
That's why I said *net* heat transfer.
No, but your retarded ass apparently thinks that that is when ISo you know some of what was known 15 years ago, so what? do you
think nothing new has happened since then?
---
Yeah right. You were a technician... oh... strike that... anYou're right, I never did get into laser discs much. No interest,
plus I was too busy designing equipment to go to the bottom of the
Marianas Trench instead of watching somebody else's stuff.
So your plastic garbage can melts at 212F, or you let the wire touch theOn Sun, 07 Aug 2005 16:10:37 GMT, "daestrom"
daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:
snip
But larger wires, and those of Al can develop such a gradient more easily.
And true, boiling heat transfer can be several orders of magnitude better,
but one then has to worry about exceeding the critical heat flux (also
known
as 'departure from nucleat boiling', 'boiling transition', or 'dryout').
Whether the water is circulating or not, and how far the bulk water
temperature is from saturation also become important (i.e. becomes a real
engineering nightmare).
A spiral of #10 bare copper wire in a plastic garbage can full of
water makes an impressive dummy load, up until the water gets hot
enough to melt the plastic can. Then the hot water gets loose. Keep a
good chair handy.
Yeah, ditto. Back on 8/1 I suspected this when I said...My conclusion from this thread is that skin effect can be important at
60 Hz in entirely practical situations, and thermal gradients in
copper or aluminum conductors are inconsequential unless the current
is high and the cooling novel.
But this discussion forced me to 'sharpen the pencil' and do the actualNow, given that both copper and aluminum are excellent heat conductors, it
might be interesting to calculate how big a temperature profile could be
expected, and from this calculate the variation in resistivity.
I suspect the work has been done before, and that the difference is rather
modest for all but the largest cylindrical conductors.
Yeah, especially when solid 'phase-changes' take place. Some steels undergoWe're doing some thermocouple stuff
just now (a simulator module and a complementary measurement gadget,
for jet engine testing) so thermal stuff is on my mind.
I've done a little superconductive/cryo work, where things are very
different. Here, the thermal conductivity of metals changes radically
as a function of temperature, so the net heat flow of, say, a
stainless or manganin leadwire from 4K up to to room temp is
determined by a complex integral (the bottom line of which,
fortunately, you can just look up.)
Bullshit. It isn't a threat, veiled or otherwise, now or ever, soThere ya go with those veiled threats again,
The shit the other asshole pulled were direct threats. Bigeven after ratting out
Steve Walz for the same thing, you effete little hypocrite.
That's funny since your position supports what I said about theOn Sun, 07 Aug 2005 16:10:37 GMT, "daestrom"
daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:6d08f1d39hgu458cpg4cjchqgr91ln1l12@4ax.com...
On Fri, 05 Aug 2005 21:17:47 GMT, "daestrom"
daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:
snip
Well I *almost* agree with you. To get a severe gradient, you do need to
run a lot of current. But it still does *NOT* matter what is in contact
with the outside surface.
Sure it does. If you run a copper wire in air, and dump in enough
current to produce a decent radial gradient, it will vaporize. You'd
have to water cool it (boiling water is ideal) to sustain the power
levels necessary for a non-trivial gradient.
Copper conducts heat about 12,000 times as well as air, and there's a
lot more air available than copper in most situations. So, very
roughly, a 1mm copper wire surrounded by a 10 mm air gap, with enough
current flowing to create a 1 deg C internal gradient, will have a
surface temp of 120,000 C.
Nah... The thermal conductivity of a film coefficient for air is not the
same as the thermal conductivity of air. The thermal conductivity is only
relavent in a very thin layer against the surface (much less than 10 mm).
Moving outward, the viscosity and velocity of the air become dominant.
Given the film coefficient of air against a vertical surface of about 25
W/(m^2-K), I make it out to only be about 8,000 C. ;-) Having forced air
convection (or a good 'stiff' wind) can improve the film coefficient to
almost 200 W/(m^2-K) (down to 1,000 C ;-). Water cooling can be as high as
5000 to 10000 W/(m^2-K) (as low as 20 C).
I did hedge my number with "very roughly", figuring I could be 2
orders of magnitude off and still make the point.
But larger wires, and those of Al can develop such a gradient more easily.
And true, boiling heat transfer can be several orders of magnitude better,
but one then has to worry about exceeding the critical heat flux (also known
as 'departure from nucleat boiling', 'boiling transition', or 'dryout').
Whether the water is circulating or not, and how far the bulk water
temperature is from saturation also become important (i.e. becomes a real
engineering nightmare).
A spiral of #10 bare copper wire in a plastic garbage can full of
water makes an impressive dummy load, up until the water gets hot
enough to melt the plastic can. Then the hot water gets loose. Keep a
good chair handy.
The industry has a long history of success using pressurized hydrogen. Most
large generators and their connections to step-up transformers are cooled
this way. Much better cooling than plain air, allowing much higher current
densities. And with the same material properties, stronger temperature
gradients.
Except all of the H2-cooled gen-xfmr leads that I've seen use hollow
conductors with H2 forced through the center as well as surrounding the
outside. Similarly, the water-cooled conductors that I've seen are those
found in generators and the water flows down the center of the hollow
conductor. Not much of a temperature profile when the cross-section is
mostly cooling water ;-)
The internal gradient is a function of the heat
generated per unit mass and the thermal conductivity of the material.
Period. Nothing else.
Not once it's gaseous.
True, but one usually designs to avoid melting, much less boiling.
Fact is, in 60hz applications, the usual design restrictions regarding
skin-effect overshadow any problems with centerline temperature concerns.
Perhaps engineers working with high-current DC applications are more
concerned with the temperature gradient issues. But I suspect it is still
small for good thermal conductors like copper.
I jumped into this fray when 'TokaMundo' said, "In a wire,....would show the
wire at the same temp from center to outer surface". I think we agree this
is wrong. And I agree that the temperature gradient is not severe for
conductors made of Cu or Al under normal circumstance such as air cooling.
But *some* gradient *must* exist, otherwise the centerline temperature must
increase (due to heat generated and not conducted away) until a gradient
begins to conduct heat away as fast as it's created by the electric current.
Wonder how bad it is for graphite rods used in electric furnaces? Of course
graphite has a much higher melting temperature so it can withstand a strong
gradient. But graphite, with its lower thermal conductivity and higher
resistivity, probably develops a very strong gradient. Coupled with the
temperature coefficient of resistivity, it might make for an interesting
current distribution. Even for DC applications.
The external medium will determine the exact temperature of the outer
surface, and by virtue of the gradient for the specific material/power,
the
centerline temperature. But the shape and relative height of the gradient
is irrespective of the external surface (as long as the thermal
conductivity
and heat produced are assumed constant).
Thermal conductivity is itself a function of temperature, so the
gradient does depend mildly on the absolute temperature of the whole
rig. Especially after the copper melts.
True. But below the melting point, it isn't hard to approximate the
variance with a low-order polynomial using temperature alone as the
independent variable. I would think this would make it relatively easy to
incorporate into the integration. Haven't tried it though, so who knows???
daestrom
My conclusion from this thread is that skin effect can be important at
60 Hz in entirely practical situations, and thermal gradients in
copper or aluminum conductors are inconsequential unless the current
is high and the cooling novel. We're doing some thermocouple stuff
just now (a simulator module and a complementary measurement gadget,
for jet engine testing) so thermal stuff is on my mind.
I've done a little superconductive/cryo work, where things are very
different. Here, the thermal conductivity of metals changes radically
as a function of temperature, so the net heat flow of, say, a
stainless or manganin leadwire from 4K up to to room temp is
determined by a complex integral (the bottom line of which,
fortunately, you can just look up.)
Yeah, the Toka guy is weird. He insists on crudely insulting anyone
who disagrees with him, and he's usually wrong. Some people seek and
need public humiliation: Usenet pain sluts, as it were.
I believe I said "near zero", Daystruck. I also believe that myBut this discussion forced me to 'sharpen the pencil' and do the actual
calculus. Turns out it is a pretty small effect in most practical
conductors (but not 'Toka's zero).
Someone out there must like them for them to be a product.But those graphite ones you mentioned in
another thread could be interesting.
That's why welding works the way it does, and soldering works theYeah, especially when solid 'phase-changes' take place. Some steels undergo
some interesting crystaline changes from 'body-centered cubic' to
'face-centered' at high temperatures and actually absorb a far amount of
heat doing it with no appreciable temperature rise. So I can imagine such
'shifts' in thermal conductivity can occur.
---On Sun, 07 Aug 2005 13:07:00 -0500, John Fields
jfields@austininstruments.com> Gave us:
So you know some of what was known 15 years ago, so what? do you
think nothing new has happened since then?
---
No, but your retarded ass apparently thinks that that is when I
stopped learning anything about it. How stupid does that make you?
Pretty fucking stupid.
---Learning never stops. At least in most cases. Your an exception.
^^^^
Get it through your head, you retarded fuck. I don't need yourNot at all. From every indication, so far, it seems you _can_
accept a modicum of instruction, but then you plateau out.
trolling, John. Grow the fuck up.John Larkin summed it up quite nicely, I think, by likening you to a
"newsgroup pain slut". Or was it "internet pain slut"? I don't
remember, but it fits, either way.
Same old fat troll fucktard bullshit. When are you going to stop
Perhaps you don't _need_ them to survive on the plnet, but you would doOn Sun, 07 Aug 2005 17:29:17 -0500, John Fields
jfields@austininstruments.com> Gave us:
Not at all. From every indication, so far, it seems you _can_
accept a modicum of instruction, but then you plateau out.
Get it through your head, you retarded fuck. I don't need your
assessments.
I suggest that you figure out who your nemisis is! He will bury yourOn Sun, 07 Aug 2005 13:07:00 -0500, John Fields
jfields@austininstruments.com> Gave us:
You're right, I never did get into laser discs much. No interest,
plus I was too busy designing equipment to go to the bottom of the
Marianas Trench instead of watching somebody else's stuff.
Yeah right. You were a technician... oh... strike that... an
assembler... oh wait... that's right... you swept the floors in the
lab. Sound familiar, fuckhead?
---On Sun, 07 Aug 2005 13:07:00 -0500, John Fields
jfields@austininstruments.com> Gave us:
You're right, I never did get into laser discs much. No interest,
plus I was too busy designing equipment to go to the bottom of the
Marianas Trench instead of watching somebody else's stuff.
Yeah right. You were a technician... oh... strike that... an
assembler... oh wait... that's right... you swept the floors in the
lab. Sound familiar, fuckhead?
On Sun, 07 Aug 2005 17:29:17 -0500, John Fields
jfields@austininstruments.com> Gave us:
Not at all. From every indication, so far, it seems you _can_
accept a modicum of instruction, but then you plateau out.
Get it through your head, you retarded fuck. I don't need your
assessments.
---On Sun, 07 Aug 2005 17:29:17 -0500, John Fields
jfields@austininstruments.com> Gave us:
Not at all. From every indication, so far, it seems you _can_
accept a modicum of instruction, but then you plateau out.
Get it through your head, you retarded fuck. I don't need your
assessments.
You're an idiot. You also made a spelling error, yet yours won't beOn Sun, 07 Aug 2005 22:43:51 +0000, TokaMundo wrote:
On Sun, 07 Aug 2005 17:29:17 -0500, John Fields
jfields@austininstruments.com> Gave us:
Not at all. From every indication, so far, it seems you _can_
accept a modicum of instruction, but then you plateau out.
Get it through your head, you retarded fuck. I don't need your
assessments.
Perhaps you don't _need_ them to survive on the plnet, but you would do
well to listen. Of course you know-it-all, so why would you "listen",
even to your superiors.