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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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Re: Optical properties of metals at high temperatures
quote:
Originally posted by DumHed:
story is being able to see through turbocharger housings when they've been run flat out for a while.
I've heard in the past that the optical properties of metal change enough at high temperatures that light can pass through,
Another possibility is that Infra Red radiation from inside the turbo, etc can cause enough of a change in colour temperature of the external metal that it looks like an image is visible through it
An interesting perspective..
If a photon from the original item gets absorbed, bounced around, and re-emitted on the other side of the opaque material, does that count as "seeing" the item that emitted the original Photon ? Does that mean the intermediate material is "transparent" ?
Semantics I suspect. If you look at something through a piece of paper and just see the shadow on the paper, are you seeing the original item ?
If by "see" you mean direct path of photons from emitter to receptor (eyes) with no absorbtion and remission along the way, then I doubt that many metals would allow that. unless they formed a very pure crystal structure (like diamond), although I suspect the atomic spacing in a metal crsytal structure would be sufficiently varied to scramble any light waves on the way through.;
That however, is just whistling in the dark, perhaps one the Kero's with their knowledge on the various phases of Metallurgy might be able to assist further ? _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Wed Nov 09, 2005 1:33 pm |
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Nick
Experienced Roboteer
Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW
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I'm no metalurgist but I rekon all metals are opaque at visible wavelengths and the only way you would "see" thru metal is after you had turned it into a cloud of plasma. Exceptions would be when the metal is only a few wavelengths thick.
Its possible to make some metals take the structural form of a glass by supper rapid cooling from the molten state, but even then, I don't recall any special optical properties.
In the case of the turbo, I agree with andrew; it could have been a thermal "shadow", caused by the turbine blades (presumably stationary?) either heating or cooling the housing, thereby altering the colour of the metal.
quote:
If a photon from the original item gets absorbed, bounced around, and re-emitted on the other side of the opaque material, does that count as "seeing" the item that emitted the original Photon ? Does that mean the intermediate material is "transparent" ?
An example of seeing through an opaque object would be an image intensifier tube. the receptor is opaque but the photons are encouraged to tunnel thru _________________ Australian 2015 Featherweight champion
UK 2016 Gladiator champion
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Wed Nov 09, 2005 2:17 pm |
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DumHed
Experienced Roboteer
Joined: 29 Jun 2004
Posts: 1219
Location: Sydney
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could the localised heating happen fast enough to be visually discernible though?
I guess in that sort of situation maybe the projectile would actually be in contact with the barrel and cause rapid frictional heating.
quote:
Originally posted by Daniel:
If the metal becomes translucent at high temperture and still remains a solid, when it gets to liquid form how would the foundarymen see where the metal is when they are pouring a casting?
Translucent materials are still visible, especially when they're glowing!
quote:
As much as car enthusiest like to gloat about their parts, turbo housings and exhuast tubes arn't made out of super rare alloys that process magical properties. Are turbo housings cast steel or aliminium?
Due to the fact that turbos glow anything up to white hot in operation I think it's fairly obvious that they're not aluminium.
They tend to be either high silicon or high nickel content steel alloys, and can run at extremely high temperatures for long periods of time.
The most expensive turbo housings are inconel, as are most modern turbocharger turbine wheels.
There are even some in various forms of stainless steel, for thinner walls and lighter weight (from the turbo Formula 1 days)
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Lets just say you are refering to steel parts. Most steels undergo a cysticallagraphic change at about 785 degress C. If the exhaust gasses are able to heat the the pipes and turbo up that far what excatly is happening to your pistons and conrods? Surely that will be a lot warmer and well into the glass transition zone (read as becoming really floppy. Not good for engine parts).
Exhaust gases can be up around those temperartures, but the internal engine parts are well cooled, and designed to transfer heat away from themselves.
Pistons are aluminium, and in most turbo engines are cooled by jets of oil from underneath.
The part that runs the hottest is the exhaust valve, which must dissipate its heat into the water cooled cylinder head via the valve guide (tube its stem runs though) and the actual valve seat when it's closed.
There are upgraded valve seats made from beryllium alloys to improve this heat transfer, and some engines (including the Nissan SR20DET) have sodium filled exhaust valves, which use the phase change of the sodium as a heat pipe through the valve stem to transfer heat from the tip of the valve to the guide, and cylinder head.
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As for Nicks thermal shodow, that sound like a good idea until you think about the turbos impeller may be spinning a several thousand rpm. It would be like watching jolts blade. Thermal conductivity isn't that fast.
I don't think it'd be possible to see the turbine spinning, because it can be running in excess of 100,000rpm under boost, and will be doing 10,000+ at idle unless it's a very large turbo (which can actually stop at idle)
quote:
So the main reason why I think you can't find any information about it on the net is because it is a myth.
Myths are the kind of thing that's all over the net though, because people always like to prove or disprove something. That's what makes it strange to not be able to find anything. _________________
The Engine Whisperer
- fixer of things
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Wed Nov 09, 2005 4:36 pm |
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Daniel
Experienced Roboteer
Joined: 30 Aug 2005
Posts: 2729
Location: Gold Coast
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quote:
Originally posted by DumHed:
Translucent materials are still visible, especially when they're glowing!
Yes, I have seen a cast steel pour and I didn't see though the metal.
quote:
Due to the fact that turbos glow anything up to white hot in operation I think it's fairly obvious that they're not aluminium.
They tend to be either high silicon or high nickel content steel alloys, and can run at extremely high temperatures for long periods of time.
The most expensive turbo housings are inconel, as are most modern turbocharger turbine wheels.
There are even some in various forms of stainless steel, for thinner walls and lighter weight (from the turbo Formula 1 days)
The nickle steel spec sheet I just looked says that nickel steel is rated to a max 870 degree C. I'd like to compare that to a color chart.
I guess the Nickel increases the phase change temperture. Taking it about that tempreture would be a bad idea. unless you know it would cool down correctly.
quote:
Exhaust gases can be up around those temperartures, but the internal engine parts are well cooled, and designed to transfer heat away from themselves.
Pistons are aluminium, and in most turbo engines are cooled by jets of oil from underneath.
The part that runs the hottest is the exhaust valve, which must dissipate its heat into the water cooled cylinder head via the valve guide (tube its stem runs though) and the actual valve seat when it's closed.
There are upgraded valve seats made from beryllium alloys to improve this heat transfer, and some engines (including the Nissan SR20DET) have sodium filled exhaust valves, which use the phase change of the sodium as a heat pipe through the valve stem to transfer heat from the tip of the valve to the guide, and cylinder head.
Ok, I know nothing about car engines.
quote:
Myths are the kind of thing that's all over the net though, because people always like to prove or disprove something. That's what makes it strange to not be able to find anything.
I thought we would have heard about engineers trying to use this sort of thing to make steath fighter planes.
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Wed Nov 09, 2005 4:57 pm |
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DumHed
Experienced Roboteer
Joined: 29 Jun 2004
Posts: 1219
Location: Sydney
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I think the effect (if it's there) is only small, and will only occur in certain conditions.
I don't think a very slightly translucent aircraft that's completely glowing red hot classes as stealth.
quote:
Yes, I have seen a cast steel pour and I didn't see though the metal.
What was behind it though?
I'm thinking that if something can be seen through the metal it could only be hotter surfaces forming internal details in the turbo / manifold.
In a stream of molten metal, even if it is translucent or even transparent all you will see is more of the same glowing metal inside it.
There's no detail to view, so it will look the same as if it was solid.
If you shine bright light on a glowing piece of metal it looks normal, because of the large amount of light being reflected compared to what is being radiated.
The dim lit situations where glowing metal is most visible would allow much easier viewing of any slight image showing through the metal.
Compare it to a dark room, viewed from outside a window where there is bright ambient light. In that case a window is effectively opaque due to swamping by reflection, but if there's more light in the room than there is outside the window is suddenly very transparent.
I think that if the properties of metal can change enough to allow some light through, then an object like a turbo, with a hollow area inside containing shapes that run hotter than the external casing would provide the perfect scenario for viewing its translucency.
I can't think of any other situations where the same conditions exist.
Another interesting thing is that one of the major reasons why metal is opaque is the sea of "free" electrons that exist in it - which is also what gives its conductivity.
As temperature increases, so does electrical resistance, so I would guess that opacity will reduce as well (maybe in too small an amount to notice though).
It could also be worth noting that nickel iron alloy is ~14 times less conductive than pure iron. _________________
The Engine Whisperer
- fixer of things
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Wed Nov 09, 2005 5:29 pm |
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