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best finish for thermal rejection ("heat paint")

Strostkovy

Stainless
Joined
Oct 29, 2017
Some further thoughts.

Radiation is only one method of heat transfer. There is also direct transfer from the surface to the surrounding air and then convection currents will carry it away. A rough surface will have more surface area and therefore would, in theory, transfer more heat that way. So a chrome surface, bright or black is not ideal for that. A rough surface, like sandpaper, would have more area and would transfer more heat. Again, the "flat" surface finish is probably better.

As for a layer that is under the final coat, it comes in play in the conduction of heat from the interior of the part to the exterior surface. This is kind of like a resistor in electronics, it opposes or facilitates the flow of the heat. But this is a different property than it's radiation properties. Those two properties may or may not track. Here the thickness of the layer also comes into play: the thicker it is the more it will block the conduction of heat. Also you should consider that the final, exterior coating must conduct the heat from the interior to it's own surface so the actual material of this layer is in play.
For conduction, the surface finish doesn't matter except for in extremely high airflows. The reason for this is the boundary layer (the air near the surface that doesn't move) is thicker than the surface features that provide the extra surface area. I spoke with a few engineers at a heatsink manufacturer and they told me that even the coarse wiggles some people put in heatsinks meant for passive cooling do absolutely nothing in thermal testing.

So long as the coatings are well bonded to the materials, they matter very little for conduction unless you have a highly optimized forced air setup. The coatings, being very thin and very wide, can conduct a lot of heat across them with minimal temperature rise, even if they are a fairly insulating material. It all ends up being negligible against the boundary layer.

Consider a block of PET plastic, at just 0.19 W/m K. This is really terrible thermal conductivity. (copper is 400 W/m K). Now apply it as a coating, 0.003" thick. That coating can dissipate 2500 watts per square meter of coating per degree Celsius of temperature rise. Running the numbers for any metal plating puts that number in the megawatts.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
One more thing to add: the material properties you find on the internet get increasingly inaccurate the more specific the property is. You can find numbers like .98 for the emissivity of aluminum oxide, yet no explanation of the testing methods or the surface finish or oxidation process used to achieve that. Be wary of too good to be true numbers. I can look up coatings in my CRC handbook later if you have a few you are considering.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Right now, I am leaning towards aluminum paint being high in thermal emissivity.

More likely high reflectivity, so it acts like a mirror. That means if it's near a cold surface (at an angle like a mirror would see it) it will look cold. If it's near a hot surface it will look hot. That is why they look darker than the rest of the post at night and brighter than the rest of the post in the daytime. They are likely reflecting IR from the sky.
 

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
Lots of interesting comments on this thread...

Just type in "Infra Red Emmisivity" into your search engine. Many interesting and useful tables of common materials will appear as if by magic.

Any paint has better emmisivity than any bare metal.
Color DOES NOT MATTER. Color is in the visible spectrum, heat is in the IR...;-)

The thinnest, lightest coating of enamal paint over bare metal is best.

Heat rejection by radiation is the FOURTH POWER of the temperature difference (K) between objects.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
Lots of interesting comments on this thread...

Just type in "Infra Red Emmisivity" into your search engine. Many interesting and useful tables of common materials will appear as if by magic.

Any paint has better emmisivity than any bare metal.
Color DOES NOT MATTER. Color is in the visible spectrum, heat is in the IR...;-)

The thinnest, lightest coating of enamal paint over bare metal is best.

Heat rejection by radiation is the FOURTH POWER of the temperature difference (K) between objects.

I don't think it's the fourth power of the temperature difference. I think it's the difference between the fourth power of the absolute temperature of each object.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
" I think it's the difference between the fourth power of the absolute temperature of each object."

That was his specific statement - unless he went back and edited. He said temp. difference in degrees Melvin.
 

bosleyjr

Diamond
Joined
Sep 30, 2006
Location
SE PA, Philly
I think that the emissivity of flat black anodized steel is around 0.85. Same for flat black "barbeque paint". For chrome, it's around 0.04. Aluminum paint is between 0.27 and 0.67 if the table I saw was accurate. If the temperature achieved is low enough so as not to destroy the paint, I'd go with flat black paint over a good high-temp primer.

But the question doesn't make sene to me. If the tubes radiate a significant amount of the heat load, they kind of have to be exposed to the exterior of the engine, right? So there will be airflow over them and conductive heat transfer may dominate radiative heat transfer. What am I missing?
 

Cole2534

Diamond
Joined
Sep 10, 2010
Location
Oklahoma City, OK
My research suggests that a dog's coat is a good insulator.

Disclaimer- optic was not weapon mounted during this testing.
 

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Strostkovy

Stainless
Joined
Oct 29, 2017
" I think it's the difference between the fourth power of the absolute temperature of each object."

That was his specific statement - unless he went back and edited. He said temp. difference in degrees Melvin.
You get different results depending on how you calculate it.

If you have 100K and 200K you have 200^4 - 100^4 = 1.5e9 units of radiant power transfer

If you have 1000K and 1100K you have 1100^4 - 1000^4 = 464e9 units of radiant power transfer

Both have the same temperature difference, but radiated power is absolute so you have to calculate it first
 

FredC

Titanium
Joined
Oct 29, 2010
Location
Dewees Texas
Took some more photos of T-posts near the house this time. Tried to keep the deer out of the cantaloupe patch. This time 5:00 in the morning. The T-posts seemed to stabilize in temp. Took a photo of a gate with tube construction, tops of the horizontal tubes are black, bottom side catching radiation from the ground are warmer, same 5:00am. Hot spot on the right is a rabbit in the background.

I get corrected here pretty often but I think surfaces that radiate heat faster also absorb it faster. I guess I could test that by taking a photo when the sun comes out later. I do have a pile of T-posts behind the shop.

Random factoid: rattle snakes even though cold blooded expend energy living and moving and can show up as warm. His head is a little messed up as I pushed a fiberglass rod through it to make sure he was dead. 3' 11" long.

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eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
I think you'd find that those metal gate tubes are the same temperature throughout if you actually checked them with a temp probe. What you're seeing is the reflectivity I mentioned previously. Take a closer look at the vertical posts, and the section where they curve around toward the top - note the black around the curve on top and no hint of temperature gradient where the horizontal posts meet the verticals. Reflectivity shows up just like that.

Emissivity shows temperature of the actual object. Reflectivity shows the temperature of whatever IR radiation is bouncing off the object and being reflected back to the camera. Have a look at the sky and the ground too. Notice that they are the same color as the top and bottom of the poles, and on the same sides? It's a tricky subject to understand without a little involved reading. It's also why emissivity/reflectivity tables exist, and are important.
 
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FredC

Titanium
Joined
Oct 29, 2010
Location
Dewees Texas
I think you'd find that those metal gate tubes are the same temperature throughout if you actually checked them with a temp probe. What you're seeing is the reflectivity I mentioned previously. Take a closer look at the vertical posts, and the section where they curve around toward the top - note the black around the curve on top and no hint of temperature gradient where the horizontal posts meet the verticals. Reflectivity shows up just like that.

Emissivity shows temperature of the actual object. Reflectivity shows the temperature of whatever IR radiation is bouncing off the object and being reflected back to the camera. Have a look at the sky and the ground too. Notice that they are the same color as the top and bottom of the poles, and on the same sides? It's a tricky subject to understand without a little involved reading. It's also why emissivity/reflectivity tables exist, and are important.
I have only been hunting and scouting with these thermal devices for about 6 months now. You see so many strange things that are hard to figure out. Something that is becoming real clear is the huge amount of thermal radiation from the ground going on in the summer and now this winterlike cold snap. You drop a cardboard box on the ground and the space between the box and the ground immediately lights up with reflected heat. Under my electric 4 wheeler the same thing, some of it is heat off the batteries and motor but most is reflection from the rather cool plastic and metal parts. These thin wall tube gates have been consistently hotter on the bottom at night where they catch the radiation from the ground to space. I am pretty certain the reason these gates show it is the thin wall conducts heat slower than the heavier thick wall H braces.
Take a look at the same gate as before, I wrapped my hand around it for 5 seconds and backed off and took pictures. 184 is one hand print, 185 is 2 of them about 1 minute apart, and 187 is 3 of them with the left one almost fully normalized. Pretty certain that the darkness of the upper side of the tube is not a reflection of the coldness of space but the immediate temperature difference of the thin tubing. Stinking cotton tail keeps photo bombing. I have a friend that wants to eat him after a couple of hard freezes.
A couple of other photos one of a dirt 4 wheeler trail and a gravel drive. The dirt "road" looks black compared to the dead grass catching thermal radiation. I assume the bare ground also cools faster than the gravel road. Would have to have them in the same frame to confirm. These thermal hunting scopes have many NUC tables and this one automatically chooses them to give an image where you can navigate and also detect the living critters. So a white object in this photo is not necessarily the same temp as a white object in another photo.




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eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
I have only been hunting and scouting with these thermal devices for about 6 months now. You see so many strange things that are hard to figure out. Something that is becoming real clear is the huge amount of thermal radiation from the ground going on in the summer and now this winterlike cold snap. You drop a cardboard box on the ground and the space between the box and the ground immediately lights up with reflected heat. Under my electric 4 wheeler the same thing, some of it is heat off the batteries and motor but most is reflection from the rather cool plastic and metal parts. These thin wall tube gates have been consistently hotter on the bottom at night where they catch the radiation from the ground to space. I am pretty certain the reason these gates show it is the thin wall conducts heat slower than the heavier thick wall H braces.
Take a look at the same gate as before, I wrapped my hand around it for 5 seconds and backed off and took pictures. 184 is one hand print, 185 is 2 of them about 1 minute apart, and 187 is 3 of them with the left one almost fully normalized. Pretty certain that the darkness of the upper side of the tube is not a reflection of the coldness of space but the immediate temperature difference of the thin tubing. Stinking cotton tail keeps photo bombing. I have a friend that wants to eat him after a couple of hard freezes.
A couple of other photos one of a dirt 4 wheeler trail and a gravel drive. The dirt "road" looks black compared to the dead grass catching thermal radiation. I assume the bare ground also cools faster than the gravel road. Would have to have them in the same frame to confirm. These thermal hunting scopes have many NUC tables and this one automatically chooses them to give an image where you can navigate and also detect the living critters. So a white object in this photo is not necessarily the same temp as a white object in another photo.




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There's not enough heat radiating from the ground to appreciably heat that thin wall tubing from that far away... aside from the fact that there wouldn't be such a sharp delineation in the temperature gradient from top to bottom of the tube due to thermal conductivity - that's reflection IMO. If you want to tell for sure, just cover the tube with a cardboard box that has the ends cut out. Stick the camera through a hole at 90° to the tube.

I've been reading the technical literature and playing with these cameras for 5 or 6 years now.
 








 
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