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best finish for thermal rejection ("heat paint")

For an aircooled IC engine.
Suppose the pushrod tubes could be had in chrome, or "black chrome" and that the pushrod tubes were considered a fair percentage of the heat radiation capacity of the engine - e.g. a VW -. Which gives the best thermal rejection?

1.) black chrome
2.)chrome painted flat black
3.) bare steel (sometimes available) painted flat black

A person on one of the forums claims that black chrome has no more heat rejection than show chrome, which of course is pretty bad.

Once surface paint is on, how much does the coat under it affect thermal radiation?

Thanks!
smt
 

Strostkovy

Stainless
Joined
Oct 29, 2017
I did a decent amount of testing at an old job and found that powder coating a heatsink, under natural convection, made no measurable difference in heatsink performance. Since this was a tight fin stack, the bulk of the heat is lost through conduction, not radiation.
I do think it is useful to know that a thin coating is pretty much nothing compared to the boundary layer of air.
Black chrome still has a poor coefficient of emissivity. I think your best option for best radiant heat loss without being worried about failing coatings at high temperature is stainless with a black oxide layer or zinc plated steel with black oxide. Accurate numbers are hard to find but black chrome is around 0.1 and black oxide is around 0.7 .
A layer of chrome under the coating will not be an issue unless it is peeling.
 
IIRC Bob Hoover covered this pretty well
I looked a while back and did not find it?
Or maybe he had only covered the paint over chrome, not the "black chrome" which probably was not available then. Will have to search again.

Strostkovy - it sounds like the commonly available, cheap bright chromed tubes painted flat black are the best option unless i get stainless and have someone black-oxide them? Not sure anyone would do it over zinc out of fear for their solution?

Thanks!
smt
 

Strostkovy

Stainless
Joined
Oct 29, 2017
I looked a while back and did not find it?
Or maybe he had only covered the paint over chrome, not the "black chrome" which probably was not available then. Will have to search again.

Strostkovy - it sounds like the commonly available, cheap bright chromed tubes painted flat black are the best option unless i get stainless and have someone black-oxide them? Not sure anyone would do it over zinc out of fear for their solution?

Thanks!
smt
Flat black paint would work well.
Black oxide over zinc is pretty common for bolts.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
I ordered the plain steel tubes.
Maybe can find someone local to run them through their hot tank.
Otherwise - flat black paint.

It's a rebuild for airplane, so the tubes will not be exposed to NY road salt, nor really much to the elements

Thanks for the input!
smt
These will have a clear view to cooler parts or the sky or ground, right? If they are enclosed near hot parts they'll just absorb heat from hot parts and radiate it back.
 

Comatose

Titanium
Joined
Feb 25, 2005
Location
Akron, OH
These will have a clear view to cooler parts or the sky or ground, right? If they are enclosed near hot parts they'll just absorb heat from hot parts and radiate it back.

The cowling of an airplane engine is typically thin, aluminum and exposed to hundred mile per hour plus winds. No worries there.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
Look at the parts in the IR region of the spectrum. The blacker they are there, the better. Many black objects, look light or white in the IR.
 

10 fingers

Hot Rolled
Joined
Mar 9, 2003
Location
Vermont
Interesting topic. Became a subject of debate in a thermodynamics class. Conclusion, regarding convection cooling, any paint, or plated coating would serve as an insulating medium. Optimum became, maximum surface area with a clean, bare finish. Not to be confused with dark surfaces that absorb radiation heat transfer.
 
DSC_0293 - Copy.JPGDSC_0338.JPG

Things are tight and there is lots of hot air under there, but also, as comatose relates, with the cowl on, generally 100+mph wind channeled into a compartment over the engine, and forced down through and out the bottom. Chrome are existing, i bent one removing heads for valve job.

Thanks all for the advice.

smt
 

Strostkovy

Stainless
Joined
Oct 29, 2017
Look at the parts in the IR region of the spectrum. The blacker they are there, the better. Many black objects, look light or white in the IR.
That's generally true, but often black shiny surfaces are terrible and almost all low gloss paints are very emissive, even if white
 

Strostkovy

Stainless
Joined
Oct 29, 2017
The cowling of an airplane engine is typically thin, aluminum and exposed to hundred mile per hour plus winds. No worries there.
If the inside of the cowling is bare aluminum it will reflect most radiated heat back into the engine. Coating the inside of the cowling black will allow it to absorb more radiated heat and conduct it away.
 

FredC

Titanium
Joined
Oct 29, 2010
Location
Dewees Texas
Photo below shows 3 T posts in a row. The tops of the post are painted with aluminum paint, I think. I will verify tomorrow. I have seen this often than the top painted surface is cooler than the rest of the green painted area many times. Looking at these posts in the daytime the lower portion with the green paint looks much darker than the lighter painted tops. The post just this side of the doe is a wooden post and does not count in the discussion.
DoeJumpingFence.JPG
 

Comatose

Titanium
Joined
Feb 25, 2005
Location
Akron, OH
Photo below shows 3 T posts in a row. The tops of the post are painted with aluminum paint, I think. I will verify tomorrow. I have seen this often than the top painted surface is cooler than the rest of the green painted area many times. Looking at these posts in the daytime the lower portion with the green paint looks much darker than the lighter painted tops. The post just this side of the doe is a wooden post and does not count in the discussion.
View attachment 379535
Not necessarily cooler, it may just look that way in a thermal camera. If the emissivity is significantly lower than the rest of the post, that will show up as cooler, regardless of the actual surface temperature. Alternatively, it may be reflecting the temperature of the sky or the ground. There are a lot of gotchas with thermography.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Not necessarily cooler, it may just look that way in a thermal camera. If the emissivity is significantly lower than the rest of the post, that will show up as cooler, regardless of the actual surface temperature. Alternatively, it may be reflecting the temperature of the sky or the ground. There are a lot of gotchas with thermography.

Yup. If you want an accurate measure of heat using a thermal camera you need to either be aiming it at something with known IR emissivity/reflectivity or use the quick trick of putting a coat of spray paint of known emissivity or even some black electrical tape over the surface. Black 3M 88 electrical tape is commonly very close to 1.0 emissivity (something like .95 IIRC). Otherwise there's no telling what the actual temperature is for sure.
 

EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
Thermal rejection? What exactly is that?

If you want to reject the heat that comes from the outside, such as heat from sunlight, then a bright chrome would do that best. It would reflect almost all the heat away.

But if you are talking about what would facilitate the radiation of the maximum amount of heat in order to cool the tubes, then a flat black finish should be best.

The physics works like this: heat absorption and heat radiation are reciprocal processes and what works well for one also works well for the other. A flat black surface is the best absorber of light and similar radiation so it also is the best radiator of that same radiation. This is why physicists talk about a "black body" radiation source.

You just need to be sure that the flat black surface characteristic extends into the infrared area of the spectrum. This is not guaranteed for all paints or surface finishes so some checking must be done. I doubt that either chrome or black chrome would meet that requirement. Both are more of an aesthetic measure than a functional heat radiating or absorbing finish.



Flat black paint would work well.
Black oxide over zinc is pretty common for bolts.
 

EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
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.
 

FredC

Titanium
Joined
Oct 29, 2010
Location
Dewees Texas
Not necessarily cooler, it may just look that way in a thermal camera. If the emissivity is significantly lower than the rest of the post, that will show up as cooler, regardless of the actual surface temperature. Alternatively, it may be reflecting the temperature of the sky or the ground. There are a lot of gotchas with thermography.
Yeah, I thought of a couple of other possibilities like that. I glanced at these posts this morning but I forgot my glasses and it was not really light yet. The posts are old, fairly certain it is aluminum paint, both top and bottom are starting to get rusty. I saw the same thing after I posted when I looked at the t-posts around the garden (not rusty). I can hang one upside down near the garden, and also get a close up to see if I can see a transition near the paint junction.

Right now, I am leaning towards aluminum paint being high in thermal emissivity.
 








 
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