How to quantify a color's effect on heat transfer.

[Indychus]

Hey,

I am working on a thermodynamic design problem, and am trying to come up with some coefficients to modify Qin through a metal surface that is painted different colors. I know that there would be a significant difference in heat through the surface if it were black as opposed to a lighter color, but i need some way to actually quantify it for use in a thermodynamic analysis. I would greatly appreciate any input on the subject.

 

[Drakkith]

This is only true in visible light. It could differ wildly in infrared. A good example is a regular black kitchen trashbag, It's black, yet it passes infrared light completely!

 

[Indychus]

That's very interesting with the trash bag, I have never considered that!

Basically, i am designing an A/C system for a small vehicle. I am trying to calculate Qin through the various surfaces, i.e. windows, roof, etc. I have no idea how to take color into account for the qin through the roof.

 

[Indychus]

Would it be correct to use the surface's emissivity coefficient? Where can I find emissivity values for different colors?

 

[Drakkith]

Would it be correct to use the surface's emissivity coefficient? Where can I find emissivity values for different colors?

That I cannot help you with. If you've already done a thorough google search then perhaps someone else here will be able to assist you.

 

[mfb]

Even with the same color (as seen by your eye), the coefficients can be different. If you want to do it properly, measure the emissivity for the full spectrum of relevant light. If some approximation is enough, measure the total emissivity in your relevant environment.

 

[Indychus]

Well thanks for the input :) I'm pretty sure i can calculate the Qin and use the emissivity coefficient to modify it based on color and material, but I'd like a confirmation of that from someone who knows more about the subject than myself. Hopefully someone will chime in.

 

[Indychus]

Even with the same color (as seen by your eye), the coefficients can be different. If you want to do it properly, measure the emissivity for the full spectrum of relevant light. If some approximation is enough, measure the total emissivity in your relevant environment.

The number does not have to be exact, a ballpark will work. Just wanting confirmation that I am approaching this the right way. Do I have to consider both the emissivity of the paint AND the underlying material?

 

[sophiecentaur]

The number does not have to be exact, a ballpark will work. Just wanting confirmation that I am approaching this the right way. Do I have to consider both the emissivity of the paint AND the underlying material?

If it is paint on metal and not too thick a layer of paint then I reckon the only relevant factor for heat transfer would be the paint emissivity.

I found this link, which suggests that, ball park, you could treat emissivity factor of 'paint' as pretty near 1. After all, you should be pessimistic in these exercises.

 

[Indychus]

Thanks for all of the help guys, I think I've got this figured out. I found the engineering toolbox chart earlier today, but was hesitant to use the 1 since I find it hard to believe that a painted surface has the same emissivity as a total black body. I found another resource that claims the most automotive paints are between 0.90-0.96. To err on the cautious side, I am going to use 0.98 in my calculations, but am also including a healthy factor of safety to ensure that it has adequate cooling.

Really, I could just cobble this together and it would probably work fine, but doing the calculations is a great exercise, and I love this stuff :)