
#1
Nov1312, 02:43 PM

P: 11

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. 



#2
Nov1312, 02:49 PM

PF Gold
P: 11,057

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!




#3
Nov1312, 02:53 PM

P: 11

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. 



#4
Nov1312, 02:54 PM

P: 11

How to quantify a color's effect on heat transfer.
Would it be correct to use the surface's emissivity coefficient? Where can I find emissivity values for different colors?




#5
Nov1312, 02:56 PM

PF Gold
P: 11,057





#6
Nov1312, 03:29 PM

Mentor
P: 10,864

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.




#7
Nov1312, 03:29 PM

P: 11

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.




#8
Nov1312, 03:31 PM

P: 11





#9
Nov1312, 05:23 PM

Sci Advisor
PF Gold
P: 11,398

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. 



#10
Nov1312, 07:47 PM

P: 11

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 0.900.96. 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 :) 


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