Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Nov 13, 2012 #1
    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. jcsd
  3. Nov 13, 2012 #2

    Drakkith

    User Avatar

    Staff: Mentor

    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!
     
  4. Nov 13, 2012 #3
    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.
     
  5. Nov 13, 2012 #4
    Would it be correct to use the surface's emissivity coefficient? Where can I find emissivity values for different colors?
     
  6. Nov 13, 2012 #5

    Drakkith

    User Avatar

    Staff: Mentor

    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.
     
  7. Nov 13, 2012 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    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.
     
  8. Nov 13, 2012 #7
    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.
     
  9. Nov 13, 2012 #8
    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?
     
  10. Nov 13, 2012 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    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.
     
  11. Nov 13, 2012 #10
    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 :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook