Just got a radiator for my '89 Cressida, but the core is bare aluminum. Every radiator I've seen has been painted black. Seems I read somewhere the reason is more than just cosmetic, that the black helps it transmit heat better. Should spray paint it matte black before installing?
My guess would be that the paint (any color) would act as a small layer of insulation. They painted brass radiators because they used to be outside the engine compartment, and they'd get fugly in a hurry (IIRC). AL doesn't have as much of a problem.
-phaeton
*sings* "....I see a row of rads and want them painted black.."
Your comments rang a bell with me too. This concept is true for some heat radiating devices, but I think it is for electronic radiating heat sinks. I wouldnt bet my life on it, but think this might be the source. Helps in the IR range IIRC.
Auto radiators work largely by convective heat transfer. Radiative is a relatively minor component.
If radiation were the main process by which the radiator (interesting misnomer in even calling it a "radiator") were cooled, then painting it black would help. But since car "radiators" are actually cooled by CONVECTION (moving cool air over the fins) the color doesn't matter at all, and paint would in fact add a little bit of insulation.
Most modern vehicle radiators that I've seen are bare aluminum, UNLESS they can easily be seen through the grille (and even then, they frequently only "dust" the front surface with black paint and the engine side is still silver).
A asked that question about my house a/c condenser to a couple of NASA engineers at JSC a few years ago. Their answer was: "it does not matter". However, if the radiator was in space with no air flow, a black body would be the most efficient heat emitter.
This is true. Although its a small effect compared to convection.
A black object will also gain heat via absorption faster than a lighter colored one. So the side of the radiator facing a hot engine will gain more heat than a lighter color.
Actually -- the radiator would radiate the same, whether in shade or full light, in space or not. But, if there was gas (ie liquid, in the general case) around it, convection would become another way to lose heat. And if it was not in darkness (at all wavelengths) then (depending on wavelength and such factors) it could gain energy by incoming radiation. This kind of exchange is going on all the time but isn't always obvious. (And let's not be forgetting plain old conduction as an energy conduit.)
The paint will act as more of an insulator than any aid to radiation. Actually, oxidized aluminum is a fairly good radiator in the infrared, the waveband of most interest at the temperature a radiator runs at. I am not so sure about the infrared emissivity of copper, I don't think it is too bad. I suspect painting was done more for appearance than heat transfer.
Most of the cooling will be by convection, which is why there is a fan. A paint coating definitely adds a thermal impedance to getting the heat into the air.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.