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I've been experimenting with radiative cooling lately and in my mind convection is always an enemy (especially forced convection since the modules are outdoors).

Looking at Newton's law of cooling, it doesn't seem to me that it accounts for surfaces made of different materials losing/gaining heat (can be wrong though, not uncommon especially when working with formulas).

Does it matter (much), that surfaces are made from different materials when we talk about convective heat gains/losses?

I'm only saying this because metal cooled more under radiation than a type of plastic I used, and to me that doesn't make much sense as the convective heat gain should, in my mind, be greater in the metal panel and therefore it shouldn't cool more than the plastic one (both had similar emissivity because of a very thin paint coating).

My experiments tell me that I'm most likely wrong in assuming the convective heat gains/losses will be greater in metals, but I thought it's most likely better to check with those who actually know what they're talking about, it may be some weird fluke in the tests.

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I'd expect metals to cool faster through convection because of a related heat property of theirs: conduction. Metals are generally good heat conductors so as heat energy is removed from the surface of the metal by air, internal heat energy in the metal can quickly flow to the surface.

The rate heat flows between two systems is dependent on their temperature difference. When air removes heat via convection, the surface of the object is cooler than the central portion of the object. If heat in the object flows slowly from the center to the surface like in many plastics then the surface stays closer to air temperature and heat transfers to the surrounding air more slowly. If the object is a metal, the heat removed at the surface is easily replaced by heat flowing from the center to the surface and the temperature delta between the surface and the air is greater so the object cools faster.

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  • $\begingroup$ Yes, but i'm talking about the maximum heat that can be transfered by the surface. Metals have low specific heat and also good conduction will replenish the losses, that makes sense and can be the explanation, those effects are easy to calculate, which is good. Anyway i would like to know more about what i was talking about. Thanks for pointing me to those other factors. Do you know if the surface material is very important or not? $\endgroup$ – mr116 May 30 '13 at 23:45
  • $\begingroup$ What i said to you was wrong, i'm testing radiative cooling, if metals cool faster by conduction they also heat faster by conduction and specific heat will only add to that effect, so the logic in my question was right and the question remains the same, is the material of the surface very important to the amount of heat that it can transfer? $\endgroup$ – mr116 May 30 '13 at 23:53
  • $\begingroup$ As far as I know, for instantaneous transfer rate (between a solid and gas) all that matters is surface area and temperature delta. Surely there are other factors but I don't think they're significant. $\endgroup$ – Brandon Enright May 31 '13 at 0:01
  • $\begingroup$ I've gone through every dimensionless number that is associated to the Heat transfer coefficient and Newton's law of cooling and really the properties of the material of the surface are never considered. Maybe because a surface is nearly 0 in thickness and therefore to calculate a surface phenomena only the properties of the surface and the medium that interacts with the surface are significant enough to produce a sizeable impact (surface area and temperature difference like you said, and also the medium, according to Newton). I guess i understood all of it now but i'll wait a bit just to... $\endgroup$ – mr116 May 31 '13 at 0:14
  • $\begingroup$ ...see someone else's opinion on this just in case it adds something of value or helps to clarify anything. Thank you for answering. $\endgroup$ – mr116 May 31 '13 at 0:15

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