Need explanation of heat transfer from metal surface 1

[sciguyjim]

Am I correct in assuming that I can get maximum heat transfer from a metal object (through convection and radiation) by having a rough, black, matte surface? I know black radiates best, but I've heard people suggest painting an object black to increase heat radiated (vs shiny silvery surface.) However, ordinary paint would also act as a slight thermal barrier. Ideally, the bare metal surface should be treated somehow to be black and rough without any coatings. Except maybe for a coating of very high surface area metal (appears black due to very tiny particles) bonded to the base hot metal object (good metal to metal heat transfer.) Right?

 

[MortenA]

I believe thermal radiation is only a major part of the total heat transfer for hot bodies (metal starts to glow). Prior to this only convection is worth considering. I dont think a coat of paint will seriously affect this.

Best Regards

Morten

 

[sciguyjim]

Ok, what are your thoughts about this: My gut feeling is that a coat of paint will have less effect on a relatively large or massive hot object, but for something tiny, that same coat of paint can now have a large effect?

 

[butelja]

What range of temperatures are we talking about? If the temperature differential between the object and its environment is large, or if the object is in a vacuum, then radiation is likely important. By contrast, if you are talking about a car radiator, then radiation is almost entirely irrelevant, with convection governing. Without more information, its hard to say.

 

[IRstuff]

Consider this: most heat sinks for electronic components and equipment are unpainted or lightly anodized aluminum. If you need to air cool something, the best approach is lots of fins and lots of air.

TTFN

 

[sciguyjim]

There are 2 areas where this topic arises often. One is with small, 2 cycle, single cylinder engines used for radio controlled model airplanes. These typically have fins around and on top of the cylinder. The other has to do with high underhood temps on certain model cars due to heat from the exhaust headers, especially if they've been upgraded. Many people can't figure out why their cars run so hot and others don't. Personally, my car is fine, with or without the silvered fiberglass blanket hiding my stock rusted exhaust headers. I'd like to understand some of the basics better. Thanks.

 

[buzz41]

sciguyjim -

I messed with radiating surfaces and coatings years ago.

What I remembered was that ALL paints are nearly black in the far infrared. Here is a link to a commercial site, the same outfit with those nasty pop-up ads, BTW. But the table is fairly complete.

http://www.x20.org/library/thermal/emissivity.htm

You will notice from the chart that most white paint is about as emmisive as black paint.

At ~400+ deg K ( engine heat ) the peak wavelength will be aprox 7 - 10 uM, a long way from visible.

At these temps, radiation cooling is fairly weak compared to forced air cooling.

 

[IRstuff]

Most heat from car parts still come and go from convection.

When you hood is closed, the blackbody behavior reaches equilibrium, since everything is enclosed, particularly if the hood is insulated. This means that the only real paths for heat exchange is air flow into the compartment and airflow across the hood. That's why turbos have ports and fins sticking out of the hood, to catch the airflow.

TTFN

 

[sciguyjim]

Buzz41 & IRstuff,

Thanks guys, you've helped a lot. I have a better perspective now.

 

[rjw57]

Another way to improve the emmissivity of a surface is to configure the surface with either trapezoidal or triangular grooves (see

http://www.virial.com/Articles/SPIE3553-247.pdf

as a reference not totally related to your application) The geometry of these grooves is dependant upon the wavelengths that you are trying to attenuate. These surface modifications are also used in conjunction with special optical finishes (see the follwing link on several paints and surface treatments:

http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm

-or-

http://www.tak2000.com/data/finish.htm

). Siegel and Howells book "Thermal Radiation Heat Transfer, 3rd Ed." refers to an article on surface conditions - Demont, P., M. Huetz-Aubert, and H. Tran N'Guyen: Experimental and Theoretical Studies of the Influence of Surface Conditions on Radiative Properties of Opaque Materials, Int. J. Thermophys., vol. 3, no. 4, pp. 335-364, 1982. I know I'm late on this post, but maybe "better late than never." Hope it helps expand the topic some...