black and white - no sun

[samv]

(this post was first posted in HVAC/R forum - but here it is again as it is more for you heat transfer experts)

My understanding is that assuming the paint and material underneith have similar properties...

1- a surface that is painted black and exposed to the sun will behave differently than the one that is painted white. The black one will absorb much more heat.
2- a surface that is painted black and is indoors and exposed to a heat source other than the sun - will behave in a similar manner as the white one

is this correct?

 

[pablo02]

what does common sense tell you? why or why not is there a difference? look at your assumptions, evaluate the type(s) of heat being generated by the heat sources, look at the differences in your two scenarios, consider how you would quantify each, and think about it...

 

[pablo02]

Let me ask a related question -- when you wrap food with aluminum foil to put a) in the oven or b) in the fridge, which side should face out -- the dull side or the shiny side? [does it really matter except to the kitchen purists?]

 

[samv]

well pablo02,

emissivity for black or white lacquer is 0.80-0.95 according to Heat Transfer by JP Holman.
however for solar radiation we all know that wearing black in the hot sun is a bad idea compared to wearing white.

agree?

 

[pablo02]

samv -- well, you kind of touched on my point, for many heat applications, I think the difference may be insignificant -- but for significant radiation sources (whether the sun or any other), there will be a noticeable difference [but as I recall, emissivity relates to the ability or a surface to radiate heat, not absorb it -- so if, these are relatively the same, the black will become hotter because it absorbs more while only releasing a similar amount]

 

[samv]

any idea where I could find info on characteristics of significant radiant sources? The difference is in the wave lenght of their radiation isnt it

 

[pablo02]

do a search on ultraviolet and infrared light (radiation)...

 

[JKEngineer]

Whether indoor or out, the response of a surface to a remote heat source will depend on the surface's emissivity and on that of the heat source and on the heat source's temperature. Emissivity affects both heat absorption by a surface as well as heat radiation by a surface.

The source temperature affects the results through the obvious radiation exchange equation involving T^4, but also through the relationship between emissivity and temperature or wavelength. For solar temperatures, the radiation peaks in the visible and the white and black paint have different effective emissivities. For a "mild" or "cool" indoor source, the radiation from the source peaks in the infrared region, at a wavelength determined by the source temperature. The emissivity of many paints are considered to be independent of the paint color for wavelengths in the 2-12 micron regime, which covers source temperatures from near room temperature to perhaps 600F ( I did not look this number up or calculate it, so I won't swear to it, but it is cited as an example). If that is the condition for the "indoor" case, then the white and black paints will behave similarly.

The other factor to consider is that the surfaces are absorbing radiated heat from the source, but also re-radiating it to the surroundings. The re-radiation and the convective losses will balance the input energy at some surface temperature (for steady state). Depending on that balance point, the emissivity of the surfaces may not be the same for the re-radiation portion of the equations, and this will also affect whether the surfaces behave differently or similarly.

HTH.

If you need detailed help, contact me directly.
Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[lilliput1]

The net energy in Btu/hr received by the surface q = qsu + qsk + qe - gs
Where qsu = from the sun
gsk = from the sky
qe = from the earth
gs = emitted from the surface
Now gs, ge and nightime gsk varies with the surface normal total emmisivity which is 0.80, 0.95 for both black & white lacquer. However qsu and daytime gsk varies with the absorptivity of the surface for solar radiation which is .97 to .99 for flat black versus 0.12 to 0.26 for flat white. So if the temperature & surface configuration of the black versus white body are the same, then the white body behaves the same as the black body when there is no sun and it is a night sky. JKEngineer gave the reason for the difference in emmissivity/absortivity depending on wavelengths.

 

[lilling]

I agree with you. This must be correct.
It is a question of difference in behaviour between long wave and short wave radiation. I have interpreted the literature just as you have done.

 

[25362]

To samv.

Bedouins indeed wear dark baggy clothes. These warm up by a few degrees more than white clothing. The warmer air beneath the garment flows upwards and out pushed by cooler external air. This draft effect cools their bodies by convection.

As for the Sun effects on the irradiated surfaces' temperature (convection excluded) I suggest a visit to thread391-73220 for a worked out example.