Emissivity coefficient of aluminum black-anodized surface on ambient temperature 1

[MEngr3239]

The emissivity coefficient of aluminum anodized is 0.77 as shown in the attachment. However, as applied the provided value to a thermal simulation case in SolidWorks, it seems 0.77 is way higher so that the simulated results for its surface temperature are very low compared to the real lab testing results. Has anyone out there experienced the same as described above? Can someone please justify whether the value 0.77 for an anodized AL surface is dependable that can be applied to the thermal simulation cases? Can you please provide the emissivity coefficient of anodized aluminum that is verified in the real world? Thank you in advance.

 

[IRstuff]

It may be more of an issue with your testing methodology and test conditions, which you've not stipulated. One obvious issue is how well you control/model natural convection, since the only clean way of doing that kind of testing is in a vacuum chamber to eliminate any hint of additional thermal loss. At normal room temperatures, natural convection dominates, but if you increase the temperature to, say, 100 C, then radiation begins to dominate.

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[EdStainless]

I have seen this testing done in a small enclosure filled with Ar to minimize heat transfer, but vacuum is clearly better.
And the walls of the chamber need to be rather cold.
The e will likely depend on the angle of viewing (the spectrum will be different at different viewing angles).
And typically you measure across a range of temperatures and either build a curve or just take an average if it is a narrow range.

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[MEngr3239]

Thank you, IR for your input. Noted well.

 

[IRstuff]

Just to add to Ed's comments, ostensibly, you can do a series of temperatures and fit the convection part fairly well, and then the only degree of freedom left for the radiative portion is the emissivity. Certainly, if you ran the test in relatively still air, you can make some assumptions about the convection component, and compensate for that, and then see how close the emissivity matches.

That said, things RARELY fit that well in real life. ;-)

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[3DDave]

What is the temperature of the surroundings in the simulation? For example, at air temperatures above freezing water can still freeze due to radiative cooling because the radiant temp (not sure if that's the right term) of the sky** can be well below zero, so less heat is returned to the water than is radiated.

In most conditions, like in a room or an enclosure, that is not true, and the cooling won't be nearly as much.

If this is that heat sink you were working on I would not count on radiative transfer as anything but a bonus; it might even be a liability if there are hotter components within view.

** Right now, with an air temp of 80F and a sidewalk temp of 80F the radiant thermometer shows the sky as 18F. Near the house and street their temp will predominate, but once the sun stops that transfer to the sky, everything starts to cool quickly.

 

[georgeverghese]

There are 2 sets of data for emissivity in Perry Chem Engg Handbook 7th edn; the ones listed for various surface finishes for aluminum generally used in engineering are in table 5-6 on page 5-28.

 

[davefitz]

The table provided indicates the e value varies with temperature ( or wavelength) - the 0.77 value is based upon an assumed surface temp of 300 K ( 80 F) . It might be worthwhile to review the values that NASA may have for anodized aluminum, likely tabulated as a function of wavelength. As I recall, most NASA data is available thru the Univ of Georgia COSMIC department.

Another factor that would affect the actual surface temp is the radiative properties of the surroundings. Refer to Siegel and Howells's "radiation heat transfer" or Mike Modest's "Radiative Heat transfer" texts

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[georgeverghese]

For solar radiation (source temp 5800degC), the 2 sets of emissivity data in Perry are in agreement for surface oxidised aluminum / anodised aluminum (ε=0.15) at a surface temp of 300degK or thereabout.

 

[pierreick]

Hi,
From Albright's chemical engineering handbook,
Oxidized metal emissivity at 25 C from 0.25 to 0.68

Pierre

 

[davefitz]

anodized aluminum has a much higher emissivity than oxidized or non oxidized aluminum- it involves submerging the piece in an acid solution, and the process is used to reduce later corrosion of the aluminum when exposed to a salt water atmosphere. As per theis link, it is approximately 0.84 .

https://buildings.lbl.gov/publications/spectral-emissivity-anodized-aluminum

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[MEngr3239]

Can someone please share the info for Emissivity Coefficients of .050" Thk. black-anodized aluminum at a temperature range of 20 to 120 degrees C? I found the emissivity coefficient as 0.8 at the ambient temperature (20 degrees C), but I need to find the emissivity coefficient at 120 degrees C so that I can create a proportional line (I assume it would be proportional) of the graph with emissivity coefficients vs. temperature 20 - 120 degrees C. Thank you.

 

[3DDave]

I doubt that the emissivity coefficient changes over such a small temperature range. It's a fraction of the expected black-body radiation which does vary per black-body equations.

Your inclusion of the material thickness suggest you don't know what emissivity is and are trying to fake emissivity using a thermal conduction model, which won't work.

 

[georgeverghese]

Can you tell us what this e is to be used for:
a) Solar radiation incident on anodised aluminum surface at 20-120degC and then re emitted out to surroundings - use 0.15 for the entire temp range
or
b)Emission from a warm anodised aluminum surface ( at 20-120degC) to the surroundings at ambient temp - use 0.7-0.8 for the entire temp range

Surface thickness has no bearing on emissivity. You've got to read up the chapter on radiation in your dusty Uni textbook or Perry chapter on Heat Transfer.

In case (a) for opaque anodised aluminum, note that total incident solar radiation splits out into 2 components at this surface : one component is absorbed and re emitted out, another is reflected back out. In case (a), emissivity is approx the same value as absorptivity, so reflected component is 1-0.15 = 0.85. Reflectance is thus the complement of absorptivity.

 

[pierreick]

Hi,
Consider this document and you may download the software in reference.
Good luck
Pierre

 

[IRstuff]

One thing to note is that a 100-K shift in temperature shifts the peak of the blackbody curve from about 10 um to about 8 um, in addition to cranking out 3.3x the amount of radiated power

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[3DDave]

Radiant heat transfer software would account for that - but, from his other thread on this same subject, he's using conduction model analysis.

 

[MEngr3239]

Hi 3DDave, the conclusion indicated in red in the attached article made me lean toward, that there might be substantial changes, but as you and georgeverghese suggested, I will be applying 0.7-0.8 to anodized aluminum surface at 20-120degC in the thermal simulation case. Will be testing the power supply sheet metal box in the lab and comparing the lab results of the anodized surface temperatures with the thermal simulation case afterward. Thanks.

 

[georgeverghese]

Obviously, with a warm metal sheet box emitting heat outward, you got radiant heat and convective heat also. In still air (or near still air), natural convection will dominate and is a function of Grashof and Prandtl numbers for air at the surface film. Natural convection is greater over vertical surfaces than across / over horizontal surfaces. Inside the box, you've got the inside film heat transfer resistance to account for, plus any radiative component for heat source emitting to the inside surface for warm surfaces not in contact with the metal box. Plenty to keep you pulling your hair out in misery.

 

[IRstuff]

MEngr3239, I don't know if you are trying to be difficult or what. This article should have been referenced from the get-go, since it affected your judgement and is a pointless reference.

> The article you cited specifically mentions that the material was HIGHLY REFLECTIVE, so not anodized
> The experimental setup makes no mention whatsoever about controlling or eliminating convection
> They used a thermograph to image the workpiece, which means they only know about the AVERAGE emissivity between about 7 to 14 microns, making their test only partially meaningful
> Their use of a "black tin box: to mitigate the effects of the radiation from the furnace walls is highly suspect, since the tin box itself radiates, and potentially could have been at a different starting temperature, and could have been thermally changed by the radiation from the furnace walls also. It is well known that something like this requires reflective surfaces, not black surfaces. Was the black from paint, or something else? Typical black paint has a wavelength dependent emissivity as well, such as Aeroglaze 306 or Krylon Ultra Flat Black 1602
> Given that they neither controlled nor eliminated convection, what do their results really mean?

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