Effects of finish of heat transfer capability

[ddelaiarro]

I'm working on a project that involves putting electronics equipment into a solid metal (aluminum) case. The cases are sealed watertight and I need to be able to transfer the heat originated at the electrical components to the ambient air. I can perform the analysis to get a pretty good idea of where I'm at before we build a prototype.

What I've been unsuccesful in finding in any information on the effect that certain types of finishes (anodizing, powder coating, chem filming, etc) has on the heat transfer process.

Does anyone have any experience on this and/or could you lead me to any literature that might be relevant?

Thanks,
Dan

 

[Bystander]

Rohsenow & Hartnett, Handbook of Heat Transfer (out of print, hit the library). The thermal conductivity of the aluminum is orders of magnitude greater than that of air; translating, the heat transfer is going to depend entirely on the rate of air flow over your package surface, and on its surface area (unless you're planning on running the package at much more than 10 K above ambient, in which case you'll be looking for materials with high room T emissivities --- black paint, white paint, just about any of the lacquers or enamels).

 

[FredGarvin]

That's a toughie. I have looked through my resources and could not find anything other than the standard emissivity vs. wavelength charts.

This may be your best bet:
http://www.iop.org/EJ/abstract/0508-3443/12/3/306

Here are other links I found (all are journal articles)

http://www.openxtra.co.uk/articles/emissivity_intro.php
http://www.springerlink.com/content/u557370371m0n614/
http://www.springerlink.com/content/h4500170m23137jv/
http://www.hthpweb.com/php/onlineac...rid=729&year=2001&issue=3&fpage=261&lpage=269

 

[ddelaiarro]

Thanks for the tips folks.

Bystander, could you elaborate on this statement? Most notably, the 10K $$\Delta$$T from the components to ambient. Some of our components have been shown to run upwards of 80*C when the ambient temp is 55*C.

unless you're planning on running the package at much more than 10 K above ambient, in which case you'll be looking for materials with high room T emissivities --- black paint, white paint, just about any of the lacquers or enamels

 

[Bystander]

qdot = k_S-B x A x emissivity x ( 4 del T x T³ ), T = ambient, A = package area, k_S-B = 5.67x10^-8W/(m²K⁴; you'll be seeing more heat loss from the package due to radiation than from conduction or convection (unless you've got a hurricane blowing air over the thing --- fin the package then). 300-400 K emissivities of metals run several hundredths to a few tenths, and organic materials, oxides (paint fillers), run half to 0.8-0.9. Gets you into the 100 W/m² neighborhood.

 

[berkeman]

I'm working on a project that involves putting electronics equipment into a solid metal (aluminum) case. The cases are sealed watertight and I need to be able to transfer the heat originated at the electrical components to the ambient air. I can perform the analysis to get a pretty good idea of where I'm at before we build a prototype.

What is the air flow situation going to be like? Can you use forced air, or are you only given still air? Will there ever be wind (like, is it outside?)?

If you are only given still air, then the shape of the enclosure and how you route the air will make a difference. If possible, you want to form a chimney with an outer package, and have the heat transfer help to generate a flow of air from bottom-to-top. What is the overall mechanical arrangement of your aluminum box with respect to the overall packaging and environment? Can you add more surface area in the form of vertical grooves or fins?

 

[Cyrus]

Its sealed watertight, so I don't see how a chimney will work. Also, adding cooling fins can decrease heat transfer if done improperly.

 

[berkeman]

Its sealed watertight, so I don't see how a chimney will work. Also, adding cooling fins can decrease heat transfer if done improperly.

I mean a chimney with the sealed box in the middle of some enclosure. If it's just plain out in the air, then vertical fins would be all that could be done to increase the heat dissipation and lower the internal temperature.

 

[Cyrus]

Fins could work, but they could also not work. So you have to be careful. You can't just slap them onto anything and expect a increase in cooling. If the fins are the wrong size or material, you will increase the thermal resistance and it will get hotter.

Also, I noticed that you said "I can perform the analysis to get a pretty good idea of where I'm at before we build a prototype."

I am going to have to say no here. You can be off as much as 50% when it comes to heat transfer, especially if your using some sort of Nusselt Number derived from equations. If you are close to the edge in terms of overheating, I would perform a lot of tests and not rely on any calculations.

 

[Bystander]

(snip)I am going to have to say no here. You can be off as much as 50% when it comes to heat transfer, especially if your using some sort of Nusselt Number derived from equations. If you are close to the edge in terms of overheating, I would perform a lot of tests and not rely on any calculations.

"50%?" or more. Thanks for getting the focus back onto the question, Cyrus.

"Finishes and emissivities?" Measure what you've got, and throw the number away, because surfaces polished for low emissivities accumulate dirt, and surfaces treated for high emissivities polish from handling, environmental effects, grain growth, or the just plain contrary nature of inanimate objects.

Figure the conductive and convective transfer you design for will halve over time due to surface fouling, and the radiative transfer to increase if you start with a high gloss finish, or decrease if you start with a flat or matte.

 

[Cyrus]

When I took heat transfer, my professor said that you are lucky if you can get the heat transfer ]to be accurate to within 20% when using the Nusseult Number based on a correlation. We did an experiment in the lab and were off by as much as 20% when we compared it to what was in our book.

How is what I said off topic? If you have experience predicting the heat transfer via the Nusselt Number and can shed some light into how accurate a prediction you get, I would be interested to know.

 

[Bystander]

(snip)How is what I said off topic? If you have experience predicting the heat transfer via the Nusselt Number and can shed some light into how accurate a prediction you get, I would be interested to know.

It wasn't --- we were wandering off into tangential discussions of emissivities --- you said, "Measure." OP wasn't about theory, it was about practice --- practice is, "Measure."

 

[berkeman]

It wasn't --- we were wandering off into tangential discussions of emissivities --- you said, "Measure." OP wasn't about theory, it was about practice --- practice is, "Measure."

Yeah, when I read Bystander's comment, I thought he was glad that you were pulling the discussion back to the OP's question -- I didn't interpret it as sarcastic. You both are being helpful to the OP.

 

[Cyrus]

He said:

I can perform the analysis to get a pretty good idea of where I'm at before we build a prototype.

I am assuming he's getting the Nusselt number from a book to do this 'analysis'.

Also, can you answer my question, out of curiosity? <Not being sarcastic here>

 

[ddelaiarro]

I want to thank everyone for their input. It seems as the consensus (and something our group had originally suggested to mgmt) is to do some actual experimentation and scrap the mathematical models. Basically, what it comes down to is that mgmt is asking us if the powder-coating we put on our products has a positive, negative, or null effect on our ability to transfer heat (given that the design does not change - i.e. not increasing surface area via size change or addition of fins). It looks as though that will be the route I will go.

Now I just have to devise an experimental procedure for the four situations (powder coat, chem film, anodize, and bare - the last as a control to judge the rest on).

Cyrus - what question are you referring to?

 

[Bystander]

"Ladeeez and gennulman --- the horses are approaching the gate, last chance to place your bets."

Powder to win, film to place, bare to show, and anodized last. Two dollar ticket.

Let us know how much money I lost.

 

[berkeman]

Remember to take the "weathering" effect into account that was mentioned in the thread. That was a great point.