Heat Rejection of Alum

[Metalguy]

I am trying to find out if a black anodized piece of die cast alum. will reject heat faster than a plain polished piece. heat cond. won't be a factor here--it's radiation vs convection. I am aware of the importance of the actual temp. for radiation heat loss, but for this case the temp. is only ~125 deg C. What temp. would be necessary for the black anod. piece (I assume there is a slight insulation effect of the anod.) to reject heat faster than the bare one?

 

[NickE]

I think that black bodies radiate heat at a faster rate than shiny ones. Not sure though, The anodizing would have a slight insulating effct over bare aluminium.

nick

 

[Metalguy]

Yes, and that's what I'm trying to find out. Flat black radiates heat about 9 times faster than white/silver, I think. Hopefully someone here can tell me about the insulating effect of anodizing.

 

[arunmrao]

This is a coincidence. I just finished teaching my daughter a school physics lesson on heat transfer. It mentions that cooking vessels are made bright and shining,so that the cooked food in the vessel loses out heat slowly,thereby the food in the vessel remains hot longer

 

[IRstuff]

The anodizing is usually irrelevant. Assume that the thermal conductivity is 10% of aluminum. Most anodic layers are less than 3 mils thick.

This means that the anodic layer is equivalent to 30 mils of aluminum in thermal conductivity. Now it just depends on how thick your thermal path is.

TTFN

 

[Metalguy]

Interesting. What I am really after is the amounts of heat lost via both methods-radiation and convection. Obviously convection depends on air flow, and in this case the air flow is something like a 110 MPH wind. Don't know whether its boundary flow or turbulent-I'll take either case. I have little doubt that radiation will dump more heat than conv. at high temps., but I need the crossover point.

 

[aviat]

This subject comes up often regarding air-cooled cylinders, which operate at approx. 260C. Painting can reduce the surfaces ability to transfer heat to the surrounding air by smoothing the surface, decreasing surface area. If you have a heat problem you could increase convection by using fins (increasing area) if needed, or using baffles to increase the speed of the air.

The following site explains this in a little detail. In his book he says that it is better not to paint black and the optimum surfaces for those temps is unpainted with Alodine treatment.

http://www.sacskyranch.com/paint.htm