aluminium cooling fins 1

[JamieMassie]

Hello

how will 50 microns of hard black anodising affect the performance of aluminium cooling fins in air

thanks

 

[sreedhara]

sreedhara ( Mechanical)

how to design aluminium fins for 180 kw heat dissipation. base temperature is 60 C and ambient is 30 C. fin type is annulus.

thanks.

sreedhara sudhakara sarma

 

[kenvlach]

Hi,
The hard black anodizing of aluminum greatly reduces the thermal conductivity but greatly improves the emissivity. Overall, the effect is beneficial. The Surface Treatment and Finishing of Aluminum and Its Alloys, 6th Edn., pp. 1092-1094 (2001) cites a number of studies:

Burr and Hay, Canad. J. Research, Sect. A, vol. 28, p. 282-286 (1950), compared the heat loss from aluminum cylinders as a function of anodic thickness (up to 20 microns) from 23 to 100^oC. Heat loss from anodized aluminum was 43 to 79 % greater than from polished aluminum.

Thermal resistance (^oK/W) as a function of anodic thickness is given by T.R. Ogden et al., Materials Letters vol. 5, no. 3, p. 84-87 (1987). Values increase linearly from 0.8 at 25 microns to about 1.15 at 50 microns. The plot extrapolates to 0.5 at zero thickness, which I presume is due to surface effects and the anodic structure itself (which has a barrier layer below the cellular structure which makes up the bulk of the thickness).

The relative coefficient of heat emissivity was studied by N. D. Tomashov, Light Metals, vol. 9, 429-438 (1946). Some values, with a perfect black body at 100:
Polished aluminum 4
As-cast aluminum 18
Anodized surface 80

Other studies show that about 90% of the emissivity is reached with an anodize thickness of 5 microns, and that there is little improvement above 15 microns thickness.

Jamie, is there a need for 50 microns thickness (such as abrasive environment)? If not, I suggest reducing the anodize thickness by a factor of 4, to 12.5 microns (0.5 mil). The only finned aluminum HXers I have ever processed with 50 microns of hard anodize were for submarines, where the environment was seawater. For air-cooled fins, 0.4-0.8 mils (sufficient for dyeing black) of either conventional or hard anodize is commonly used.

sreedhara, if the fins are inside an outer tube, all I can suggest is applying 10 microns anodic thickness prior to assembly.

P.S. Most of the above info is also in The Surface Treatment and Finishing of Aluminum and Its Alloys, 5th Edition, which is clearance priced at $199 from ASM Int.

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=11399

 

[JamieMassie]

thanks, I may well reduce the thickness
J.

 

[ko99]

kenvlach,
Excellent data. Just one question on the thermal resistance: Does your source specify the heat source area? Or perhaps the units are actually K/W/area? Thanks

sreedhara,
180kW with only a 30C rise is 0.000167 C/W! I hope you have space for a very large heat sink, and I hope the heat source is spread over a large area. Size and fin design also depend on airflow. Kevin O'Connor

http://www.ECooling.biz

 

[IRstuff]

Depends on what you are doing. The units are correct for lumped thermal resistance calculations where the thermal conductivities and surface areas are known and lumpable.

W/m-K is unit for thermal conductivity, which, when multiplied by Area/thickness, results in thermal conductance, whose inverse is thermal resistance. TTFN

 

[kenvlach]

ko99,
Good point re units.
The plotted results from Ogden et al. (1987) in The Surface Treatment and Finishing of Aluminum and Its Alloys, 6th Edn. has 'Thermal Resistance (K/W)' for the y-axis.
However, the text mentions that the thermal conductivities found were 0.5-1.0 W/m/K, which is more than an order of magnitude less than for bulk, polycrystalline alumina.

Thus, the thermal resistance has units m-K/W (or m²K/m/W, with area in the numerator & thickness in the denominator).

 

[ko99]

Thanks kenvlach

Ogden's test area must have been small (about .4x.4&quot. He probably used K/W instead of K/W/in2 because of the y-intercept (.5 K/W at zero thickness)

The actual contribution of the anodized layer to overall performance depends on other factors. Using some typical values as an example, if the sink has 20in2 surface area, sink-to-air of 2 K/W, and 2in2 interface with the heat source, then even 50 microns will be negligible. Kevin O'Connor

http://www.ECooling.biz