How to specify emissivity

[dkusmer]

I have a thinwall metal tube (2"OD X .032"wall X 24"long). The metal has an emissivity of 0.54. There is vacuum on the OD (outside diameter) and air on the ID (inside diameter). The OD of the metal tube is plated with silver (emissivity = 0.06). How do I specify this in Simulation CFD (i.e. emissivity=.06 on the tube OD and emissivity=.54 on the ID). Thanks.

Heat is being radiated thru the vacuum to the OD of the metal tube. As the tube heats up it transfers that heat to the air inside the tube which eventually heats the components inside the tube.

Regards, Dan

[Royce_adsk]

The easiest way to do this is to split the tube into 2 parts so that you can define 2 different materials. Most of the material properties will be the same except for emissivity.

-Royce

[dkusmer]

Royce,

Thanks, great idea, especially if most of the material properties will be the same except for emissivity. However, my life isn't quite that simple.  I know the plating (silver) is only .001" thick compared to the .032" tube thickness but silver's thermal conductivity is ~30X greater than the metal tube. I'm not sure how it will affect my analysis.

If I make the tube out of 2 equally thick parts (each part .016" thick) do you think I should adjust the silver's conductivity to account for the fact that in reality it is only .001" thick? What would you make the material's conductivity?

Thanks, Dan

[Royce_adsk · Accepted solution]

Sorry for the delay.

Another option you can do is apply a surface material to where you have the silver plating.

Using the solar heating example I show how adding a silver roof using a surface material can reduce the temperature on the roof while using solar heating.

Would that approach work for your analysis?

[dkusmer]

Royce,

That is what I am looking for but I can't the option for adding a surface material in 2D axisymmetric. Do you know if it is available for 2D axisymmetric?

Regards, Dan