Thermal couductivity of granular material

[asimpson]

What would be the relative thermal conductivity of granular material to solid? Roughly? Assuming dry and in air. What would be the effect on grain size or shape.

Specifically I am looking at iron filings to solid iron which I measured to have a relative density of less than 50% I would imagine thermal conductivity to be dramatically reduced.

Any thoughts?

Thanks

 

[IRstuff]

Nothing obvious comes to mind, but you can probably make use of thermal spreading resistance concepts coupled with thermal contact resistance concepts to build a model:

http://www.mhtl.uwaterloo.ca/intro_disk.html

http://en.wikipedia.org/wiki/Thermal_contact_conductance

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[Mechomatic]

I agree with IRstuff. Contact resistance and contact surface area between grains are factors which take this far beyond the usual practice of determining thermal conductivity. If it's critical to have a fairly accurate model, I suggest setting up your own experiment to test different temp gradients with multiple grain sizes. If you're just looking for a ballpark figure, do a volumetric average of conductivity (to over-estimate k) and/or knock off a percentage to account for contact resistance. I don't have a percentage estimation for contact resistance in that situation, but I'm sure you could cobble something together that would be "close enough." Again, though- run your own experiment if you need an accurate model (and have the time and resources).

 

[asimpson]

Thanks guys

 

[IRstuff]

There seems to be a lot of literature on heat transfer related to "porous media." It's not immediately obvious if that's applicable, but there may be some applicable tidbits.

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[dvd]

My comment about conductivity with a granular material is that there will be some settling/consolidation of the granular mat'l and, if in a horizontal plane, the mat'l will pull away from the upper surface (vertical plane will produce hot spot at the top). This will greatly reduce the heat flux in/out of the mat'l. So if one is depending on the granular material to remain in intimate contact with both surfaces, it won't be reliable.

 

[dvd]

... so convection may end up dominating, and free convection within a porous media may be important to understand.