aluminum foil embedded in thermal insulation blanket

[brin]

we have an insulation blanket used on an aircraft anti-ice duct (eg: bleed air at 550 deg F). The blanket has a layer of aluminum foil between the inside fabric and the glass fibre insulation) that causes problems during manufacture. The foil tends to crease and then tears the inside fabric layer.

We would like to reposition the foil layer into the glass fibre layer so that it does not contact the inside fabric layer. However, we are not sure what effect this will have on the blanket thermal insulation properties.

Is there a simple analytical method to determine the effectiveness of the foil (eg: as a radiant barrier)in 1) immediately adjacent to the heat source, and 2) with a .38 inch layer of glass fibre between the foil and the heat source?

thanks

 

[IRstuff]

The usual Planck's Law and Stefan-Boltman equations should do it. Ostensibly, the higher the temperature the radiant source, the more effective the radiant barrier.

TTFN
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[racookpe1978]

A "radiant barrier" in direct contact with the hotter (or colder) surface will NOT serve as a radiant barrier, but only as one additional (and usually very poor insulator at that!) convective barrier: By stopping air (or the surrounding gas around the hot surface) from flowing easily when heated (cooled) there is always a little insulation effect. But, to be useful, the radiant barrier MUST be separated from the hot surface (2-3 inches separation zone is my training, but I have no specific references available) AND surrounded by a isolation mechanism/material that reduces convection across the radiant barrier "gap".

 

[IRstuff]

YEah, I have my doubts as well. Nonetheless, multilayer insulation (MLI) is used in certain applications, and it consists of nothing more than layers of foil separated by thin insulation netting, and it seems to work OK.

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[brin]

Thanks IRstuff & racookpe1978. I understand the point about coupling the radiant barrier to the hot surface. This has been a point to ponder because it seems that the foil would simply conduct heat direct from the duct surface without acting as a radiant barrier.

I'll keep digging thru my old textbooks.

 

[IRstuff]

http://en.wikipedia.org/wiki/Multi-layer_insulation

describes the MIL that I referred to. The spacing between foil layers is extremely short, comparable to a moderate cell foam insulation.

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[brin]

interesting article. It makes the statement about "MLI being arbitrarily close to each other as long as they are not in thermal contact". Therein lies the crux of my problem- would a .0068 inch layer of 7628 glass cloth be considered sufficient to prevent thermal contact?

I appologies for having to ask what may be an obvious point.

Thanks

 

[IRstuff]

MLI foil layers are not in "contact", but are separated by low-conductivity nettings that are.

Your glass cloth is not fundamentally different than the MLI separators, aside from pore size. Additionally, the typical reflective foils are so thin that they really don't make that much difference conduction-wise. It's not like you're shorting out a bunch of insulation.

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