[berniebenz]

Originally Posted by AC_Hacker

In the link that michael pointed to, there was the sentence, "The layers of MLI can be arbitrarily close to each other, as long as they are not in thermal contact."

That material was designed for use in space, where, except for conduction thermal radiation is the only mode of heat gain or loss.

The nature of thermal radiation on earth is the same, but heat loss will have the additional mode of convection. Convection depends on the flow of fluids (air can be considered a fluid, since it flows) and fluid flow is inhibited by friction.

So, if the gap between layers, radiant insulation layers that is, is very small, the fluid friction will inhibit convection losses. The larger the space, the lower the fluid friction, the greater the convective losses.

So, the "at least 3/4 inch" idea might have a problem.

Also, regarding wavelengths, check this out:



So, a long wavelength is 1mm.

If your wavelength idea is correct, you need a maximum space of 1mm to allow the reflective surface to do it's reflecting magic. As your space becomes larger than that, fluid resistance drops, convective currents form... the larger the space, the greater the convection.

I believe I have heard this referred to as "convective pumping".

Best,

-AC

Well spoken, AC! In covering the general case of friction's affect on natural convection heat transfer. Later in this thread we discuss the special case of controlling downward heat flow,