## Thermal Conductivity of Porous Material

does anyone know where this equation regarding the thermal conductivity of porous material K=K$_{}0$[(1-P)$/(1-B P$)] comes from? I know B has to do with thermal density, but I have no idea what P is? let alone the origins of the equation. This has to do with beds of spherical material.

background: I'm doing a lit review for a graduate student and I'm stuck on this particular equation, and she doesn't know much about it either.

thanks!
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 Tags conductivity, heat, porous, thermo