compare diffusion resistance factor and diffusion coefficient

[mschiere]

Dear all,

I'm calculating moisture diffusion in spruce (Fichte) under its fiber saturation point. In literature sources I find values D (Fick's model) referred to as the diffusion coefficient, and WUFI uses value mu as the diffusion resistance factor. Is there an equation to easily compare the two?

Thanks in advance,

Marcus

[Thomas]

Hi Marcus,

the diffusion coefficient delta as used by WUFI describes the vapor diffusion flow g resulting from a given partial pressure gradient:

g = - delta * dp/dx

(Various versions of Fick's law may use the concentration gradient or the mass fraction gradient or other gradients instead. The respective deltas have to be converted to the delta relevant for the description using the partial pressure gradient).

The mu value simply describes how much smaller the delta of a given material is than the delta of stagnant air:

delta =: delta_air / mu

mu = delta_air / delta


Please see also the topic #17 "Water Vapor Diffusion" in WUFI's online help.

Regards,
Thomas