diffusion flux density

[stergiani]

Hei,

I am working on a case with a CLT wall standing on concrete floor (which is insulated from the inside) and I want to check the hygrothermal conditions on the base of the CLT.
I was checking my fluxes close to the bottom of the CLT and i noticed that my diffusion flux density was at 0. I then realized that the graph was taking quite wide range, and the diffusion (and capillary flux) density is just in the order of magnitude of E-8. After reading about diffusion and liquid transport on the relevant literature, it seems to me that the diffusion flux is normal to be that low. Could someone verify that to me or let me know if I am wrong and what could be the mistake? My heat transfer coefficients are properly set for both indoor and outdoor boundaries, the μ-values for the materials are defined by the database and all Sd values are set to 'no coating'.
Thank you in advance
S

[Thomas]

Hi stergiani,

yes, that's the right order of magnitude for a vapor diffusion flux.

Suppose you have a layer of stagnant air, 0.1 m thick, with 20°C/80% RH on one side and 20°C/30% RH on the other side. The mu-value is 1, so the diffusion coefficient is 2e-10 kg/(m s Pa), and the vapor pressure difference across the layer is 0.8*2339 - 0.3*2339 = 1170 Pa, and the resulting diffusion flux density is

g = 2e-10 * 1170 / 0.1 = 2e-8 kg/(m2 s)

The diagrams are currently not 'smart' enough to handle these small numbers in the auto-scaling of the y-axes. Our programmers will have to tweak this a little bit...

Regards,
Thomas

[stergiani]

Thank you, Thomas