liquid transport coefficient

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Tobias
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liquid transport coefficient

Post by Tobias » Fri Oct 08, 2021 2:37 am -1100

Hi, I have a question concerning the liquid transport coefficients ( DWs and DWW). I've noticed that in many materials data sheets these coefficients are not given and so there values are assumed to be zero by WUFI ( i think). I wanted to know if, in this case, it is consered that in the material, the liquid water cannot spread. The liquid water content observed in this material is therefore only due to the condensation of water vapour ?
Kind regards

Thomas
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Re: liquid transport coefficient

Post by Thomas » Sun Oct 10, 2021 12:38 am -1100

Hi Tobias,

yes, if no liquid transport coefficients are given, they are assumed to be zero. (Strictly speaking, they are assumed to be 1e-16 m²/s for numerical reasons, but that is practically zero.) So in these cases water transport can only happen by vapour diffusion, the strength of which is determined by the mu-value of the material.

If at some point in time the water content distribution in such a material is different from the initial distribution, this can only have been caused by vapour diffusion or by the activity of some moisture source.

Kind regards,
Thomas

Tobias
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Re: liquid transport coefficient

Post by Tobias » Sun Oct 10, 2021 8:31 pm -1100

Hi Thomas,
Thank you for your reply !

Tobias
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Re: liquid transport coefficient

Post by Tobias » Sun Oct 10, 2021 9:54 pm -1100

Hi!
Another questions concerning, this liquid transport coefficients.
If these are assumed to be zero for some matérials, is it because it is these coefficents haven't been measured or is it because zero is a good approximation of DWS and DWW for these materials ? I've noticed that it was possible to generate artificial DWW and DWS, knowing the absorption coefficent A of the material. In order to carry out the most realistic simulation possible, should these coefficients be left to zero or should new coefficients be generated ?
Kind regards
Tobias

Thomas
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Re: liquid transport coefficient

Post by Thomas » Mon Oct 11, 2021 1:12 am -1100

Hi Tobias,

the liquid transport coefficients are set to zero if the material is not capillary-active. In these materials there is no or only negligible liquid transport, for example mineral wool insulation, polystyrene insulation, plastic membranes, glass etc.

As a special case, there are a few materials in the material database which are capillary-active, but for which the liquid transport is treated together with vapour transport. In these cases, the liquid transport coefficients are set to zero and the mu-value (the diffusion resistance factor) is variable, depending on moisture content.

The reason for this is that during a vapour resistance measurement some small amount of liquid transport occurs simultaneously, but the measurement is usually analysed as if only vapour transport had occurred. This results in a variable mu-value which therefore includes the effect of liquid transport at low moisture levels. It could be argued that it is better to model liquid transport separately, but then the mu-value would also have to be modified so as to remove the liquid transport component from it.

An example for this are the "hardwood" and "softwood" materials in the database. The material info says:

"Hardwood / softwood:
These material data are primarily meant for the calculation of diffusion processes (data set without liquid transport coefficients; liquid transport modelled by variable mu-value). To be used in cases without capillary water uptake by the wood, e.g. for drying-out of roof boards and for similar wood layers which are not directly exposed to weather."

On the other hand, the "Spruce, radial" is a wood material which does have liquid transport coefficients. The material info says:

"Oak/spruce radial/longitudinal:
The liquid transport coefficients have been determined from the moisture behaviour of specimens exposed to natural weather. The moisture-dependence of the mu-value is already included in the liqid transport coefficients. Dww has been set equal to Dws in order to describe swelling phenomena. Great dependence on swelling condition! To be used for wooden parts in direct contact with rain water (e.g. framework constructions) and for analysing drying processes at high moisture contents. Should, however, be used with caution and only for approximative calculations, because of large uncertainties."

Kind regards,
Thomas

Tobias
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Re: liquid transport coefficient

Post by Tobias » Mon Oct 11, 2021 2:08 am -1100

Hi Thomas !
Thank you so much for your detailed and clear answer !
Does it implies that for the materials, for which the liquid transport coefficients are set zero, the liquid water brings by the rain on the exterior surface doesn't "enter" at all ? I mean, for some of these materials , the liquid water transfer is partly taken into account with the variable mu-value like you said. So, the capillary capacities of these materials are taken into consideration when there are vapour transfers in the materials. Nonetheless, in the case of the rain, the source of humidity on the surface isn't water vapour but it is almost only liquid water so eventhough there is a variable mu-value, I don't see how liquid water can be carried inside the material from the outside.
Kind regards
Tobias

Thomas
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Re: liquid transport coefficient

Post by Thomas » Tue Oct 12, 2021 9:44 pm -1100

Hi Tobias,

yes, including the liquid transport in the mu-value only works if the vapour and the liquid fluxes have comparable intensity. These materials are intended to be used under indoor conditions where they are not exposed to rain water. Under these circumstances, no major capillary uptake of liquid water takes place anyway, and the modification of the mu-value takes into account the very small liquid flow which takes place under indoor conditions.

This means that although some these materials are able to show capillary activity when exposed to liquid water (e.g. rain), this property is neglected in these particular datasets because the material is supposed to be used under circumstances where it is not expected to be exposed to liquid water (and because no liquid transport data were available). That's why the info text for hard- and softwood says, "To be used in cases without capillary water uptake by the wood".

If you need to take rain absorption into account for your purposes, you'll have to use a different (but sufficiently similar) material, or you'll have to modify the existing material by providing liquid transport data.

Kind regards,
Thomas

Tobias
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Re: liquid transport coefficient

Post by Tobias » Wed Oct 13, 2021 9:41 pm -1100

Hi Thomas
Thank's for your answer !
Kind regards
Tobias

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