Modeling materials with insufficient data

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Bonilauri
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Modeling materials with insufficient data

Post by Bonilauri » Sun Apr 22, 2018 5:13 am -1100

Hello,
I'm using WUFI Pro version 6.1.

In the material database, I noticed that several generic materials only have limited data. e.g. the moisture-depentent thermal conductivity.

From the class I took at Fraunhofer last year, I remember that you can use "basic info" for a material, including e.g. the dry thermal conductivity (measured at RH = 0%).

How does the software handle the change in material performance, when not enough data is provided? Does WUFI implement the formulas of ISO 10456 to adjust the thermal conductivity to different levels of moisture content, if the only values for the material refer to dry conditions (RH=0%)?

Thank you,
Enrico
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Christian Bludau
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Re: Modeling materials with insufficient data

Post by Christian Bludau » Sun Apr 22, 2018 7:56 pm -1100

Dear Enrico,
no, WUFI does not add values except the moisture storage function (which is based on the porosity then).
For the hygrothermal functions like thermal conductivity - moisture dependent the dry value is used constant over the water content as shown in the diagram.
If you like to use another, you can unlock the material and generate that (here a supplement would be used) or you can enter your own table.
Christian

Thomas
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Re: Modeling materials with insufficient data

Post by Thomas » Sun Apr 22, 2018 10:40 pm -1100

Hi Enrico,

the "basic data" are always required. If one of those is missing, the simulation will not run because then some term in the transport equation is left undefined.

On the other hand, if some of the "hygrothermal functions" are left undefined, the simulation can run nevertheless. It is up to the user to decide which of the hygrothermal functions are essential for the simulation at hand, which ones are "nice to have" but not essential, and which ones are irrelevant for the present case.

For example, the liquid transport coefficients of a material must be specified if the material is capillary-active and its liquid transport affects the result of the simulation - otherwise the simulation would not be realistic. On the other hand, a material may be capillary-active, but its position in the assembly or the dry boundary conditions may keep it dry so that in the present case no liquid transport occurs in the material - in this case it is not strictly necessary to determine and to enter the liquid transport coefficients.

Another example: The thermal conductivity of most materials depends on their moisture content. However, usually the effect is small and does not significantly affect the conclusions drawn from the simulation (e.g. "component passes / fails"). In this case, the moisture-dependence of the thermal conductivity is just "nice to have", but not essential. It may be included if the data are available, but if the data are not available, it may be left out without seriously affecting the simulation result.

On the other hand, if you wish to examine the insulation quality of very wet walls, it may be necessary to take the moisture-dependence of the thermal conductivity into account.

A third example: WUFI allows to specify a moisture-dependent diffusion resistance factor (µ value). In the case of moisture-adaptive vapor retarders this variability is essential for their function and must be included in the data set. In many other materials, some variability is present but not essential; in these cases the variability may be left out, using only a constant µ value.

A data set in which some of the hygrothermal functions are left unspecified should therefore not be considered incomplete. In many cases, some the hygrothermal functions are just refinements which are not essential for a proper description of the hygrothermal properties of a material. These can then safely be left out of the data set.

If you are not sure whether a hygrothermal function is essential for the simulation at hand, you can use WUFI to do some test calculations with and without the respective functions.

Regards,
Thomas

Bonilauri
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Re: Modeling materials with insufficient data

Post by Bonilauri » Mon Apr 23, 2018 3:20 am -1100

Thomas and Christian,
Thank you both for the clarifications!

Enrico

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