XML material data queries: Drying factor, thermal conductivity and water content

[Peter Mayer]

I wonder if you could explain some of the material data properties which are included in the xml material files. It would be helpful to know how to manage these properties when creating user defined materials.

Scalar properties
1) Drying factor [id 11]: what is this and how is it used in the wufi calculation?
2) Thermal conductivity values: what is the difference between:
• Thermal conductivity, dry, 10°C [id 4]
• Thermal Conductivity, Design Value [id 15]
3) Water content: what is the difference between:
• Reference water content [id 6]
• Typical Built-In Moisture [id 13]

Functions
Function id 3: should the name be “Liquid transport coefficient, redistribution”?
The name “Thermal conductivity moisture dependent” is used for both function id 3 and id 4.

[Thomas]

I wonder if you could explain some of the material data properties which are included in the xml material files. It would be helpful to know how to manage these properties when creating user defined materials.

Scalar properties
1) Drying factor [id 11]: what is this and how is it used in the wufi calculation?

Hi Peter,

it had been expected that it might be possible to describe the characteristics of the drying process by a single number, similar to the A-value which describes the rate of capillary water uptake and which can be used to estimate the liquid transport coefficients for suction. The drying factor would then have been useful to estimate the liquid transport coefficients for redistribution. However, it has not been possible as yet to develop a satisfactory estimation method, and the drying factor is currently not used

2) Thermal conductivity values: what is the difference between:
• Thermal conductivity, dry, 10°C [id 4]
• Thermal Conductivity, Design Value [id 15]

The "thermal conductivity" which is entered as one of the basic material parameters is used by WUFI as the thermal conductivity of the material.

You have the option to add a moisture-dependence and/or a temperature-dependence (as optional Hygrothermal Functions). In these cases, the number entered as the basic value also automatically appears in the respective tabulations and is then more specifically identified as the thermal conductivity in the dry state (with the other table entries describing values for other moisture states) and/or the thermal conductivity at 10°C (with the other table entries describing values for other temperatures).

If you make no use of these options, WUFI uses the value entered in the text field as a constant number. It is up to you to decide whether you wish to use a value measured at 10°C in the dry state, or a value measured at some representative temperature in some representative moisture state, or whether you want to use a slightly increased value to take typical moisture contents into account, etc. WUFI will happily accept everything it is given by you, no questions asked. You decide what makes sense for the investigation at hand.

In some countries "design values" are prescribed for design calculations. These are fixed numbers (they have no explicit moisture or temperature dependence), but they usually include some increase to take typical moisture contents into account. Details of how the design values are to be derived vary strongly from country to country. If you are doing design calculations for which you are required to use the design values rather than measured values, you can park this design value in the text field "Thermal Conductivity, Design Value". To use it, check the option "Excluding Temperature and Moisture Dependency of Thermal Conductivity" in the Numerics dialog (yes, the labelling of that option is not very clear). If you don't use design values, leave that parameter field empty.

3) Water content: what is the difference between:
• Reference water content [id 6]
• Typical Built-In Moisture [id 13]

The moisture storage function and the two kinds of liquid transport coefficient (suction and redistribution) can optionally be described by model functions rather than tabulations. The reference water content is one the parameters specifying these functions.

If you use the model function for the moisture storage function, the number entered in the text field "Reference Water Content" (visible once you check "Approximate") will be the function value for 80 % RH.

If you use the model function(s) for the liquid transport coefficient(s), this is also one of the required parameters. It is supposed to be the moisture content at 80 % RH, but you can enter any value you deem appropriate for the purpose (unless you also use it to model the moisture storage function - then it must be the value you want for the moisture storage function at 80% RH).

If you don't use any of these model functions, a value entered in the text field is still present in the background and in the XML file, unused. It reappears in the respective text field(s) once you reactivate the function(s).

The name derives from the fact that, as measurements have shown, the typical moisture content in the walls of a house exposed to typical weather is more or less equal to the sorption moisture resulting from exposure to 80% RH. This so-called "reference water content" is (or used to be) known for many construction types, and the idea is that in case the user does not know the material data for some material, the reference water content often is known, and WUFI can assist the user by deriving material functions from this limited information.


For each simulation you have to specify some appropriate initial water content. If you don't have specific numbers but the initial situation is a "normal" equilibrium water content, a good guess for the initial water content is the sorption moisture corresponding to 80% RH (this being the typical moisture content in this situation, see above). On the other hand, if you wish to determine the drying-out time of a freshly-built house with still wet walls, the initial water content should be the "built-in moisture". If you need this value repeatedly, you may "park" it in the text field "Typical Built-In Moisture". If you select the option "In each layer" for the "Initial moisture in Component" (in the dialog "Initial Conditions") a button "Assign Typical Built-In Moisture" allows you to transfer to the table of initial moisture contents all the "Built-In Moisture" values which have been specified in the material data sets for the layers. If a layer has no "Built-In Moisture" specified in the material data set, its entry in the table remains empty and you have to enter a value by hand.

This edit field only serves as a convenient memo pad for this occasionally needed value. The calculation result is independent of whether this field contains a value or not. If you never use built-in moisture contents as initial moisture conditions, leave this parameter field empty.

Functions
Function id 3: should the name be “Liquid transport coefficient, redistribution”?
The name “Thermal conductivity moisture dependent” is used for both function id 3 and id 4.

In the XML file I just exported from WUFI 6.3 to check this, id 3 has this name, and I can't see any problem with the names for the thermal conductivities. Maybe you saw this in an older version of WUFI and these have been fixed now.

Kind regards,
Thomas

[Peter Mayer]

Hi Thomas,

Many thanks for your detailed and clear answers.

With regards to the least important point: 'XML function names for ids 3 and id 4.' I am using WUFI Pro 6.3.

I've included an xml extract for: 'fired clay brick used for cellars ' to illustrate the duplicate use of the name "Thermal Conductivity, moisture-dependent" that I am seeing.

wuf_xml_material_data_extract_fired_clay_brick_used_for_cellars.jpg (24.97 KiB) Viewed 10385 times

Peter

Best regards

[Thomas]

Ah, I see. Thanks, I have alerted the programmers to look into this issue...

Regards,
Thomas