Temperature dependent thermal conductivity

[james bertram]

Guten morgen Fraunhofer people

I have been researching temperature effects on thermal conductivity of building materials (paper released by Building Science Corp in US), and this led me to to have a look at the option for temp effects on thermal conductivity.

Available thermal conductivity data seems to be for a mean temp (Australia = 23 C).

Can you clarify, are values used in wufi "mean temp" values. If so, does the correction factor adjust the mean temp value?.

Thanks you again,
James

[Thomas]

Guten Abend James,

unless a moisture- or temperature-dependence of the thermal conductivity is explicitly specified in the material data set, WUFI uses a constant thermal conductivity (the value entered in the respective data field).

A constant thermal conductivity may be used if it can really be considered constant for the material at hand, or if the user decides that the variation (small or large) of the conductivity does not affect the result of the intended simulation and it is sufficient to use a representative constant value. The "representative value" will usually be some kind of "mean value" but other choices are possible and it is for the user to decide which choice will be appropriate for the simulation. WUFI does not expect a constant value to comply with some specific definition - if you have reason to tell it to use a constant value it will simply trust you and use that value.

If you wish to use a temperature-dependent thermal conductivity, you can enter a table which specifies the thermal conductivities for different temperatures.
Alternatively, if you are satisfied with a simple linear variation of conductivity, check the box "Generate" and enter the "temperature-induced thermal conductivity supplement" which specifies by how many W/mK the thermal conductivity increases for each degree of increased temperature. The value entered in the "thermal conductivity" field is then assumed to be the value for 10°C, and the conductivities for higher and lower temperatures are extrapolated from there, using the specified supplement.

Kind regards,
Thomas

[james bertram]

Hello Thomas - an erudite answer as always...thank you.

But I am still a bit

Does this mean that the temp values shown on the "Thermal Conductivity, temperature dependent" table and graph are determined by the method of calculation:
1 "Mean value" = temp values are mean values?
2 "Other method" = temp values are as for that method?

I would have thought WUFI inputs and calculation protocols would be based on one or the other?

Thank you for your support
James

[Thomas]

But I am still a bit

Hi James,

then I'm afraid I don't really understand what the question is aiming at.

If the question is whether WUFI assumes or requires that the thermal conductivity to be used for a material has been determined with some specific measurement method, then the answer is: no, it doesn't.

The material parameters provided by the user should simply be "adequate for the task". Depending on the relevance of a particular material parameter for the simulation results, it may be sufficient to guess it, or to take it from published tables describing similar materials, or to measure it with crude and quick methods. If the result depends very sensitively on a certain parameter, it may be necessary to measure this parameter very carefully.

As to the thermal conductivity in particular, experience shows that water contents usually only depend weakly on the thermal conductivities of the materials in the assembly. It is then sufficient to simply use "reasonable" numbers.

If you are worrying whether a measured conductivity which is stated to be valid for a certain mean temperature should be adjusted for use in a simulation in which a different mean temperature prevails: this is usually not necessary. If you are not sure how sensitively a simulation result depends on a certain simulation parameter and how accurately this parameter needs to be determined, simply run a few test simulations with variations of this parameter. In most cases you will find that the simulation result only depends so weakly on these variations that your assessment of the simulation (e.g. construction fails / doesn't fail) would remain unaffected. If you find that varying a parameter does make a significant difference, you can use the test calculations to find out how accurately that parameter needs to be determined.

In the unlikely case that you find it necessary to adjust your conductivity to the mean temperature occurring in the simulation, you have to do this by hand and enter the result in WUFI.

Kind regards,
Thomas

[james bertram]

Thomas - thank you for taking the time to explain further.

Your response clarifies all.

Best wishes
James