Properties of water in Wufi5

[Michel]

In Wufi5, it is possible to change the thermal conductivities of building materials.
But I have not found how the properties of water and water vapor change with temperature in the software.
Assuming that accurate values are available in the software, what is the max. temperature which can be realistically used ?
Thanky

[Thomas]

Assuming that accurate values are available in the software, what is the max. temperature which can be realistically used ?
Thanky

Dear Michel,

the properties of water are hard-coded in the software. The values used in WUFI have been selected as appropriate for the range of conditions usually encountered in building physics, but not necessarily far beyond that.

For example, the thermal capacity of the water is treated as a constant value, its (very small) temperature dependence is ignored. The temperature dependence of the surface tension is also ignored, whereas the temperature dependence of the viscosity is taken into account as a quadratic polynomial. The saturation vapor pressure is computed via a Magnus-type formula, etc.

These values should be appropriate for the temperatures usually occurring in a building component, but their quality will slowly degrade towards much higher or lower temperatures.

However, the limiting factor for the accuracy of the calculation results will usually not be the limited applicability of the properties of water under non-standard temperature conditions. The limitations will usually arise from measured data involved in determining the properties of the water or from unmodelled transport phenomena which may become dominant at high temperatures.

Measured data:
The vapor transport in a porous material is described by the vapor diffusion coefficient in the pore air, multiplied by a 'diffusion resistance factor' which describes the influence of the pore structure on the vapor transport. The vapor diffusion coefficient is computed from the empirical Schirmer equation, the diffusion resistance factor must be measured.
While the accuracy of the Schirmer equation will certainly degrade at higher temperatures, the inaccuracy involved in the measurement of the diffusion resistance factor will probably dominate anyway in the calculation results.

WUFI does not provide a value for the thermal conductivity (and its temperature dependence) of the water. It is assumed that these are appropriately accounted for by the measured moisture-dependent thermal conductivities of the building materials provided by the user. Any inaccuracy in these measurements will directly affect the calculation results (however, experience shows that in general the resulting moisture contents depend only weakly on the thermal properties of the materials and thus also weakly on inaccuracies of these properties).

Unmodelled transport phenomena:
At higher temperatures, transport phenomena may become important which have not been taken into account in WUFI (since WUFI is intended to be a simplified model applicable under ordinary standard conditions). For example, at higher temperatures, the partial vapor pressure developed by evaporating water may be an appreciable fraction of the barometric pressure, leading to water transport driven by pressure differentials (in addition to vapor pressure gradients or water content gradients).
The temperature-induced shrinking and swelling of materials may lead to changes in the pore volume and squeeze water out of or suck water into the material, etc.


How important the effect of these limitations on the calculation results can be depends therefore on the amount of water in your construction, on the materials containing the water and the temperatures occurring in these materials as dictated by their location in the building component.

I'm afraid there is no simple answer to your question; the accuracy of the results will gradually decline with temperatures rising above the usual temperature level, but the details will depend very much on the specific construction type, materials and boundary conditions of the individual case under investigation. Maybe my description of the involved factors may help you a bit to get a 'feel' for what is to be expected.

Standard EN 15026 states: "The hygrothermal equations described in this standard shall not be applied in cases where [...] daily mean temperatures in the component exceed 50 °C.". But this is, of course, only a very rough estimate intended as a very rough guideline.

Kind regards,
Thomas