WUFI

Whether the program takes into account a difference in heat of phase transition at negative and positive temperatures. If yes, completely
whether the moisture in a material is considered frozen at temperature below zero?

Mikhail wrote:Whether the program takes into account a difference in heat of phase transition at negative and positive temperatures.

Dear Mikhail,

the latent heat of melting and the latent heat of evaporation are treated as constant (heat of melting: 333 kJ/kg, heat of evaporation: 2500 kJ/kg), their dependence on temperature is neglected.

If yes, completely whether the moisture in a material is considered frozen at temperature below zero?

The freezing-point depression in the small capillaries is taken into account. That is, even at temperatures below zero only part of the water contained in a porous material is frozen. How much water is frozen at a given temperature depends on the pore structure of the material. For details, please see Dr. Künzel's PhD thesis, section 2.3.6 "Moisture transport below the freezing point":
http://www.hoki.ibp.fhg.de/ibp/publikat ... tion_e.pdf

Kind regards,
Thomas

Dear Thomas!

I spend calculations of protecting designs at low temperatures (up to-40 C).
Whether for me it is important to know brings an essential error ignoring
phase transition water - ice in such calculations. Therefore I have questions:
1. Really in Dr. Künzel's PhD thesis has shown a basic opportunity
to enter for each material some function of quantity of a moisture-
Ice from temperature. A question - whether it is realized somehow in the program?
2. Yes, really heat of phase transition ice - water is essentially less
heat of phase transition of waters - pairs. However, enthalpy some ice is much lower enthalpy water,
therefore it is necessary to take into account this moment at the decision of the equation of balance enthalpy,
which is realized in the program.

Hi Mikhail,

the partial freezing of water at temperatures below 0°C is implemented in WUFI precisely as described in Dr. Künzel's thesis.

To confirm this, you can do a calculation with a material which is filled with water and which is slowly cooling below 0°C. Set up a "component" consisting of some material which can hold a lot of water (for example, some brick). Set the initial water content as high as possible and apply a high sd-value at the surfaces to prevent the water from evaporating. Set the initial temperature at 20°C and the ambient temperatures somewhere below 0°C (for example, -10°C or -20°C). Then start the calculation and examine the heat flowing out of the material.

As long as the temperature is above freezing, the heat flowing out of the material is simply determined by the combined heat capacity of the material and the contained water.
As soon as the material goes below 0°C, you will see that latent heat is added to the amount of heat flowing out of the component. However, not all latent heat is released at 0°C. Only the water in the largest pores freezes at 0°C; the water in the smaller pores freezes at lower temperatures. So you will see some latent heat continuously being released while the temperature drops lower and lower, showing that there is still some liquid water which is freezing at that particular sub-zero temperature.


Concerning the temperature-dependence of the latent heat of melting or evaporation:
Please keep in mind that WUFI is primarily a simulation tool for moisture transport, not for heat transport, and that it is a simplified model, adapted to the conditions and accuracy requirements usually encountered in building physics. In our experience, the moisture conditions resulting from a simulation usually depend only relatively weakly on the exact temperature conditions, so that it is permissible to use slightly simplified models for the temperatures.

If you are mainly interested in the moisture conditions, these simplifications may be acceptable for your purposes. If you are interested in the exact thermal behavior of your building component, they may not be acceptable, in particular if your quite extreme temperatures are considered.
If you have experimental or theoretical test cases where you are confident that they show you with sufficient accuracy what should happen, you can use them to test whether the WUFI results are useful for your investigations.

Regards,
Thomas