Comparative analysis between Wufi and Other software

[rozumjak]

Hello,

In my thesis, I am doing comparative analysis of two simulation tools (Wufi Pro (1D) and HeMoT (a tool developed at Czech Technical University in Prague)).
My outputs are wall sections at different times (profiles) and monitoring positions within a simulated wall during the last simulation year (temperature and moisture fields).

While the temperature profiles agree almost perfectly for both tools there are considerable differences between moisture profiles simulated by the two tools. RH computed by Wufi tends to be much higher than RH computed by HeMoT. Interestingly, this is apparent mostly during winter time, while during the summer time are the RH profiles in good agreement.

The main difference between the tools is that Wufi does take into consideration both liquid transfer coefficient for suction and redistribution, while HeMoT considers only liquid transfer coefficient for suction. This could explain why wall simulated by Wufi stays moist longer than wall simulated by HeMoT (thus higher RH for Wufi simulation). But does it also explain why this happens only during winter time (colder half of the year). I dont have enough brain capacity for this one.

My supervisor says that another reason for different RH profiles could be the so called "weigh function", which they introduced in their program and which might not be present in Wufi calculation model. This function blocks water vapor diffusion resistance factor from the system of equations for situations when material is in over-hygroscopic state and liquid transfer coefficient when material in under-hygroscopic state.

Could you tell me if Wufi uses a different approach for this problematics, which could explain the differences in RH profiles? E.G. computing with both mentioned coefficients regardless of the RH situation?

Thank you very much
Jakub

[Manfred Kehrer]

I think our Chzec Collaboration Partner Petr Slanina can help you here the most. You find him at www.WUFI.cz

[rozumjak]

Ok, thanks..

But do you know how Wufi treats the moisture transfer in over- and under-hygroscopic range (the last two paragraphs of my initial question)?

Jakub

[Christian Bludau]

Dear Jakub,
probably there is a difference in the input of the material data.
You can find the background of WUFI in
"Künzel, H.M.: Simultaneous Heat and Moisture Transport in Building Com­ponents – One- and two-dimensional calculation using simple pa­rameters. Dissertation Universität. Stuttgart 1994"
Christian

[Thomas]

The main difference between the tools is that Wufi does take into consideration both liquid transfer coefficient for suction and redistribution, while HeMoT considers only liquid transfer coefficient for suction. This could explain why wall simulated by Wufi stays moist longer than wall simulated by HeMoT (thus higher RH for Wufi simulation). But does it also explain why this happens only during winter time (colder half of the year). I dont have enough brain capacity for this one.

Hi Jakub,
if you set the liquid transfer coefficients for suction and redistribution equal in WUFI (that is, you use the same tabulated values for both) then you can test whether distinguishing between suction and redistribution makes a difference in your case or not.

My supervisor says that another reason for different RH profiles could be the so called "weigh function", which they introduced in their program and which might not be present in Wufi calculation model. This function blocks water vapor diffusion resistance factor from the system of equations for situations when material is in over-hygroscopic state and liquid transfer coefficient when material in under-hygroscopic state.

WUFI automatically reduces vapor transport if the water content is above free saturation. See the online help topic "Reference | Material data | Basic material data - Diffusion resistance factor dry":

"Please note that even if you do not explicitly use a moisture-dependent µ-value, WUFI will treat it as moisture-dependent for moisture contents above free saturation wf:
WUFI will reduce the permeability delta/µ in proportion to the moisture excess over wf, until it reaches zero at maximum saturation wmax (that is, µ reaches infinity). This reflects - in a first approximation - the fact that at very high moisture contents even the larger capillaries become clogged with water and can't contribute to vapor transport any more."

The behavior of liquid transport at high water contents depends on the tabulated data you entered for the liquid transfer coefficients. If there are non-zero values at water contents above free saturation (or above any other threshold) , WUFI will apply liquid transport, if the table contains zeros for any water content, liquid transport wil lbe suppressed at these water contents. Please see the discussion in the online help topic "Reference | Material data | Liquid Transport Coefficients":

"The last entry is also used for all higher moisture contents up to wmax. Although there is little capillary conduction above free saturation [2], there might be other transport mechanisms which could approximately be described by finite liquid transport coefficients (convection due to gravitation or pressure differentials etc.)."

"To allow for this kind of transport processes (if desired), WUFI accepts arbitrary liquid transport coefficients for this moisture region, too. In general, you will rarely encounter moisture contents in this region which is difficult to treat in calculation. By setting the last entry to zero, the capillary conduction in the moisture region above the last entry may be suppressed."

Regards,
Thomas

[rozumjak]

Thank you very much Thomas,

you helped me a lot.