[BLDG-SIM] Fwd: Re: Fwd: Moisture flux in internal walls

[Curtis Pedersen]

>X-Sender: liesen at gold.me.uiuc.edu
>Date: Mon, 04 Feb 2002 12:36:20 -0600
>To: BLDG-SIM at gard.com
>From: "Richard J. Liesen" <r-liesen at uiuc.edu>
>Subject: Re: Fwd: [BLDG-SIM] Moisture flux in internal walls
>Status:  
>
>Hello,
>Your assumption is correct once you reach Steady-State (SS) for any 
>given surface for thermal and moisture fluxes.  Since you are using 
>the Moisture Transfer Function (MTF) solution you now have a storage 
>term for both moisture and heat inside the wall.  In many typical 
>cases you will not obtain steady state conditions for a long enough 
>time to reach a constant moisture mass flux rate in a wall; the time 
>constant for moisture transfer can be much longer than for heat 
>flux.  You would have to setup special runs with specified and 
>constant boundary conditions to obtain this type of result, ie. 
>Constant outside temps and moisture, constant heat and moisture 
>loads inside the zones, and constant and non-varying temperature and 
>moisture control profiles varied enough between the 2 experimental 
>zones to force the heat and mass flux in the same direction for a 
>long period of time and then watch the fluxes slowly go to SS. 
>Given this experimental simulation run you would see both the heat 
>and mass fluxes from inside to outside go to the same value.  In 
>normal simulations you rarely achieve these controlled conditions 
>and would rarely see the fluxes go to SS.  Keep in mind when using 
>the MTF calculation that the thermal flux and temperature profiles 
>also affect the moisture flux and profile, and vice-versa.
>
>The sign convention is the same for both the thermal and moisture 
>fluxes; it is positive into the surface from the outside air and 
>positive from the surface into the zone air.
>
>The sign convention for flux has nothing to do with absorption and 
>desorption, which is the internal storage process for moisture. 
>Absorption is when the water vapor changes to a liquid in the pore 
>structure in the layer and then releases the heat of vaporization. 
>Desorption is when the liquid changes to a water vapor in the pore 
>structure in the layer and then requires the heat of vaporization.
>
>
>Thanks
>Dr. Richard Liesen
>Associate Director, Building Systems Laboratory
>Department of Mechanical and Industrial Engineering
>University of Illinois at Urbana-Champaign
>
>
>At 10:28 AM 2/4/2002, you wrote:
>>Date: Mon, 04 Feb 2002 09:29:16 -0500
>>From: habaza11 at cs.com
>>To: <BLDG-SIM at gard.com>
>>Subject: [BLDG-SIM] Moisture flux in internal walls
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>>
>>Sirs;
>>When an internal wall is shared between two zones. I assume that 
>>the external layer of this wall will be the internal layer of the 
>>same wall in the other zone that shares the same wall, and visa 
>>versa.
>>Thus the moisture flux of the external surface of this wall should 
>>be equal to the moisture flux of the internal surface of the same 
>>wall from the other zone. If this assumption is correct, why the 
>>moisture flux of the internal surface of this wall does not equal 
>>the moisture flux of the external surface of the same wall from the 
>>other side in E-plus.
>>Attached please find the input file for more details.
>>Second; when moisture flux is negative does this mean absorption or 
>>desorption?.
>>Thank you.
>
>Thanks
>Dr. Richard Liesen
>Associate Director, Building Systems Laboratory
>Department of Mechanical and Industrial Engineering
>University of Illinois at Urbana-Champaign
>www.bso.uiuc.edu
>support at blast.bso.uiuc.edu


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