Re: [EnergyPlus_Support] moisture flux in internal walls

[Linda Lawrie <linda@xxxxxxxxxx>]

At 12:22 PM 2/4/02, habaza11@xxxxxx wrote:
>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.

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.

(Supplied by Linda Lawrie)


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