SebSta am 13. Juni 2013 um 12:45 Uhr

2013-06-13T12:45:43Z

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	[[AirLayers \| Air layers]] can therefore only approximately be		[[Details:AirLayers \| Air layers]] can therefore only approximately be
	simulated by treating them as a 'porous' material. It is possible to allow		simulated by treating them as a 'porous' material. It is possible to allow
	for the amplifying effect of convection on heat and moisture transport by		for the amplifying effect of convection on heat and moisture transport by

SebSta: hat „FAQ General:WaterContentofanAirLayer“ nach „FAQ:General:WaterContentofanAirLayer“ verschoben

2013-06-13T12:25:58Z

hat „FAQ General:WaterContentofanAirLayer“ nach „FAQ:General:WaterContentofanAirLayer“ verschoben

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Len am 2. Oktober 2008 um 08:06 Uhr

2008-10-02T08:06:56Z

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	= (6): Water Content of an Air Layer		= (6): Water Content of an Air Layer =

	<B>I did a WUFI calculation with an assembly that includes an air layer. However, I get		<B>I did a WUFI calculation with an assembly that includes an air layer. However, I get

Len: Die Seite wurde neu angelegt: = (6): Water Content of an Air Layer I did a WUFI calculation with an assembly that includes an air layer. However, I get completely unrealistic water contents for ...

2008-10-02T08:06:42Z

Die Seite wurde neu angelegt: = (6): Water Content of an Air Layer I did a WUFI calculation with an assembly that includes an air layer. However, I get completely unrealistic water contents for ...

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= (6): Water Content of an Air Layer

I did a WUFI calculation with an assembly that includes an air layer. However, I get
completely unrealistic water contents for the air layer. What went wrong?


WUFI was developed to simulate the hygrothermal processes in porous building
materials. The detailed simulation of heat and moisture transport in air layers
(including convection, turbulence etc.) is much more complicated and is outside
WUFI's scope. Furthermore, it does not make much sense to try and implement these
inherently two- or three-dimensional processes in a one-dimensional simulation program.


[[AirLayers | Air layers]] can therefore only approximately be
simulated by treating them as a 'porous' material. It is possible to allow
for the amplifying effect of convection on heat and moisture transport by
employing appropriate effective
[[Details:BasicMaterialData | heat conductivities]] and
[[Details:BasicMaterialData | vapor diffusion resistance factors]].


However, the [[Details:MoistureStorageFunction | moisture storage function]] of
an air layer can only very crudely be approximated by the moisture storage
function of a porous material. The latter is largely temperature-independent
(and implemented as such in WUFI), so that the functional dependence of the
moisture content in air on the relative humidity and temperature cannot be
reproduced. 
Furthermore, the default moisture storage function used by WUFI for materials
for which the user has not defined one assumes that capillary condensation will
occur in the material already at relative humidities less than 100%, which is
not true for an air layer (it has been modeled after the moisture contents of
dense mineral wool).


As a result you will get unrealistically large moisture contents for air layers.
Note, however, that WUFI uses the relative humidity as the driving potential
for moisture transport and computes the water content as a secondary
quantity from the resulting relative humidity (using the moisture storage function
of the respective material). 
So the resulting distribution of relative humidity should in general be quite
realistic, its temporal behavior will just be damped much more than in reality
(the moisture content acts as a 'capacity term' for moisture transport in the
same way the heat capacity acts as a capacity term for heat transport). If
short-term fluctuations don't play a major role, the general trend in the
behavior of the relative humidity should be tolerably realistic. 
This also means that quantities that depend on the relative humidity in or
near the air layer (e.g. mould growth rates) can be evaluated more
realistically than quantities that primarily depend on the moisture content
(e.g. heat conductivity, heat capacity).


Please note that the unrealistically large moisture capacity of an air layer
may also affect other layers. If you are interested in the moisture
distribution in an assembly that contains an air layer, the air may (or may
not) take up more moisture than realistic, so that less moisture remains for
distribution among the other layers.


You may mitigate these problems by explicitly defining a slightly more realistic
moisture storage function for the air layer. To this end, use a linear function
like

<TABLE>
<TR><TD><TT>phi: </TT></TD><TD><TT>w: </TT></TD></TR>
<TR><TD><TT>0 </TT></TD><TD><TT>0 </TT></TD></TR>
<TR><TD><TT>1 </TT></TD><TD><TT>wf</TT></TD></TR>
</TABLE>

with a low value for wf (the numerics may not be able to cope with
very low values, you'll need to experiment a bit) (*). This avoids the spurious
capillary condensation.


Also see the next question for a related problem.


(*) Note, however, that the porosity and thus wmax should
remain high. If the water content exceeds wf, WUFI reduces
the vapor permeability, in proportion to the excess, to reflect the fact that
the pore volume gets increasingly filled with water and thus vapor transport
decreases. At w=wmax the permeability reaches zero (all pores
are filled). For vapor-permeable materials like air layers or mineral wool
where moisture transport occurs mainly via vapor transport, wmax
should therefore remain at a realistic value.

FAQ:General:WaterContentofanAirLayer - Versionsgeschichte

SebSta am 13. Juni 2013 um 12:45 Uhr

SebSta: hat „FAQ General:WaterContentofanAirLayer“ nach „FAQ:General:WaterContentofanAirLayer“ verschoben

Len am 2. Oktober 2008 um 08:06 Uhr

Len: Die Seite wurde neu angelegt: = (6): Water Content of an Air Layer I did a WUFI calculation with an assembly that includes an air layer. However, I get completely unrealistic water contents for ...