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The water vapor transfer may be described in analogy to the heat transfer:

2008-08-13T10:55:49Z

Die Seite wurde neu angelegt: = Water Vapor Transfer Coefficients = <P> The water vapor transfer may be described in analogy to the heat transfer: </P> <TABLE> <TR><TD WIDTH="10"> </TD> <TD CO...

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= Water Vapor Transfer Coefficients =
<P>
The water vapor transfer may be described in analogy to the heat transfer:
</P>

<TABLE>
<TR><TD WIDTH="10"> </TD>
<TD COLSPAN="4">
g<SUB><SMALL>v</SMALL></SUB> = <FONT FACE="SYMBOL">b</FONT><SUB><SMALL>p</SMALL></SUB> ·(p<SUB><SMALL>a</SMALL></SUB> - p<SUB><SMALL>s</SMALL></SUB>)
</TD>
</TR>
<TR><TD COLSPAN="5"> </TD>
<TR><TD> </TD>
<TD>g<SUB><SMALL>v</SMALL></SUB></TD>
<TD>[kg/m²s]</TD>
<TD>:</TD>
<TD>water vapor flux density</TD>
</TR>
<TR><TD> </TD>
<TD><FONT FACE="SYMBOL">b</FONT><SUB><SMALL>p</SMALL></SUB></TD>
<TD>[kg/m²sPa]</TD>
<TD>:</TD>
<TD>water vapor transfer coefficient</TD>
</TR>
<TR><TD> </TD>
<TD>p<SUB><SMALL>a</SMALL></SUB></TD>
<TD>[Pa]</TD>
<TD>:</TD>
<TD>ambient water vapor partial pressure</TD>
</TR>
<TR><TD> </TD>
<TD>p<SUB><SMALL>s</SMALL></SUB></TD>
<TD>[Pa]</TD>
<TD>:</TD>
<TD>water vapor partial pressure at the surface</TD>
</TR>
</TABLE>
 <BR>
<P>
The reason for a resistance affecting water vapor transfer is the same as with
[[Details:HeatTransfer | heat transfer]], namely the effect of a boundary
air layer close to the wall surface.
</P>
<P>
Since the ambient conditions affect both boundary layers in analogous ways, there are
similarity relations between water vapor transfer and heat transfer, so that
<FONT FACE="SYMBOL">b</FONT><SMALL>p</SMALL> may be obtained from the following equation
(with the numerical value valid only for the SI units quoted above):
</P>

<TABLE>
<TR><TD WIDTH="10"> </TD>
<TD COLSPAN="4">
<FONT FACE="SYMBOL">b</FONT><SUB><SMALL>p</SMALL></SUB> = 7·10<SUP>-9</SUP> · <FONT FACE="SYMBOL">a</FONT><SUB><SMALL>c</SMALL></SUB>   [1]
<TD>
</TR>
<TR><TD COLSPAN="5"> </TD></TR>
<TR><TD> </TD>
<TD><FONT FACE="SYMBOL">a</FONT><SUB><SMALL>c</SMALL></SUB></TD>
<TD>[W/m²K]</TD>
<TD>:</TD>
<TD> convective component of the heat transfer coefficient</TD>
</TR>
</TABLE>

<P>
This requires to estimate the convective component
<FONT FACE="SYMBOL">a</FONT><SMALL>c</SMALL> of the heat transfer coefficient
<FONT FACE="SYMBOL">a</FONT>. WUFI uses the following estimate (in FORTRAN notation):
</P>

<TABLE>
<TR><TD><FONT FACE="COURIER NEW">ALFAWC = ALFAW - 6.5 </FONT></TD>
<TD><FONT FACE="COURIER NEW">! EXTERIOR </FONT></TD>
<TR>
<TR><TD><FONT FACE="COURIER NEW">ALFAEC = ALFAE - 4.5 </FONT></TD>
<TD><FONT FACE="COURIER NEW">! INTERIOR </FONT></TD>
<TR>
<TR><TD><FONT FACE="COURIER NEW">ALFMIN = 3.5 </FONT></TD>
<TD>  </TD>
<TR>
<TR><TD><FONT FACE="COURIER NEW">BETAW = 7.E-9 * DMAX1(ALFMIN, ALFAWC)</FONT></TD>
<TD><FONT FACE="COURIER NEW">! EXTERIOR </FONT></TD>
<TR>
<TR><TD><FONT FACE="COURIER NEW">BETAE = 7.E-9 * DMAX1(ALFMIN, ALFAEC)</FONT></TD>
<TD><FONT FACE="COURIER NEW">! INTERIOR </FONT></TD>
<TR>
</TABLE>

<P>
The suffix <FONT FACE="COURIER NEW">W</FONT> refers to the exterior surface;
<FONT FACE="COURIER NEW">E</FONT>, to the interior surface;
and <FONT FACE="COURIER NEW">C</FONT>, to the convective component.
</P>
<P>
For example, the water vapor transfer coefficients 8.0E-8 (exterior) and
2.2E-8 kg/m²sPa (interior) result from WUFI's default heat transfer resistances
1/<FONT FACE="SYMBOL">a</FONT> of 0.056 and 0.13 m²K/W, respectively.
</P>

=== Literature: ===
<TABLE>
<TR><TD VALIGN="TOP">[1]</TD><TD>Illig, W.: Die Größe der
Wasserdampfübergangszahl bei Diffusionsvorgängen in Wänden von Wohnungen,
Stallungen und Kühlräumen,<BR>
Gesundheitsingenieur 73 (1952), H. 7/8, p. 124-127.</TD></TR>
</TABLE>