Details:WaterVaporTransfer
Water Vapor Transfer Coefficients
The water vapor transfer may be described in analogy to the heat transfer:
|
gv = bp ·(pa - ps) |
||||
| gv | [kg/m²s] | : | water vapor flux density | |
| bp | [kg/m²sPa] | : | water vapor transfer coefficient | |
| pa | [Pa] | : | ambient water vapor partial pressure | |
| ps | [Pa] | : | water vapor partial pressure at the surface | |
The reason for a resistance affecting water vapor transfer is the same as with heat transfer, namely the effect of a boundary air layer close to the wall surface.
Since the ambient conditions affect both boundary layers in analogous ways, there are similarity relations between water vapor transfer and heat transfer, so that bp may be obtained from the following equation (with the numerical value valid only for the SI units quoted above):
|
bp = 7·10-9 · ac [1] | |||||
| ac | [W/m²K] | : | convective component of the heat transfer coefficient | ||
This requires to estimate the convective component ac of the heat transfer coefficient a. WUFI uses the following estimate (in FORTRAN notation):
| ALFAWC = ALFAW - 6.5 | ! EXTERIOR |
| ALFAEC = ALFAE - 4.5 | ! INTERIOR |
| ALFMIN = 3.5 | |
| BETAW = 7.E-9 * DMAX1(ALFMIN, ALFAWC) | ! EXTERIOR |
| BETAE = 7.E-9 * DMAX1(ALFMIN, ALFAEC) | ! INTERIOR |
The suffix W refers to the exterior surface; E, to the interior surface; and C, to the convective component.
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/a of 0.056 and 0.13 m²K/W, respectively.
Literature:
| [1] | Illig, W.: Die Größe der
Wasserdampfübergangszahl bei Diffusionsvorgängen in Wänden von Wohnungen,
Stallungen und Kühlräumen, |
