Post
by Thomas » Tue Apr 19, 2022 9:52 pm -1100
Hi WEI LI,
yes, for use with the transport equation the \(d_\mathrm{cavity}\) should be removed.
In the transport equation, the water content \(w\) is a volume density (kg/m³), so any moisture source term should be a volumetric source density (kg/(m³ s)). \(w\) describes the moisture density at one point in the volume, and \(S_w\) describes the moisture source density at that point.
If we compare two layers containing air change sources, with the air change rates being the same but the second layer twice as thick, then we expect the absolute amount of moisture released by the source in the thicker layer to be twice the amount released in the thinner layer because the thicker layer has twice the volume. But since the double amount is spread over twice the volume, the volumetric source density remains the same. So for a given air change rate the source density is independent of the volume and thus independent of the layer thickness.
The layer thickness \(d_\mathrm{cavity}\) should be included if the total moisture released in a given cross section of the wall is to be described (that is, in each square meter of the wall this or that amount of moisture is released). But this is not what is required for the transport equation.
Best regards,
Thomas
