WEI LI wrote: ↑Mon Feb 21, 2022 10:05 pm -1100
Hi Thomas, thanks for your reply.
In the current case (a porous material with some liquid moisture content and some vapour content) the relative humidity is more or less fixed because it must correspond to the liquid moisture content (via the moisture storage function), and the liquid moisture content will usually only change slowly (usually by evaporation or condensation which are relatively slow processes).
Does "the current case" refer to the case I mention on the original post? The "metal-air-metal' structure?
It refers to the behavior of a porous hygroscopic material between impermeable surfaces which contains some liquid water and some vapour.
I remember I have seen it from somewhere in the Wufi help document, says something like: the water content (I assume it means the liquid moisture content) is just a secondary value which is derived from the RH by the moisture storage function. In my opinion, WUFI calculates the RH field, by solving the moisture transport equation, and then calculates the water content (I mean the water content in post process, which is liquid water content) by the moisture storage function. PLEASE CORRECT ME IF I AM WRONG.
Yes, we are using relative humidity and temperature as the primary variables describing the hygric and thermal states of the materials. The transport equations (which include processes such as liquid transport, vapour diffusion, moisture storage capacity, evaporation and condensation, moisture sources and sinks, heat flow, heat storage capacity, release and consumption of latent heat, heat sources and sinks, etc.) then determine the values for the relative humidity and the temperature at the end of the next time step. Then the secondary variables, such as liquid water content, are computed from relative humidity and temperature. This is possible because there are unique mathematical relationships between all these quantities.
However, you said the RH is more of less fixed because of the liquid mositure content is changing very slowly. This is quite conflicting with my understanding, or not?
There is no contradiction. There are always situations where the water content or the temperature remains more or less constant for a while (for example, if the component surfaces are impermeable or only poorly permeable to moisture or heat transport, or if the boundary conditions are such that they do not give rise to moisture or temperature changes of the construction). The fact that a variable remains constant under certain conditions does not invalidate its usability as a variable in general.
...the liquid moisture content will usually only change slowly (usually by evaporation or condensation which are relatively slow processes)
In the
particular case I considered, a case where the construction surfaces are impermeable to liquid or vapour transport (the case you considered initially), the
total amount of moisture (liquid + vapour) is fixed because of the impermeability of the surfaces. However, the
relative amounts of liquid versus vapour may still change when the temperature changes. The liquid amount cannot change much because it has only the limited pore space into which it can evaporate. Because relative humidity is connected with the liquid amount through the moisture storage function, it cannot change much either under these circumstances. But some of the liquid water
does evaporate, in order to create a new equilibrium with the vapor pressure in the pore air. This explains why you saw a change in the liquid water content with changing temperature even though your construction was tight on all surfaces. The
small change in liquid water content also causes (via the moisture storage function) a
small change in relative humidity, but unless the porosity is not very large, only a small change in relative humidity is required to reach the new equilibrium.
All of this refers to the behaviour of porous materials. Air layers are not porous materials (they don't have liquid water content, so they don't really have a moisture storage function). But until a new material type "air layer" with specifically tailored mathematical properties will be introduced in a future WUFI version, we have to use porous materials which behave as close to real air layers as possible. It works reasonably well for most cases, in particular if the function of the air layer is just to serve as some moisture or heat flow resistance in the component. But don't expect completely realistic behaviour under all circumstances.
Could you please inform me where I can learn more about how WUFI is taking care of the evaporation and condensation thing? Like which part of the help document I can found them?
There are no specific routines in WUFI whose task it is to treat evaporation and condensation. These phenomena are automatically taken into account by the transport equations: the solution for the new time step is a new moisture field in the construction, and the changes of liquid moisture content which are not caused by liquid transport may be said to be due to evaporation or condensation.
Maybe we are talking about two different things. From your original question it seems that you are interested in condensation which occurs in an air layer bounded by an impermeable surface (e.g. a metal sheet). That problem is conceptually relatively simple: when the vapour pressure in the air is greater than the saturation vapour pressure at the surface (i.e. when relative humidity at the surface reaches or surpasses 100 %), condensation occurs. The condensation rate is determined by the difference of the two vapour pressures and the vapor diffusion surface transfer coefficient. There is either a layer of liquid water or there isn't.
On the other hand, in porous materials there is
always some liquid water content present in larger or smaller quantities, and it increases whenever relative humidity increases, even at relative humidities far below 100 %. So in general, condensation in porous materials behaves quite differently from condensation in an air layer and you cannot learn much about condensation in air layers by investigating condensation in porous materials.
Kind regards,
Thomas
