Learning Forum

[Dominik Buksa]

Hi. I would like to ask if there is a wall drying model in ansys? I would like to model a wall (rock) that will have a certain humidity and I want a model that, under the influence of lower humidity at the inlet, will dry my wall (rock). Has anyone done something similar?

[Rob]

Flow in the rock may be difficult, porous media may not be fully suitable. Looking at moisture fraction in the air space is fairly straightforward, but you then need to work out how to get moisture off the wall. 

[Dominik Buksa]

I wanted to introduce air with lower humidity into the "inside of the cave", and in fact this air "dries" the rocks (depending on the season). But I have no idea how with the ansys software I could dry out the wall.

[Rob]

You have two parts to this problem. Flow in the domain strips moisture from the rock at a rate that's determined by what is available & air conditions (temperature, speed & humidity). On the rock side you have diffusion of moisture which is a function of what's in the rock (moisture, porosity & permeability) and moisture concentration on the surface. 

Modelling the whole lot isn't going to be easy, and you almost certainly won't know about the cave geology. 

Fixing a moisture fraction at the wall may be easier for you to model. But that then depends on exactly what you're trying to prove. 

[Dominik Buksa]

I understand, thank you for your help!
When I set the moisture fraction on the wall, how do I , "dry" it? I haven't found anything like that in the ansys documentation. In the ,,species" boundary conditions I can set the h20 fraction to my desired value, but it is a fixed value and I would like it to change within the dry air flow.

[Rob]

You could have a look at the wall reaction models: water (wall) + energy becomes water-vapour (gas)  or similar. The kinetics would need some work. Eulerian Wall Film with momentum off may also be viable, but the time scales for that may be too long due to the way the model works.