# Modelling of condensation in air

1. Mar 22, 2009

### ponjavic

Hi, I am trying to simulate condensation of water in air.

I am using the shan-chen multi-phase multi-component model in order to achieve this (a variation of the Lattice Boltzmann Equation, not too important). The goal ultimately is to study the effect of condensation in fuel cell membranes.

I am struggling a bit with some physical concepts.

Modelling water/vapour makes sense to me and I have successfully used the single-component multi-phase model to simulate condensation of water droplets in vapour.

Taking air into consideration it becomes a bit more difficult. Basically the air contains vapour so the air is actually random gases + vapour so these are two components. The vapour itself consists of two phases vapour and liquid. Ultimately I want to show how condensation causes water droplets to form. However due to the nature of the multi-component model all I seem to get is separation of the two components (basically fluid-fluid separation). How do I within the H20 fluid distinguish between vapour and liquid?

For those of you not familiar with the Lattice Boltzmann Equation, what I have for each component is a probability distribution function which allows recovery of the macroscopic density at each lattice.

If this post is in the wrong forum I apologize, please move it.

2. Mar 25, 2009

### lalbatros

ponjavic,

Modelling water condensation of water in air can be seen from many point of view.
You should give more background information for a suitable answer.

My usual point of view could be that of an engineer interrested by condensation or drying of dusty gases.
In this case the "psychrometric" empirical approach is very useful. (1)
The psychrometric ratio gives a way to calculate mass transfer from heat transfer.
Many correlations for heat transfer are available, like for spherical particles in a gas.
On this basis it is possible to calculate condensation or evaporation rates on or from existing particles. These particles could be any solid material or pre-existing droplet. Typical applications could be drying of particulate materials or spray-based gas conditioning tower.

However, this topic is much broader in practice as well as in theory.
Application in meteorology needs a detailled study of the nucleation process and could involve fine partcile and pollutants.
For a theoretical physicisit, the sponteneous nucleation will be an application of fluctuation theory.

I guess that your point of view if more about CFD modelling.
Or would it theoretical physics?

I think you should either ask your question in a more specialised forum, or you should give more background information, specially on the motivation of your question. Sometimes knowing the motivation could help a non-specialist to give a useful answer.
(also sometimes exposing the question more clearly will lead you to an answer at the bottom of the page)

(1) http://en.wikipedia.org/wiki/Psychrometrics

3. Mar 25, 2009

### Andy Resnick

Have you accounted for interfacial tension in the condensed droplet? I wonder if you are having a problem with the time evolution of the system as well. Have you run the simulation 'backwards' and looked for evaporation effects?

Reading your post carefully again, are you not able to assign the water to a vapor or liquid phase (based on density) consistently?

You are working on a difficult problem, for sure.