Learning Forum

[user deleted]

Good afternoon. I am modeling an external chemical transition in ammonia vapor. A primary liquid (water1) with a viscosity of 0.05 (kg/m*s) enters the gaseous medium (gas1). When (water1) comes into contact with (gas1), a third substance (water2) with a very high viscosity of 5 (kg/m*s) appears at the boundary of these phases.

Question: how can this process be modeled? I used the VOF model (three phases) and species transport with a constant transition mechanism. I can't get water2 at the phase boundary. Tell me, how can I simulate the appearance of water2 on the border of gas1 and water1?

[Ahmed Hussien]

This particular model presents a challenging scenario. There are two possible options to consider. The first option involves defining two phases, namely "Liquid" and "Gas," and specifying two species for the liquid phase, "water1" and "water2." The alternative option is to define three phases, as you have already done. Regardless of the chosen option, it will be necessary to write a User-Defined Function (UDF) to describe the mass transfer that occurs between the phases.

 

Within the UDF, it is crucial to identify the cells located at the interface and incorporate source and sink terms for the different phases. This ensures that water1 and gas1 are consumed while water2 is produced. It is important to consider that when water2 is generated at the interface, water1 and gas1 are no longer in direct contact. This raises the question of how to define this interface in such a scenario.

[user deleted]

Ahmed, good afternoon and thank you for answering my question. Which of the two methods that you described earlier seems the simplest to you? I looked through the user manual for UDF and found "DEFINE_SR_RATE" (example 1). The function determines the reaction rate at the junction of two phases. Is this UDF not suitable for my task? Could you tell me to recommend a UDF from which I could take a piece of code, rework it and continue?

Ps

The reaction proceeds without the release/absorption of heat (heat can be neglected). The thermal gradient at the interface of the two phases is zero.

[Ahmed Hussien]

The "DEFINE_SR_RATE" defines the surface reaction rate between two species within the same phase. Your problem is more complicated as it involves different phases. In your case, I would suggest using "Define_Source" where you define the mass source for each phase based on the mass transfer mechanism. I am not sure about the exact implementation as it should depend on your chemical resctions.  

[user deleted]

Good afternoon. Is the phase transition from "water1" to "water2" implemented based on the concentration of the substance "water1"?

Program logic :(I assume this)   
#include "udf.h"
if the concentration of "water1" in cell N is <=1, then ....
Define_Source (mass transition)
{
can there be an Arrhenius coefficient here?I do not know how to describe the phase transition rate. 
}
return the concentration in the cell

How to find the zone between "water1" and "gas1" is a solvable task. How to determine the reaction rate is a problem.