## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Clarification on Volume Fraction during Condensation Process in ANSYS Simulation

• Arun mathew
Subscriber

Hello,

I am currently working on a simulation that involves the condensation process, and I have some questions regarding the volume fraction of liquid water. I would greatly appreciate any clarification or insights you can provide.

In my simulation, I have a chamber with a base temperature set below the saturation temperature. The initial temperature of the moist air is above the saturation temperature, and the volume fraction of the liquid water is initially set to 0. As condensation occurs, water content accumulates on the bottom surface, and the contour plots show the volume fraction of water changing from 2.94e-9 to 1.37e-6.

Evaporation-condesation,species transport model.

I have two specific doubts that I would like to address:

1. If water condenses at the bottom wall, should the volume fraction of the liquid water be 1 at the bottom surface?
2. I believe that condensation typically occurs at the solid interface, so why is there a change in the volume fraction of liquid water in the middle of the chamber?

3. Any insights or explanations regarding these questions would be highly appreciated.
• Rob
Ansys Employee

Depending on how much liquid condensed you'll see the volume fraction in a cell, so if only a small fraction of the cell contains liquid the value to report will be under one.  Which multiphase model are you using for the simulation? Are you running double precision?

• Arun mathew
Subscriber

I am currently running a simulation with double precision, using the mixture model with species transport. The involved phases are air+vapor and liquid water. I have set the temperature of the tank's bottom surface to 15°C, and all other walls are adiabatic, with a saturation temperature of 30°C.

I initiated the model with a constant temperature of 31°C. The volume fraction of liquid water is 0, and the volume fraction of water vapor in phase 1 is 0.01 (resulting in a volume fraction of air of 0.99).

I have attached the volume fraction contours at the start of the simulation and after 20 seconds. In theory, water vapor condenses at the bottom wall where the initial temperature is 15°C. My simulation replicates this behavior.

However, I have a question concerning the volume fraction of liquid water at the cell along the bottom wall during the condensation process.

Shouldn't the volume fraction of liquid water in the cell next to the bottom wall reach a value of 1 before condensation occurs in the cell above it? If the volume fraction of liquid water is less than 1, does it mean that the remaining volume in the cell is occupied by phase 1 (air+vapor)?

• Rob
Ansys Employee

Yes, but you're assuming the air temperature doesn't drop due to the wall temperature. The condensation model in multiphase just looks at the cell conditions, so I suspect you're forming a "fog".

• Arun mathew
Subscriber

When I examine the temperature contour of the mixture of air vapor and liquid water, I notice that it eventually reaches the temperature of the bottom wall. This makes me believe that the air temperature is also decreasing.

I'm also unsure about whether fog is being created. Does the creation of fog result in a change in the volume of liquid near the upper wall(away from the bottom wall), changing it from its initial value of 0 to a certain amount?

• Rob
Ansys Employee

It shouldn't do immediately, but as vapour condenses the amount of vapour in the domain will reduce. If there are no flow boundaries how is the gas volume being modelled?

• Arun mathew
Subscriber

I've just assigned the top wall pressure inlet. However, I'm still having trouble understanding the contour. The provided image displays the volume fraction contour at 17 seconds.

In the volume fraction (VF) contour, the variation in water liquid due to condensation is evident, ranging from 1.89e-5 at the bottom to 0 at the top.

I'm uncertain about the physical interpretation of this contour. When the volume fraction is less than 1 at the bottom wall, does that imply that the remaining volume of cells at the bottom wall is filled with phase 1 (which is air and water vapor)? If that's the case, how does the liquid water appear in the cells above those adjacent to the bottom walls?