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Multiphase mixture model mass conservation not obeyed

• friday.abolorunke
Subscriber

Hi All,

I am modeling a two-phase problem with interfacial species mass transfer in a closed domain rectangular domain with impermeable BC & zero diffusive flux on all the walls.

Model: Mixture model for two phase. Species mass transfer for mass transfer mechanism.

Issues:

The mass of Vapor co2 = 2.5e-7g and Liquid ethanol = 2.9e-7g. Initial total mass = 5.4e-7g

Vapor co2 converts into liquid co2 via dissolution and goes into the liquid phase. So, in this system the total mass of the liquid increases while the vapor phase decreases. After I ran the simulation to steady-state I calculated the mass of the liquid, vapor and species to see if the mass was conserved. I expect the total mass to be the same (remember domain is closed).

At steady state, Mass of liquid co2 = 1.3e-7g, mass of liquid etoh = 3.45e-7g and mass of vapor co2 is 2.45e-7g. Final total mass = 7.2e-7g. Not equal to 5.47g

Total mass in the system increased. Another strange observation is that the mass of liquid Etoh increased too. I expected that to remain the same because the only mass transfer in the system is due to dissolution of vapor co2.

Please, what could be the reason behind this mass imbalance. Seems my system is gaining mass. Could I be doing something wrong. Any advise? asides from refining the mesh.

Regards,

Friday

• Rob
Forum Moderator
What is the gas density? How good (or likely bad) is the convergence? Why do you need to spam our feeds? We work office hours in various timezones and will get to your thread as we drop into the forum.
• friday.abolorunke
Subscriber
Hi Rob Sorry for spamming your feed and that of the other guys. I did not know it would have that effect.
The initial density of the gas phase is 141.3kg/m^3 and the liquid is 731.6kg/m^3. At the end of the simulation, i.e; at steady-state when the mass fraction & pressure field is uniform, the density of the gas phase becomes 132.45 kg/m^3 and the liquid 772.8 kg/m^3 and the volume of the liquid phase increased too, by about 50%. I am using Peng-Robinson EOS to compute the gas and liquid density as the density is dependent on pressure and composition. The system is isothermal, so the temperature is constant.
As per convergence, I think the convergence is good. The continuity, species & volume fraction residuals are below 1e-7, while velocity residual is below 1e-5.
Thank you for your time.
Regards Abolorunking
• Rob
Forum Moderator
No worries. Using the @ means we get a notification, one or two is fine, but it's become a habit and 15-20 notifications means the panels get overwhelmed. If one of us has missed a reply for a few days it's a different matter, except it's buried in the unnecessary notifications....

Back to the problem.
How are you dissolving the gas? Real gases aren't great around the phase change point. I tend to use ideal gas (if possible) and then set liquid species densities with temperature (liquids don't compress much), the phase density should follow with composition. If you monitor phase mass, species mass and mass transfer during the simulation is it a gradual loss or intermittent?
• friday.abolorunke
Subscriber
The phase change from gas (co2) to liquid (co2) is by concentration driven species mass transfer. The liquid phase has a mixture of ethanol & co2, while the gas phase ethanol & co2 too. These mixtures deviate from ideal gas behavior that's why I used Peng-EOS for both of them. And indeed the Peng-Robinson works well for this system as I have compared it with my experimental density data for ethanol & co2 mixtures. Based on your recommendation, I will set up a another case and use ideal gas for the vapor phase & Peng-Robinson for the liquid phase and see what happens.
To your second question:
It's a gradual gain in mass for the liquid phase & liquid species, but gradual loss in mass for vapor species & phase. Overall the simulation is gaining and not losing mass.
Ethanol gaining species mass gets me really confused because I did not specify any mass transfer for that. I expected the total mass of Ethanol to be the same, while the mass of liquid co2 increases and vapor co2 decreases.

• Rob
Forum Moderator
Check the molecular weights etc. Also how well resolved is the mesh? The fact that all materials are gaining mass tends to suggest it's numerical.
• friday.abolorunke
Subscriber
Molecular weights are good.
Number of elements: 8800. Size of the domain: 22e-5 m by 1e-5 m. Element size: 5e-7 m
I will refine the mesh and change the element size to 1e-7 m and let you know the outcome. Thank you so much.

• Rob
Forum Moderator
You're welcome. Don't forget about adaption, might save you some work.
• friday.abolorunke
Subscriber
Hi Rob I know it took a while to get back. I ran two different cases. Indeed mesh adaptation and increasing the overall mesh density gave me a better result. I was able to match my bench mark with a deviation under ~3%. Meanwhile, I used a different set up for the other two cases. I used UDF to model the interfacial mass transfer for dissolution and evaporation and the VOF model.
Thank you Rob, & DrAmine and Kremella for all the help. This model is the core of what my PhD is about. I started with zero experience in multiphase flow, udfs, interfacial mass transfer and with the help of this forum I am able to bring the project to a close, at least that is what I think haha.
I still have more questions though. Regards Abolorunking
• DrAmine
Ansys Employee
Out of curiosity: Are you using Mixture Model with Slip ( I think so to use species mass transfer) or using your own mass transfer mechanism?
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