Learning Forum

[Joseph Omandac]

Hi!

I am trying to model a salinity gradient solar pond, which works simply by having three different layers of saline solution across its depth; 20% salt solution at the bottom, 10% at the middle, and freshwater at top. The working principle states that natural convection separately occurs within the three different layers, diffusion between the layers are minimal, allowing it to the mixture to reach high temperatures and the component to hold high amounts of heat.

However, I still do not know the proper way to model a salt solution, considering the concentration of salt in the fluid.

I tried using multiphase VOF model for the three distinct fluids, having to define their density, Cp, viscosity, and thermal conductivity in the materials. But this does not consider the concentration of salt; I think it views the substances as three different immiscible fluids.

Should I use the Eulerian model instead? How about Species Transport?

Any hints or directions to lead me to modelling the proper salinity gradient in Fluent would be very much appreciated.

Thanks!

 

Best,

Joseph

[Karthik R]

Hello,

I think you should be able to use Eulerian MP model for this. You should switch on species transport and create a mixture template. You can assign different densities to the various species in your model (and you could make them a function of temperature, which might be important for your model) and use a 'volume-weighted-mixing-law' to model the mixture phase. Once you enable gravity, you should hopefully see stratification. I hope this helps you move forward. 

You are absolutely correct, VOF is meant for immiscible fluids and is not useful for your modeling requirements.

Thank you.

Best Regards,

Karthik

[Joseph Omandac]

Hi, Karthik

Thanks for looking into my problem. I just reviewed the Fluent Theory Guide to be familiar with the Eulerian model.

Last question: What should I input to the Specified Operating Density in the Cell Zone conditions of the setup? I just know that this value is supposed to be zero for natural convection involving air. What specifically does this value and the operating temperature do to the simulation?

Always grateful for your support. Thanks!

 

Best,

Joseph

 

[Karthik R]

Hello Joseph,

Operating Density is an extremely important parameter when it comes to modeling natural convection flows. You will need to set this parameter depending on your model.

If you are using a Boussinesq model for 'Density', the operating density needs to be set to 0 as this model uses 'Operating Temperature' for its calculations. However, if you are using any other model for density, you might want to set the 'Operating Density' appropriately. I strongly suggest that you read through section 12.2.4.4 (titled Steps in Solving Buoyancy-Driven Flow Problems, Fluent Users Guide R18.2) for more information on this. I am attaching a small snip of the same for your reading here.

I hope this helps.

Best Regards,

Karthik

[DrAmine]

How big is your density ratio?