How to use concentration difference to drive a flow?
 

Hello community,

I am relatively new to ANSYS Fluent and currently working on a 2d fluid simulation problem that involves transient analysis of a channel with two inlets and a porous zone. I am seeking guidance on setting up the simulation correctly, especially regarding the concentration boundary conditions for the second fluid.

I have a channel where water enters through one inlet at a slow velocity. Simultaneously, another fluid enters through a second inlet (which is at the beginning of the porous zone), and this fluid needs to mix with the water after it passes through the porous zone. The concentration of the second fluid is 1 at the second inlet and decreases along the porous zone, leading to diffusion into the water. The goal is to monitor the increasing concentration of the second fluid over time and plot the results.

Questions and Challenges:

1. Which Fluent model is most suitable for this scenario?
I have tried to use Multiphase models: VOF, Mixture, Eulerian. But they don't seem to be working. Basically, the residuals keep oscillating.

2. How should I define the concentration boundary conditions for the second fluid at the second inlet and the porous zone?
I have been trying to define the second inlet as a concentration inlet. I have tried using mass-flow-inlet. But the problem is that I am not sure of the mass flow rate. The second fluid needs to diffuse through due to concentration difference. So, I don't know the mass flow rate.

I would greatly appreciate any guidance or suggestions from experienced users on how to correctly set up the Fluent simulation for this two-phase flow with species transport. Any insights into model selection, boundary conditions, or general best practices for similar scenarios would be immensely helpful. Thank you in advance for your assistance!

Are the fluids miscible? Do they mix? If yes, do not use multiphase models, use species transport models and concentration will mean concentration and they are the same phase. If they are not miscle, then use multiphase models, but then one substance cannot diffuse into the other (because they're not miscible) and concentration is meaningless because they are different phases.

Hi,
Thank you for your response.

Both of the fluids are non reacting liquids. One is water, and the other is slightly more viscous Newtonian liquid. Water inlet is a velocity inlet while the other liquid's inlet is at the beginning of the porous zone where the liquid 2 concentration is 1. Initially there is no liquid 2 in water, this concentration difference causes liquid 2 to slowly diffuse through the porous zone into the water flow. At the outlet all the normal gradients are zero. I wish to plot the increase of concentration of liquid 2 in water against time at the outlet. I am attaching the photo of my domain for a clearer explanation.