Diffusive-Convective Fluid flow during Carbon Dioxide Dissolution in Water
 

I want to simulate the dissolution of carbon dioxide in water. I have a 2D square (fluid: water) where the top boundary is a CO2 inlet. This is what’s supposed to happen (attached picture).

1. Carbon dioxide will get into the water through diffusion, the only mass transfer process.

2. As CO2 mixes with water, the water density increases.

3. The higher-density water moves down. This will initiate mass transfer through convection.

I have set up species transport, energy, and viscous flow (laminar) to simulate this. I have specified the top boundary as a CO2 inlet (I set it as a mass inlet).

I’m facing a problem in setting the boundary condition for the inlet. I need to provide a mass flux value for the inlet; I know at first there is diffusion (mass flux for diffusion is calculated through Fick’s law, so I can provide the diffusive flux through a named expression). But how do I model the convective flux? Or will Ansys automatically incorporate the convective flow, so I should only provide the diffusive flux?

Any suggestion/insight is highly appreciated. I’m happy to provide any additional info.