[yifei98]

I am seeking assistance with an issue encountered during my Eulerian multiphase simulation in ANSYS Fluent. Specifically, I had this error "Turbulent Viscosity limited to the viscosity ratio of 1e+5".

Below is a detailed description of my setup and the problem I faced:

Simulation Setup:


Geometry: The model consists of a cylindrical liquid region (water) with a cylindrical gas sparger at the bottom to introduce CO₂ bubbles into the stagnant liquid. This is a 3d multiphase simulation that intends to focus on the interfacial mass transfer simulation so I used the constant bubble diameters and neglected the detail bubble simulation. ​

Phases Defined:

Mixture 1 (Liquid Phase/Primary Phase): Comprises water (l) and dissolved CO₂ [CO₂(aq)], the CO2(aq) is manually defined as a fluid by inputting all the physical parameters. ​
Mixture 2 (Gas Phase): Consists of CO₂(g) and water vapor [H₂O(g)].​
Mass Transfer Mechanism: Implemented using Henry's law with the van't Hoff mole fraction correlation. The overall mass transfer coefficient is determined by constant overall coefficient(Sherwood number * diffusion coefficient) / constant bubble diameter. The mass transfer occurs from phase 2 [CO₂(g)] to phase 1 [CO₂(aq)].​

Boundary Conditions:

Inlet: Defined as a velocity inlet with turbulence settings specified via intensity and length scale (length scale is 2 micrometers, corresponds to the average diameter of the pores on the gas sparger; intensity is 0.7%, I determined based on realistic 50 sccm volumetric inlet flow rate) using equations I = 0.16*Re_D^(-1/8). The volume fraction of liquid at the inlet is set to 0, and CO₂ is set to 1.​

Outlet (top surface of the liquid region): Set as a degassing boundary, allowing only gas to exit.​

Turbulence Model: Utilizing the k-ε RNG model with standard wall functions.​

Chemical Reactions(desolvation of co2 into h2o): Not included at this stage to focus on achieving convergence with a simpler simulation.​

Issue Encountered:

During the simulation, the following warning is repeatedly displayed: "Turbulent viscosity limited to viscosity ratio of 1.000000e+05."​

Steps Taken to Resolve the Issue:

Solver Adjustments:

1. Switched to the coupled solver and adjusted the scalar and coupled parameters.​
2. Initially ran the simulation with only turbulence equations enabled, then enabled all equations.​
Mesh Quality Verification:

Orthogonality: Average: 0.99, Minimum: 0.70, Maximum: 0.999.​
Skewness: Average: 0.071, Minimum: 0.00097, Maximum: 0.73.​
These metrics should suggest that the mesh quality is acceptable.​

Despite these efforts, the warning persists. I understand that this issue may be related to numerical instabilities or modeling inaccuracies. I have come across suggestions to use Full Approximation Scheme (FAS) multigrid methods to address such problems; however, FAS is only compatible with the density-based solver, which is not applicable to the Eulerian multiphase model. Also, there are other suggestions online to increase the limit for viscosity ratio, but I think that's not correct since high turbulence is not expected in this scenario since we only have low gas flow rates bubbling in a stagnant liquid region.

Could someone please guide potential causes for this warning in the context of my simulation setup? Additionally, any recommendations on strategies to mitigate this issue and achieve convergence would be greatly appreciated. Thank you for your support.