Learning Forum

[abdullahhassan]

Hello,

I have been trying to model the evaporation of water inside a tube using the VOF model (Implicit, dispersed) and water as a material. Initially, I started doing with a 2D model of the tube which works fine in general, but as soon as I turned on multiphase modeling, I received two errors 

1- "turbulent viscosity limited to viscosity ratio of 1.000000e+05 in XXcells" and

2- temperature limited to 5.000000e+03 in XXcells 

and continuity started to rise with every iteration until I received a floating error.

I improved the mesh near the walls or increased the overall structured cell count, but the error persists. 

Then, I tried a smaller 3D tube as I had some experience with 3D phase change simulation. 

Initially, I used a tube with polyhedra meshing (0.015m dia inside & 0.5m length) but the error persists regarding turbulent viscosity and temperature reached 5000K. 

Then, I meshed the geometry through ICEM to make a better structural mesh, but the error persists. 

 

Note: I have increased the turbulent viscosity limit, initialized with k = 1, and epsilon with 100, but it keeps on giving a floating error. 

One thing is that while using the SIMPLEC scheme, it diverges very quickly, but with PISO, it takes around 50 to 100 more iterations to diverge. 

 

Can you tell me what the possible error in this will be? 

Mass flow inlet = 13kg/s, 

Turbulence = K-epsilon , realizable with enhanced wall treatment,

[Rob]

Why are you using VOF if the flow dispersed? 

[abdullahhassan]

I am using dispersed because I think, we should use it when one fluid is significantly dominant in volume as in my case, liquid is dominent but yes i want to convert 100% liquid to vapor as well (please correct me if I missunderstood).

I used VOF to increase the computation time. 

I have started simulation with VOF sharp but the results are same. No simulation runs more than 40 - 50 timesteps.

 

[Rob]

Look at how the VOF model works. Then review Mixture and Eulerian options. 

[abdullahhassan]

Hi Sir,

I have checked the VOF model and also tried multiple settings in the viscous tab, BC, or initialization, but the issue of "turbulent viscosity & temperature limited to 5.000000e+03" still persists. 

Currently, after setting everything, I have turned off all the equations. First, I converged the flow, and then I turned on the Energy to make it converge as well, but the simulation diverges after a few iterations. It seems that the multiphase is not the problem, but the energy equation is the problem. 

I have used multiple mesh size but I don't understand what is the real reason. can reverse flow also lead to this temperature limitation or turbulent viscosity warning?

 

[Rob]

Can you post some images of the flow & phase distribution? If you just look at temperature what do you see, ie are you trying to flash the liquid by mistake?