[ortb]

Dear CFD community,

I have a problem to analyze, which involves a hot gas stream flowing over a surface of molten slag in a container. The question is whether the gas momentum will transfer (at least partly) to the slag to prevent local freezing. Further, we want to get an idea about the shear-driven flow pattern inside the container.

I tried doing it with VOF in Fluent, and it seems to work, although the high viscosity ratio of ~10^5 causes issues. The simulation develops very slow on the slag side and turbulence convergence is bad, probably due to the interpolation of properties (density, viscosity) at the VOF surface.

Another idea I had would be to separate the domains of gas and fluid entirely and use a scheme where I would transfer the wall shear stress of a gas-only simulation as a boundary condition to the slag surface and do it iteratively.

Do you have any advice on how to tackle this properly? Which models would you recommend? Any literature you can think of?

Best regards and thank you for your help!
ortb

[FMDenaro]

Originally Posted by ortb

Dear CFD community,

I have a problem to analyze, which involves a hot gas stream flowing over a surface of molten slag in a container. The question is whether the gas momentum will transfer (at least partly) to the slag to prevent local freezing. Further, we want to get an idea about the shear-driven flow pattern inside the container.

I tried doing it with VOF in Fluent, and it seems to work, although the high viscosity ratio of ~10^5 causes issues. The simulation develops very slow on the slag side and turbulence convergence is bad, probably due to the interpolation of properties (density, viscosity) at the VOF surface.

Another idea I had would be to separate the domains of gas and fluid entirely and use a scheme where I would transfer the wall shear stress of a gas-only simulation as a boundary condition to the slag surface and do it iteratively.

Do you have any advice on how to tackle this properly? Which models would you recommend? Any literature you can think of?

Best regards and thank you for your help!
ortb

Since you have a high viscosity ratio, do you have some experimental/numerical evidence that the stream of gas is able to modify the geometry of the surface of the fluid face by tangential stress?


Would be possible to approximately model the fluid as a solid and work as in CHT problems?

[ortb]

Originally Posted by FMDenaro

Since you have a high viscosity ratio, do you have some experimental/numerical evidence that the stream of gas is able to modify the geometry of the surface of the fluid face by tangential stress?

Yes, we are sure that there is in fact a motion. The viscostiy of the slag is not that high at around 0.85 kg/(m-s).

Originally Posted by FMDenaro

Would be possible to approximately model the fluid as a solid and work as in CHT problems?

That is exactly what I did as a first approximation. It has shown freezing to a higher degree than is usually observed in such systems. Therfore, we felt that the slag should be modeled as a proper fluid.


Do you think the coupling approach is appropriate?

[arjun]

Originally Posted by ortb

Dear CFD community,

I have a problem to analyze, which involves a hot gas stream flowing over a surface of molten slag in a container. The question is whether the gas momentum will transfer (at least partly) to the slag to prevent local freezing. Further, we want to get an idea about the shear-driven flow pattern inside the container.

I tried doing it with VOF in Fluent, and it seems to work, although the high viscosity ratio of ~10^5 causes issues. The simulation develops very slow on the slag side and turbulence convergence is bad, probably due to the interpolation of properties (density, viscosity) at the VOF surface.

Another idea I had would be to separate the domains of gas and fluid entirely and use a scheme where I would transfer the wall shear stress of a gas-only simulation as a boundary condition to the slag surface and do it iteratively.

Do you have any advice on how to tackle this properly? Which models would you recommend? Any literature you can think of?

Best regards and thank you for your help!
ortb

High viscosity causes high pressure values so any force calculations or momentum flux calculations will be wrong here.


If you are modelling solid region as high viscosity then the results should be wrong.

[FMDenaro]

I think you have no other way that a VOF for the coupled problem, using a very small time-step.

[ortb]

Originally Posted by arjun

High viscosity causes high pressure values so any force calculations or momentum flux calculations will be wrong here.


If you are modelling solid region as high viscosity then the results should be wrong.

I think you are right. I found an analytic solution of the shear flow problem, which yields way lower boundary velocities than my simulation.

[ortb]

Originally Posted by FMDenaro

I think you have no other way that a VOF for the coupled problem, using a very small time-step.

That is extremely expensive and still does not solve the material property problem. Now I will try to export shear stresses and apply them as a boundary to a second simulation.

Thank you all for your input.

[arjun]

Originally Posted by ortb

I think you are right. I found an analytic solution of the shear flow problem, which yields way lower boundary velocities than my simulation.

We did a ice melting problem long time ago in Wildkatze solver that previously the person tried with fluent and comsol. In both solvers that high viscosity issue created problems. In Wildkatze we then created a method without increasing viscosity. That lead to much better solution.

[ortb]

Originally Posted by arjun

We did a ice melting problem long time ago in Wildkatze solver that previously the person tried with fluent and comsol. In both solvers that high viscosity issue created problems. In Wildkatze we then created a method without increasing viscosity. That lead to much better solution.

Fluent is my only realistic option. I will try the workaround via shear stress profiles and report the results.