Learning Forum

[sachintom1992]

I am simulating subcooled water flow boiling in annulus geometry. The experimental case considered for validation has high void fraction flow regimes i.e. slug regime (up to area-averaged void fraction of 60 %). So flow regime-dependent closure correlations are used within AIAD framework using UDFs.  I am using Ansys Fluent 2023 student version.

When I am enabling the population balance model and performing the run the cases are diverging. I made interfacial heat transfer liquid side default model (Ranz-Marshall) without UDF and Drag, Interfacial area as UDFs. Then it is running without any divergence. But I am unable to use UDF for interfacial heat transfer liquid side. 

Is it any bug there? With Ranz-Marshall udf also, the case is diverging when enabling the coupling of wall boiling and population balance model.

Thanks and regards,

Sachin      

[Rob]

I'm not aware of any bugs. Typically convegence issues are linked to mesh quality (resolution & cell checks) and time step size. 

[sachintom1992]

The divergence is not happening with the default Ranz-Marshall model. When the same correlation is given as UDF, it diverges. 

Also, one more query with respect to coupled wall boiling and population balance model. In the population balance equation (PBE), the condensation source term is there, and in fluent PBE, that term is not considered from the manual. Is the condensation term needs to be given as a source term here?

[Rob]

Check how Fluent handles the phase change limiters: the latent heat energy will cause evaporation/condensation which is counter to whatever just happened. That can cause stability issues. 

Not sure for PBE, the change of bin is down to "stuff" so it depends what is activated. Bubble size will change, but several of the kernels are set for bubble size change due to shear.