## Fluids

#### How can I calculate the interfacial area in only liquid phase in my bioreactor?

• Roya
Subscriber

Hello everyone,

I am trying to calculate the interfacial area for my aerated bioreactor. The bioreactor is filled with liquid up to H and the rest is filled with air (Figure below). There is air injected at the bottom of bioreactor in the form of single-size bubbles. I would like to calculate the interfacial area for the liquid phase (media-bubbles). I would like to know how I can calculate it?

• Rob
Ansys Employee
How are you injecting the bubbles?
• Roya
Subscriber
I inject the bubbles through the inlet which is at the bottom of the tank (the ring sparger). I use Eulerian multiphase model (dispersed) for modeling the flow.
• Rob
Ansys Employee
OK. Currently you'll need to split the domain to have a separate fluid region in the liquid part and then get the bubble volume fraction in that zone. A UDF would work but you'll need to limit the region that it looks in to be below the free surface height. We have asked for some more tools, and they're in development.
• Roya
Subscriber
I don't know if I got what you mean, but if I understand you correctly:
I have already used a region_0 (Adapt/Patch) to specify the region at the top which is filled with air. Is that what you mean?
Even if I separate the fluid region from the air region, the surface doesn't remain unchanged as the fluid is being stirred and vortexes appear at the top surface.
• Rob
Ansys Employee
Sort of. We can also use those registers to split the cells into different zones. If you're getting vortexing that makes it a little more complicated as you'd need to adjust the split every time you wanted the data. may have another idea but I think you will need a UDF and limit the data gathering to below the free surface.
• Roya
Subscriber
Dear Rob,
How can I simply write a UDF and define interfacial area as a parameter that I want to be calculated, But ask the code to only calculate it where volume fraction of air is less than one (which is where the liquid phase exists)?
I don't think DEFINE_EXCHANGE_PARAMETERS really work here as it takes the UDF to the phase interaction tab which is not what I need. I am not activating any mass transfer model, so basically using interfacial area option in phase interactions tab is meaningless for me.
Do you think that it would solve the problem or would it cause issues and is inaccurate?

Kind regards Roya
• Rob
Ansys Employee
It'll be one of the volume of fluid macros, use https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v221/en/flu_udf/flu_udf_sec_define_mass_transfer.html as a starting point. You're not changing phase, but do want to dump the data to a UDM.
The problem you'll face is distinguishing the volume fraction caused by bubbles and the region around the interface.
• Roya
Subscriber
Dear ROB Is there any UDF in Fluent in which can calculate the volume fraction of one phase in another phase? Then I can use the DEFINE_REPORT_DEFINITION_FN and calculate the interfacial area in a second UDF.
Regards Roya
• Rob
Ansys Employee
Isn't C_VOF the volume fraction?
• Roya
Subscriber
Yes, it is. But it only gets the value of volume fraction correctly if it is specified for the volume fraction in one phase. In my case, I have a region at the top which is filled with air. And I don't want that section to be involved in the calculation. I want the volume fraction of air only in liquid phase. Is that also still under develop in ANSYS? Or is there any UDF available for that?

• Rob
Ansys Employee
That's the volume fraction, whether it's in the air (only) or a fraction in the liquid it's still the same macro.