cell zone creation for degassing udf implementation
 

For my two phase counter flow, I need to apply degassing boundary condition as provided in Ansys Fluent udf manual example 2.
As this udf will be hooked in cell zone on specific cells. I am analyzing a cylindrical column.

Question is that how should I create cell zones? Is there any need to define interface between these zones.

There could be two options:
1. Mesh cylinderical column with one block and then in Ansys Fluent mark required region and seperate cells from that region it requires. this will create an interior surface. Does it sufficient or I require to define interface here.

2. Mesh cylinderical column with two two blocks. one block defining one zone (fluid_1) for the region where Udf will be hookd for degassing. And another block for remaining body as second zone (fluid_2). Does it require an interface surface to be created ?

Reagards