I have a simple shell tube heat exchanger multiphase simulation Ansys Fluent problem with two different fluids- A & B and a solid material for tube wall.

Fluid A (inside tube) is not undergoing any phase change. So multiphase modelling is important only for Fluid B which is in shell.

Once I define the all the materials and the phases for fluid B (viz. liquid and vapor), in Cell Zone Conditions, I can not assign Fluid A to the tube fluid domain. All I can see to assign are the phases of fluid B only.

How can I assign the fluid A to the tube fluid domain?