I am simulating a double pipe heat exchanger. In the inner tube which is shown by red color, refrigerant 134a is being condensed (changing phase from vapor to liquid). The green color shows copper tube while blue shows water in the outer pipe (single-phase).

When I turn the multiphase model on it takes all the materials i.e. R 134a, copper tube, and water as two-phase mixtures as indicated by cell zone conditions in the bar on the side. I only want R134a to be multiphase while other zones to be single phase. Kindly guide me on how can solve this problem.

Note: I have tried defining water as a separate phase but it doesn't work.