Rob
Ansys Employee

This looks to follow from this  https://studentcommunity.ansys.com/thread/how-to-select-and-specify-material-for-voids-after-boolean-subtract-operation/  where Peter told you that you need to subtract and retain the PCM balls from the flow region. 


If you label these then you can separate the two zones with walls (if the PCM is retained in a shell) or to patch phases if they aren't.