Fail to define a single phase fluid after turning on VOF
 

Dear all,
I am trying to model a double pipe heat exchanger with the heat transfer fluid flowing inside the inner pipe to melt the PCM in the annulus. Air also exist in the annulus part, so I used VOF model to model this part. After I activating VOF model, when I define the inner fluid, it regarded inner heat transfer fluid as mixture of two phases defined in VOF model. I want to define inner fluid as another material. How can I solve this problem? Do I have to write UDF to solve it?