Thank you Rob. In my real system, there is a copper inner pipe body with a thickness of 1mm, so the heat would be absorbed from the phase change material (so that it will solidify) through this copper inner pipe and after through the fluid domain. I'm confused if i mesh this copper inner pipe as in the image and i will have 2 interfaces soli/fluid and solid/pcm or i just ignore it and i put an inner pipe with zero thickness with shell conduction and i have one interface. I don,t know if the second approach works or not. need help for that