Abstract: Melting of unfixed solid phase change material(PCM) inside a cylindrical tube was investigated.An analysis model of contact melting inside a cylinder tube with constant heat flux was established.Control equations of melting were established with Nusselt's liquid film theory,and dimensionless melting equations were obtained theoretically.Melting velocity,thickness and pressure distribution of molten layer were discussed,and they were compared with melting under Dirichlet condition to study melting law with Neumann condition.It was found that molten film thickness increases as height of solid decreases,and melting velocity decreases during melting process.Heat flux has great effect on melting while heat flux is low.

Key words: contact melting, heat transfer, phase change, heat flux