Abstract: A numerical model is established using the enthalpy-porosity approach to study the heat transfer characteristics of a tube-and-shell phase-change heat exchanger filled with paraffin wax RT50. The influence of exchanger placement forms,tube diameters and fin structures on the phase change process of RT50 is analyzed. The results depicts that the vertical heat exchanger has a faster melting rate than the horizontal one as the tube diameter is large. However,the opposite results are obtained in case the tube diameter is small. For the horizontal exchanger,the heat conduction is dominant at the beginning and end of the melting process,while the natural convection plays a more important role at the intermediate stage of melting. Besides,the duration of the melting is mainly determined by the natural convection. In addition,the installation of fins on the outside of the tube can significantly increase the heat transfer coefficient and shorten the melting time. Compared with finless tube,the utilization of ring-fin tube in the horizontal heat storage device can shorten the melting time by 31.6%,and the utilization of straight-fin tube can shorten the melting time by 42.1%. Therefore,the straight fins have a better effect than the ring fins. The main reason is that straight fins can strengthen the natural convection during the melting process.