This paper presents a two-dimensional model for the phase change, conduction based heat transfer problem around a tube immersed in the pcm. The energy equation is written in the enthalpy form, and the heat and flow problems are coupled by an energy balance on the fluid element flowing inside the tube. The numerical solution is based upon the average control volume technique and the ADI finite difference representation. The results obtained show the effects of the variation of the ratio of the radius of the inner to the outer tube, Blot number, Stefan number and the working fluid inlet temperature on the solidified mass fraction, NTU and effectiveness.