In the present study, melting behavior of RT50 as a phase change material in a shell and tube heat exchanger is considered. Therefore the effects of parameters including a geometrical property (eccentricity) and flow specifications (mass flow rate and inlet temperature of heat transfer fluid) on different decisive parameters of the PCM are investigated. The selected parameters which are critical for every storage systems are liquid fraction, melting time and thermal storage performance. Enthalpy porosity method is used for the modeling of phase change process and pure conduction and natural convection are considered in the simulation. The differential governing equations are solve using. SIMPLE algorithm in which momentum and energy equation are discretized employing QUICK differentiating scheme. The grid size and the time step were chosen after careful examination of the independency of the results to these parameters. The details of the process progression are presented via temperature contours and streamlines. The study declares that the initial stages as well as ending portion of melting process are dominated by weak mechanism of conduction. Results show that the average temperature of the PCM isn't a function of eccentricity as long as there is solid PCM above the inner tube. Generally by increasing the eccentricity the rate of heat transfer and average temperature at the last stages of melting process increases. The Results indicated that although increasing eccentricity and inlet temperature of the heat transfer fluid influences the rate of heat transfer in the heat exchanger especially when both variations are considered simultaneously, mass flow rate does not leave such a significant impression on the rate even in eccentric cases. (C) 2016 Elsevier Ltd. All rights reserved.