In a circular tube of radius Ro, which was initially filled with PCM/composite PCM in solid state at constant temperature (To). A higher temperature (Tw > Tm) was applied to the outer surface of the tube when time t > 0. The PCM inside the tube changed from solid to liquid, absorbing a large amount of latent heat. Natural convection originated driven by the buoyancy force generated by density changes, which caused by temperature differences. The heat transfer mechanisms (heat conduction and
