Melting of the ice/water as the phase change material in a horizontal single-tube annulus is sluggish when the stable stratification exists at the bottom of the configuration. To obviate this problem, three heat transfer enhancement techniques could be implemented using the enthalpy-based lattice Boltzmann method with the double distribution function method to accelerate the process. The multifarious arrangements of the tubes in this horizontal annulus are investigated to expand the region affected by the natural convection. Also, the dispersion of the Cu nanoparticles in the base PCM could boost the thermal conductivity and melting rate. Finally, the metallic porous matrix made of nickel–steel alloys and saturated with the base PCM could be used to enhance the thermal conductivity of the base PCM. The solid–liquid phase change process is defined as the constrained melting of ice-water in the tube heating mode. There is a thermal equilibrium between ice/water and the nickel–steel porous matrix and the Cu nanoparticles. The Prandtl number, Stefan number, Rayleigh number, and Darcy number are 6.2, 1, 104–105, and 10−3, respectively. The volumetric concentric of the nanoparticles is between 0 and 0.02 and the porosity ranging from 1 to 0.9 in the representative elementary volume scale.
