Thermo-magnetic convection regulating the solidification behavior and energy storage of Fe3O4 nanoparticles composited paraffin wax under the magnetic-field

Solidification is an essential process in phase-change energy storage and magnetic fields have great effects on it. While the majority of previous studies focused on non-uniform magnetic fields, this study investigated the differences using uniform magnetic fields. The thermo-magnetic convection model and the enthalpy porosity method describe the motion behavior of Fe3O4 nanoparticles under a uniform magnetic field and phase change process. The properties of Fe3O4/paraffin phase change nanocomposites are measured, and a visualization platform is built to verify the numerical model. The effect and mechanism of direction and intensity of magnetic induction, nanoparticle concentration on solidification performance, heat transfer, and energy release are investigated. Results showed that magnetic field could promote or inhibit solidification depending on its direction. The degree of global regulation increased with the concentration of nanoparticle and magnetic induction intensity, while the local nonuniformity increased or decreased under forward or reverse magnetic fields. The solid phase fraction and energy release of the cavity under positive or negative magnetic fields increased or decreased maximally by 29.2%, 19.23% or 4.6%, and 3.88% compared with paraffin. Essentially, the regulation was the result of different motion behaviors of nanoparticles in the liquid phase controlled by Kelvin force. In addition, methods and optimization suggestions for regulation were proposed, which were expected to provide a reference for the design of controllable energy storage devices.