Vortex formation evolution in melting of metal-foam-embedded nano-enhanced phase change materials regulated by dual-directional Helmholtz coil magnetic fields

A numerical study is conducted to investigate the regulation mechanism of dual-directional magnetic fields generated by Helmholtz coils on the vortex modes and melting performance of nano-enhanced phase change material (NEPCM) embedded in metal foam. The enthalpy-porosity method, Darcy-Forchheimer model and local thermal non-equilibrium model are used to describe the melting process, flow in porous media and heat transfer, respectively. The proposed model is then validated against experiments, then the effects of magnetic numbers (〖Mn〗_y and 〖Mn〗_z) and Rayleigh number (Ra) on fluid flow, melting heat transfer and energy storage characteristics are illustrated and discussed. Results show that the applying a magnetic field parallel to buoyancy (z-direction) can effectively promote NEPCM melting. At low Ra, with an increase in the Mn, the complete melting time of NEPCM is shortened, and the heat storage efficiency can be increased by up to 4.613%, whereas the heat storage capacity is reduced by 2.814%. The competitive effect of dual-directional magnetic fields can effectively modify this mechanism. When natural convection is weak, the regulation of NEPCM melting by a magnetic field orthogonal to buoyancy (y-direction) exhibits a specific force-loop phenomenon. Under the coupled regulation of dual-directional magnetic fields, a new magnetic-force phenomenon emerges, in which the competition between the z- and y-direction magnetic fields disrupts the original force-loop structure of the Kelvin force. At high Ra, the magnetic field cannot change the vortex morphology, and the heat storage efficiency can be increased by up to 12.9%, with the heat storage capacity improved by 5.7%.