Modelling and Simulation of Flow and Heat Transfer of Ferrofluid under Magnetic Field of Neodymium Block Magnet

•The effects of the magnetic field of block neodymium magnets on ferrofluid flows were numerically investigated.•The permanent magnet can significantly affect the fluid flow and heat transfer characteristics.•Ferrohydrodynamics effects are completely dominant and magnetohydrodynamics effects are ignorable.•Applying the magnetic field increases Nusselt number at Re=250, where the heat transfer is low, by a factor of 2.•The applied magnetic field remarkably increases the skin friction of walls. Neodymium magnets are the strongest type of permanent magnet commercially available. This investigation aims to numerically study the behavior of ferrofluids in the presence of neodymium block magnets which could be used in a wide range of applications. The problem formulation is derived using the principles of ferrohydrodynamics (FHD) and magnetohydrodynamics (MHD), and the finite volume method is employed for solving the equations. The flow of water-Fe3O4 magnetic nanofluid at 250≤Re≤2300 in a three-dimensional channel under heat flux exposed to a block neodymium magnet is considered. The results indicate that the magnet can significantly affect the flow field and heat transfer while FHD effects are completely dominant and MHD effects are ignorable. In the presence of the magnet, a secondary flow is created, which is more significant for low Reynolds numbers. Applying the magnetic field increases the heat transfer so that at Re=250, where the heat transfer is low, it can increase the Nusselt number by a factor of 2. Moreover, the magnetic field substantially increases the wall skin friction. Considering both the increments of heat transfer and friction, the Reynolds number of 1500 has the maximum thermal performance factor. With increasing Reynolds number or distance between the magnet and channel, the magnetic effect decreases. It is found that the thermal performance factor is increased by reducing the distance of the magnet and channel. In addition, if the height of the magnet is decreased by half (from 1 cm to 0.5 cm), the thermal performance factor improves by 6%.