Anisotropic behavior of magnetic nanofluids (MNFs) at film boiling over a vertical cylinder in the presence of a uniform variable-directional magnetic field

Abstract

Brownian motion and thermophoresis are two primary sources of nanoparticle migration in nanofluids which have substantial impacts on thermophysical properties of nanofluids. In addition, orientation and intensity of an external magnetic field influence the thermal conductivity of MNFs and make them anisotropic. An external magnetic field is also able to amend the thermophyscial properties of magnetic nanofluids (MNFs) to improve the thermal performance. The motivation behind this study is the need to examine the anisotropic behavior of thermal conductivity and its effects on flow field and heat transfer characteristics at film boiling of MNFs over a vertical cylinder in the presence of a uniform variable-directional magnetic field. The results have been obtained for different parameters, including the Brownian motion to thermophoretic diffusion NBT, saturation nanoparticle concentration ϕsat, Hartmann number Ha, magnetic field angle α, ratio of film thickness to cylinder radius ε, and normal temperature difference γ=(Tw−Tsat)/Tw. A closed form solution for the nanoparticle distribution is obtained and it has been indicated that the heat transfer rate is improved further when an external magnetic field is aligned in the direction of the temperature gradient. Furthermore, it is shown that the larger nanoparticles intensify the nanoparticle migration and enhance the heat transfer rate.