Natural convection and entropy generation of a ferrofluid in a square enclosure under the effect of a horizontal periodic magnetic field

The current work is concerned with the study of the impacts of a periodic magnetic field on natural convection and entropy generation of Fe3O4-water nanofluid flowing in a square enclosure. The magnetic field is applied as a sinusoidal function of they coordinate. The Galerkin finite-element method is employed to numerically solve the model governing partial differential equations. To evaluate the correctness and accuracy of the results, comparisons have been performed between the outcomes of this work and previously published results. The governing dimensionless parameters and their corresponding ranges are the Rayleigh number (Ra = 10(3)-10(6)), Hartmann number which indicates the amplitude of the periodic magnetic field (Ha = 0-50), the period number (lambda = 0.1-1.0) and the nanoparticles volume fraction (phi = 0-0.08). The outcomes of the study are presented in the forms of streamlines, isotherms and entropy generation contours as well as the local and average Nusselt numbers. Regardless of Ha and lambda values, the average Nusselt number Nu(avg) and the total entropy generation S-t associated with the periodic magnetic field are found to be higher than those corresponding to the uniform magnetic field. Additionally, When lambda = 0.25 and Ra = 10(6), for Ha < 10, using the Fe3O4 nanoparticles enhances the total entropy generation S-t, however, when Ha > 10, the effect of the existence of the nanoparticles on S-t would be inverse. (C) 2018 Elsevier B.V. All rights reserved.