Laminar Natural Convection of Water-Fe3O4 Magnetic Nanofluid Over an Annular Finned Vertical Cylinder with a Non-Uniform Magnetic Field

Abstract

This study investigates the laminar natural convection of water-Fe3O4 nanofluid with a nanoparticle volume fraction of 4 % around an annular finned vertical cylinder under the influence of a non-uniform magnetic field generated by an electric conductor wire. The three-dimensional problem is solved using the finite volume method, and the effects of fin numbers (2, 4, 8, 12), magnetic field intensities (2 x 106, 5 x 106, 7.5 x 106), and Rayleigh numbers (5 x 104-107) on heat transfer and fluid flow characteristics are analyzed. Results demonstrate that the addition of fins enhances cooling efficiency by generating secondary flows and increasing velocity gradients, particularly at higher Rayleigh numbers. In the absence of a magnetic field, increasing the number of fins from 2 to 12 results in a 28 % to 44 % increase in heat flux, while in the presence of a magnetic field, this increase ranges from 44 % to 150 %. Furthermore, increasing the magnetic field intensity from zero to 7.5 x 106 leads to a heat flux improvement of 26 % to 148 % for a simple cylinder, and 21 % to 277 % for a cylinder with four fins. The study also highlights the non-linear interaction between Rayleigh numbers and magnetic field intensity, where heat transfer initially increases with Rayleigh number but decreases under high magnetic fields due to the suppression of convective flows. These findings underscore the critical role of fin configuration and magnetic field intensity in optimizing heat transfer, with the most effective results occurring at specific Rayleigh numbers and magnetic field strengths.