Magnetic-Field-Controlled Thermohydrodynamic Behavior of Fe3O4/SWCNT Hybrid Ferrofluid in a Cavity with a Rotatable Elliptical Barrier

Abstract

Magnetic regulation of ferrofluid transport can modulate heat and fluid motion without mechanical actuation, but the combined effect of field strength and an orientable internal obstacle remains insufficiently resolved. This study numerically examines laminar natural convection of a 1.3 vol% Fe3O4/SWCNT (Single-Walled Carbon Nanotube)–water hybrid ferrofluid in a square cavity containing a rotatable elliptical barrier under a vertical magnetic field. Eight barrier orientations (0–315° in 45° increments) are evaluated at Ra = 103 and Ha = 20 and 80 using the finite element method in COMSOL Multiphysics. The governing formulation was corrected to use a symmetric viscous operator and the Lorentz term associated with a vertical magnetic field; the base-fluid Prandtl number calculated from the tabulated properties is 6.07. An independent, differentially heated square-cavity benchmark reproduced the reference values for maximum horizontal and vertical velocities and the average Nusselt number with relative errors below 0.76%. The computed fields show that barrier orientation controls the locations and signs of the velocity extrema and the pressure range, whereas the reported temperature profiles change comparatively little between the two Hartmann numbers. The 90° orientation gives the minimum reported Bejan number, while 45° gives the maximum. Because Be is a ratio, these extrema identify changes in the relative contribution of thermal irreversibility and do not, by themselves, establish a minimum or maximum of total entropy generation. The results demonstrate a coupled geometric–magnetic redistribution of the local thermohydrodynamic fields and identify 90° as the preferred orientation only under the Bejan-number criterion adopted here.