This study investigated the effect of an omnidirectional magnetic field on the heat transfer performance of a saline water-Al₂O3 magnetic nanofluid (MNF) cavity. The cavity was a quarter-circle with a cold left wall and a heated right wall, whereas other surfaces were insulated (adiabatic). All surfaces were assumed constant. The governing equations were solved numerically based on control volume and SIMPLE algorithm. The nanoparticle concentration ranged from 0 to 0.04 percent, the Hartmann number (Ha) was between 0 and 9, and the Rayleigh number (Ra) was 10⁵. Findings indicated that increased nanoparticle concentration and magnetic field intensity enhance the reductive effect of the Lorentz force. Increasing the magnetic field intensity has an insignificant effect on the thermal performance of the fluid with zero nanoparticle concentration. As the nanoparticle concentration increases, the electrical conductivity of the fluid increases too which leads to a significant increase in the reductive effect of the magnetic field so that a 2% increase in nanoparticle concentration at Ha = 4.5 reduces heat transfer by more than 45%, whereas this reduction is higher than 65% at a nanoparticle concentration of 4% and Ha = 9.