Abstract: The direct numerical simulations have been conducted to investigate the flow and heat transfer characteristics of the Rayleigh-Bénard convection system under a horizontal magnetic field. The study is conducted in a rectangular cavity with an aspect ratio of 1:4:1.5, covering a Rayleigh number range of 5×10⁵ ≤ Ra ≤ 5×10⁶ and a Hartmann number range of 10² ≤ Ha ≤ 10⁴. The aim is to analyze the impact of the magnetic field on the flow and heat transfer characteristics. The results indicate that the competition between buoyancy and Lorentz forces is the key mechanism regulating both the flow structure and transport characteristics. Under weak magnetic fields, vigorous thermal plumes serve as the primary carriers of heat transfer, which is associated with a thin thermal boundary layer and pronounced flow instabilities, with viscous dissipation being the main source of energy loss. In moderate magnetic fields, the flow exhibits pronounced quasi-two-dimensional characteristics, and while the thermal boundary layer thickens, the flow remains susceptible to instabilities, with viscous and Joule dissipation being of comparable magnitudes. Under strong magnetic fields, the flow transitions completely to a quasi-two-dimensional state, maintaining a stable, vertically stacked double-vortex structure; at this stage, Joule dissipation supplants viscous dissipation as the dominant mechanism of energy loss. Furthermore, scaling relationships between the Nusselt number (Nu) and the Reynolds number (Re) with respect to the Rayleigh number (Ra) were obtained, thereby confirming the asymptotic power-law behavior under strong magnetic field conditions.

Key words: horizontal magnetic field, Rayleigh-Bénard convection, transport characteristics, scaling law