Aspect-ratio-driven pattern selection and heat/mass transfer in liquid metal double-diffusive convection under uniform magnetic fields

Double-diffusive convection of low-Prandtl-number liquid metals under magnetic fields is numerically investigated in a laterally heated rectangular cavity, with particular emphasis on the coupled effects of aspect ratio and magnetic-field orientation (β=0°,90°). High-order numerical schemes are employed to resolve flow topology and transport characteristics over a wide range of aspect ratios (A∈[1,12]) at Ra=104 and 105. At the moderate Rayleigh number (Ra=104), the flow remains in steady or periodically oscillatory states for all aspect ratios considered, and the heat and mass transfer rates exhibit pronounced non-monotonic dependence on aspect ratio. These extrema are shown to be closely associated with transitions in the number of convection rolls, highlighting the role of flow structural reorganization in transport enhancement. At the higher Rayleigh number (Ra=105), the flow dynamics become significantly more complex, with increasing aspect ratio leading to successive transitions from steady convection to periodic oscillations and eventually chaotic behavior. The onset of chaos is attributed to the combined effects of intensified buoyancy forcing and geometric stretching of the flow domain, which promote nonlinear interactions that cannot be fully suppressed by magnetic damping. The magnetic field orientation is found to play a crucial role in modulating flow stability and transport efficiency, with vertical magnetic fields sustaining more coherent convective motion and yielding higher heat and mass transfer rates than horizontal fields due to the anisotropic action of the Lorentz force.