Effect of Convective Boundary Conditions and Enclosure Orientation on Natural Convection Heat Transfer of Nanofluids using the Lattice Boltzmann Method

The orientation of thermal systems and the nature of convective boundary conditions significantly affect their thermal performance, particularly in the presence of nano-enhanced heat transfer fluids. In this study, the effect of enclosure orientation on heat transfer characteristics is investigated for a cooling nanofluid (Al₂O₃/water) in a square enclosure. Two configurations are analyzed, where the active walls are either parallel or orthogonal to the direction of gravity, by considering a 90° rotation of the baseline enclosure. The governing conservation equations are solved numerically. The D2Q9 two-dimensional Lattice Boltzmann model is employed to simulate heat transfer and fluid flow. The influence of key parameters is examined, including the Rayleigh number (Ra = 10³, 10⁴, 10⁵, 10⁶), nanoparticle volume fraction (ɸ = 0%, 2%, 4%, 6%), and Biot number (Bi = 0.5, 5, 50, 100). The results highlight the impact of nanofluid viscosity on thermal performance. Moreover, the configuration with active walls parallel to gravity-featuring a heated bottom wall and a cooled top wall-yields a 10.5% higher average Nusselt number. These findings contribute to the design of more efficient nanofluid-based cooling and heating systems.