This paper is a numerical study conducted to investigate the conjugate flow and heat transfer occurring in three-dimensional (3D) natural convection. A cubical enclosure partially filled with porous block (central cubic) and considered in local thermal equilibrium with the fluid. The physical case considered concerns the existence of a horizontal temperature difference across the enclosure, between the left and the right wall, with the other external surfaces being adiabatic. Under these conditions, flow inside the enclosure is generated by the density (temperature) difference across the enclosure and the interaction between the solid porous blocks and the fluid. The Nusselt number on the hot and cold walls is presented to illustrate the overall characteristics of heat transfer consequence of the constrained flow inside the enclosure. The study focuses on the fluid flow and heat transfer evolution versus the dimensionless thickness of the inserted porous layer (0% ≤ η ≤ 100%) and the relative thermal conductivity of the solid matrix to that of the fluid $(10−3≤λ̃≤103)$. The obtained complex flow structure and the corresponding heat transfer (velocity, temperature profiles) are discussed in a steady-state situation. The numerical results are illustrated in terms of isotherms, velocity, streamlines fields, and averaged Nusselt number. Thus, the results of this work can help developing new tools and to optimize the overall heat transfer rate, which is important in many electronic energy components and other energy recovering systems.

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