This article presents numerical simulation results of natural convection of two immiscible gas/liquid fluids, air and Al₂O₃-water nanofluid, within a rectangular enclosure which was heated partially from side walls. The finite volume approach is used to solve the governing equations of momentum and energy. The SIMPLE algorithm is used to correlate between the velocity and pressure fields and satisfying the continuity equation. After validating the numerical method, the variations of the solid volume fraction of the nanofluid (0 ≤ ϕ ≤ 0.2), the Rayleigh number (10³ ≤ Ra ≤ 10⁶), the aspect ratio of the enclosure, the heater’s length and locations on side walls were studied in details, while the free surface level of the nanofluid was considered to be constant (H_nf = 0.8H). The results show that the interface between two immiscible fluids exerts a different condition within the liquid region especially on the streamlines and the formation of circulating zones in corners.