<p>This study numerically investigates mixed convection heat transfer in a ventilated square cavity filled with an MWCNT–Fe₃O₄/water hybrid nanofluid and partitioned by dual porous layers with distinct properties. Unlike previous works that considered uniform porous media or single-phase nanofluids, the present study focuses on the combined influence of hybrid nanoparticles and multi-layered porous structures under varying Reynolds (50–1000) and Grashof numbers (10³–10⁶). Parametric analyses were performed for Darcy number (0.01–100), porosity (0.5–1.8), thermal conductivity ratio (0.2–5.0), and porous thickness (0.1–0.5). The results show that increasing Reynolds number strengthens vortices and enhances thermal dispersion, while higher Grashof numbers shift the system to buoyancy-dominated convection with significantly improved heat transfer. Porous layer thickness has little effect at low Gr but suppresses convection when enlarged (a = 0.5), reducing the average Nusselt number due to flow resistance. This suppression is alleviated by higher Darcy numbers or thermal conductivity ratios, which improve permeability and heat diffusion. At high Gr, increasing the conductivity ratio leads to conduction-dominated transfer. The hybrid nanofluid consistently augments thermal performance across regimes. Overall, the study highlights the synergistic role of hybrid nanofluids and dual porous layers in enhancing mixed convection. The findings provide practical guidance for optimizing porous media and nanofluid design in advanced thermal management applications.</p>