Heat transfer and flow in MgO-water and TiO2-water nanofluids in rectangular cavities: Role of aspect ratio and thermal efficiency

Geometrical confinement strongly affects heat transfer and flow resistance in nanofluidic cavities, yet systematic assessments across a wide range of aspect ratios (AR) remain scarce. This study numerically investigates laminar mixed convection of MgO–water and TiO₂–water nanofluids (φ = 2%) in lid-driven rectangular cavities with AR = 0.5, 1, 2, 4, and 8 at Re = 100. The governing equations are solved using an in-house finite-volume Fortran code with a second-order characteristics-based scheme, validated against benchmark data. The flow structure transitions from two counter-rotating vortices at high AR to a single dominant vortex at AR = 1, and finally to a boundary-layer-dominated regime at AR = 0.5. The maximum average Nusselt number for pure water (Nu = 16.40) occurs at AR = 0.5, while MgO–water achieves its highest relative enhancement at AR = 8 (Nu = 5.28, 37% improvement), with the friction penalty governed by f_max = 4/AR. Using the Thermal Efficiency Parameter (TEP), which couples heat-transfer enhancement with pumping-power penalty, MgO–water yields TEP > 1 only at AR = 8 (TEP = 1.06), is neutral at AR = 2, and detrimental elsewhere; TiO₂–water remains neutral or detrimental at all AR. These findings demonstrate that nanoparticle addition is not universally beneficial and depends strongly on cavity geometry, providing practical design guidelines for compact heat exchangers, electronic cooling, and solar thermal collectors.