Numerical Simulation of Mixed Convection Heat Transfer of ZnO–Water Nanofluid in a Rectangular Cavity with Moving Walls

This study numerically investigates steady, two-dimensional, laminar mixed convection of a ZnO–water nanofluid in a rectangular cavity with an aspect ratio of 2. The upper and lower horizontal walls move in opposite directions with equal dimensionless velocities, while being maintained at different temperatures; the vertical sidewalls are stationary and adiabatic. The governing equations are solved using a characteristic-based numerical method combined with artificial compressibility, and the nanofluid is modeled as a single-phase mixture with effective thermophysical properties. Simulations are conducted at Re = 16, Gr = 4, and Pr = 6.161 for nanoparticle volume fractions of 0%, 2%, 4%, and 6%, with viscous dissipation neglected. The main contribution of this work is the comparative assessment of local and average heat transfer and wall friction on both moving walls, providing insight into the different responses of the hot and cold boundaries to nanoparticle addition. The results reveal that increasing the nanoparticle concentration enhances heat transfer along the upper wall, increasing its average Nusselt number from 11.16 for pure water to 11.87 at 6% concentration. In contrast, the average Nusselt number on the lower wall decreases from 8.48 to 7.16 at 2% concentration, followed by a slight recovery to 7.40 at 6%. Meanwhile, the average friction coefficient increases from 1.00 to 1.65 on both walls. These findings demonstrate that nanoparticle addition produces distinct thermal responses at the two moving boundaries while consistently increasing the hydrodynamic resistance.