Numerical Investigation of Viscous Dissipation Effects on Mixed Convection of a Water-Based TiO₂ Nanofluid in a Lid-Driven Cavity with an Aspect Ratio of L/H = 4

This study numerically investigates mixed convection in a rectangular cavity filled with a water-based titanium dioxide TiO2 nanofluid, focusing on the influence of viscous dissipation on the flow and temperature fields. The cavity has an aspect ratio of 4, with its upper and lower walls moving in opposite directions at equal dimensionless velocities. The vertical sidewalls are stationary and thermally insulated. The governing equations are solved using the finite volume method, with the Reynolds, Grashof, and Prandtl numbers fixed at 49, 16, and 6, respectively, corresponding to a Richardson number of Ri = Gr/Re² ≈ 0.0067, i.e., a forced-convection-dominated regime with a weak buoyancy contribution and the nanoparticle volume fraction maintained at 0.03. The Eckert number is varied from 0.0 to 0.3 to examine the contribution of viscous dissipation to the thermal behavior. Numerical convergence, grid independence, and validation against previously published skin-friction data are considered to assess the numerical results. The results show that the streamline patterns and velocity profiles remain qualitatively similar across the investigated Eckert numbers, indicating a limited influence of viscous dissipation on the overall flow structure. In contrast, the temperature profiles exhibit noticeable changes as the Eckert number increases. The dimensionless temperature generally rises over a substantial portion of the cavity, demonstrating the importance of viscous heating in the energy balance. These findings indicate that viscous dissipation affects the thermal field more strongly than the overall velocity distribution under the present operating conditions.