Numerical investigation of mixed convection heat transfer of nanofluids in a lid-driven trapezoidal cavity

Mixed convection heat transfer in a two-dimensional trapezoidal lid-driven enclosure filled with nanofluids heated from below is numerically studied. The governing equations for both fluid flow and heat transfer are solved by using the finite volume method (FVM). The bottom wall of the enclosure is heated while the upper wall is cooled at lower temperature and the other two sidewalls are adiabatic. Four types of nanofluids (Al₂O₃, CuO, SiO₂, and TiO₂ with pure water) with nanoparticle volume fraction (ϕ) in the range of 1–4% and nanoparticle diameter in the range of 25–70 nm were used. This investigation covers Richardson number and Reynolds number in the ranges of 0.1–10 and 100–1200, respectively. The trapezoidal lid-driven enclosure was studied for different rotational angles (Φ) in the range of 30°–60°, different inclination sidewalls angles (γ) in the range of 30°–60° and various aspect ratios (A) ranged from 0.5 to 2. This investigation is also examined the opposing and aiding flow conditions. The results show that all types of nanofluids have higher Nusselt number compared with pure water. It is found that SiO₂–water has the highest Nusselt number followed by Al₂O₃–water, TiO₂–water, and CuO–water. The Nusselt number increases as the volume fraction increases but it decreases as the diameter of the nanoparticles of nanofluids increases. The Nusselt number increases with the decrease of rotational angle and inclination angle from 30° to 60° and with the increase of aspect ratio. The results of flow direction show that the aiding flow gives higher Nusselt number than the opposing flow.