Effect of surface roughness on heat transfer and entropy generation of mixed convection in nanofluid

In this study, mixed convective heat transfer and entropy generation of Al2O3-water in a lid-driven square cavity with roughness elements on the bottom surface have been studied. The vertical sidewalls of the cavity are adiabatic, and horizontal walls are maintained at a constant temperature, while the top wall is moving at a constant velocity. Various Reynolds numbers, Rayleigh numbers, and nanoparticles concentrations have been considered. The wavy bottom wall of the cavity is determined by the number and amplitude of the roughness elements. It has been observed that the larger amplitude reduces the heat transfer rate while increasing the total entropy generation and the average Bejan number (Be-avg). Conversely, the roughness elements reduce the heat transfer rate and total entropy generation while increasing Be-avg. It has been further observed that the amplitude has a greater effect on the entropy generation and Be-avg than the number of roughness elements. In addition, increasing the Reynolds and Rayleigh numbers increases the average Nusselt number and total entropy generation, while reducing Be-avg. Addition of nanoparticles in a base fluid increases the heat transfer while minimizing the total entropy generation. Be-avg rises with nanoparticle concentrations. The lowest entropy generation for nanofluid could be achieved at the low Rayleigh number and Reynolds number with a fixed number and amplitude of the roughness elements. In the cases studied, total entropy generation is greatly affected by heat transfer irreversibility while fluid friction irreversibility plays a minor role in the total entropy generation enhancement.