Enhancing the convective heat transfer in vertical and horizontal rectangular enclosures using nanofluids: The crucial role of aspect ratio

Abstract

This study extensively examines double-diffusive natural convection of a water nanofluid containing Al2O3 nanoparticles in both horizontal and vertical rectangular enclosures subjected to uniform thermal and solutal fluxes along the horizontal direction. The key contribution of this research is to emphasize the significance of aspect ratio as a critical factor for enhancing heat transfer. Neglecting this factor could have substantial implications for the advancement and practical application of nanotechnology in heat transfer. The study also examines the impact of other relevant parameters namely nanoparticle concentration, thermal Rayleigh number, and three distinct models to calculate effective viscosity and thermal conductivity. A comparison among these models highlights a notable disparity between models based on experimental data (model II and III) and that based on theoretical assumptions (model I). It was revealed that the thermal Rayleigh number manifests a notable impact on both heat and mass transfers, especially for horizontal enclosures, and an augmentation in the aspect ratio improves the convective heat and mass transfers. However, for lower Rayleigh numbers, the aspect ratio effect diminishes. The incorporation of nanoparticles might deteriorate up to 21% or enhance up to 12% heat transfer according to the dominant transfer mode, we obtain the enhancement in conduction and deterioration in convection. Model I gives higher Nusselt and Sherwood numbers, followed by models II and III, but model I showed a peculiar behavior compared to models II and III. A critical aspect ratio is identified which defines the regions where nanofluid enhances heat transfer compared with clear water.