article · Physics of Fluids
This research assesses double-diffusive natural convection in horizontal and vertical rectangular enclosures filled with an aluminium oxide and water nanofluid. Subjecting the enclosures to uniform thermal and solutal fluxes, the investigation highlights the enclosure aspect ratio as a crucial determinant of heat transfer performance. Increasing the aspect ratio enhances convective heat and mass transfers, although this effect weakens at lower thermal Rayleigh numbers. The thermal Rayleigh number itself significantly influences transport processes, particularly within horizontal enclosures. Crucially, the addition of nanoparticles does not universally improve performance: it enhances heat transfer by up to 12 percent when conduction dominates, but impairs it by up to 21 percent when convection dominates. Furthermore, the analysis identifies a critical aspect ratio that distinguishes operating regimes where nanofluids outperform clear water, while noting significant divergences between theoretical and experimentally based property models.
Using nanofluids is often assumed to boost cooling and heating efficiency automatically. This work shows that without matching the fluid to the correct enclosure geometry and flow conditions, adding nanoparticles can actually degrade thermal performance by over 20 percent. Understanding these geometric thresholds helps engineers prevent costly performance drops in fluid-based thermal management systems.
This work represents early-stage, theoretical and computational research. The findings could inform thermal engineers and designers developing heat exchangers, cooling enclosures, or solar thermal hardware by helping them choose suitable geometry and fluid mixtures. However, the abstract indicates this is fundamental parameter modelling rather than a practical device trial, placing it at a distance from direct commercial deployment.
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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.
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DOI: 10.1063/5.0186490
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