article · International Journal of Thermofluids
This research investigates natural convective heat transfer using an aluminium oxide and water nanofluid inside rectangular enclosures. Numerical simulations were conducted using the finite difference method, incorporating temperature-dependent viscosity and thermal conductivity. The analysis examined how nanoparticle volume fraction, nanoparticle size, and enclosure aspect ratio influence heat transfer rates and fluid flow patterns. The findings show that nanofluids improve heat transfer only when the aspect ratio remains below specific critical thresholds. When the aspect ratio exceeds these values, pure water actually outperforms the nanofluid, resulting in reduced heat transfer performance. This decline occurs because increases in fluid viscosity outweigh thermal conductivity gains when convection dominates conduction. Additionally, the critical aspect ratio depends on the nanoparticle volume fraction and increases with larger nanoparticle sizes, while smaller nanoparticles generally favour improved heat transfer.
Nanofluids are widely studied to improve cooling and thermal management in engineering systems. However, previous studies have shown conflicting results regarding their effectiveness. By identifying exact enclosure shapes and particle sizes where nanofluids help or hinder heat transfer, this work clarifies why performance varies and helps prevent counterproductive designs in thermal equipment.
This work is relevant to thermal engineering designers and developers of cooling systems using fluid-filled enclosures. It provides numerical criteria for selecting enclosure dimensions and nanoparticle properties to avoid performance losses. The findings represent early-stage simulation research, meaning practical implementation would require experimental validation in physical systems before integration into commercial thermal management products.
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Significant research has been performed on natural convection involving nanofluids inside rectangular enclosures since they are extensively used in thermal engineering. However, a significant gap remains in comprehending the conflicting results in nanofluid behavior observed between experimental and numerical studies. This study seeks to bridge this divide by examining natural convective heat transfer within vertical and horizontal enclosures filled with Al2O3/Water nanofluid. Using temperature-dependent viscosity and thermal conductivity, we aim to elucidate the key parameters influencing nanofluid enhancement. The results obtained through numerical simulations, using the finite difference method, showed the impact of different parameters namely nanoparticle volume fraction, 0 ≤ φ ≤ 0.05, nanoparticle size, dnp = 13, 29, and 45 nm, and aspect ratio, 0.25 ≤ A ≤ 4, on various factors, including heat transfer rates, maximal stream function, and temperature and stream function contours. The outcomes revealed that the use of nanofluids leads to improvements in heat transfer, but only when the aspect ratio is below certain critical values, Acr = 0.56, 0.48, and 0.53 for φ = 0.01, 0.03, and 0.05 respectively. Beyond these values, pure water performs better than the nanofluids in terms of heat transfer where the enhancement drops to -8%, -23%, and -22% when φ = 0.01, 0.03, and 0.05 respectively. Additionally, this critical value relies on the nanoparticle volume fraction and it rises as the nanoparticle diameter augments. This effect is believed to be caused by the high enhancement in viscosity compared to thermal conductivity when the convective mode dominates the conductive one. Furthermore, a larger aspect ratio leads to better heat transfer and the smaller nanoparticles are favorable for better heat transfer for a particular nanoparticle volume fraction. These findings carry significant implications for the field of nanofluid engineering, emphasizing the indispensable consideration of aspect ratios in optimizing thermal performance.
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DOI: 10.1016/j.ijft.2023.100501
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